MARIA CRISTINA GAMBI · CARLA MORRI
■ Invertebrate vagile fauna
Vagile fauna indicates all mobile
animals, i.e, those free to move
autonomously or, if sedentary, capable
of restricted movements in time and
space. In this section, we examine
vagile invertebrates, which include
many of the most typical species
faithfully associated with seagrasses in
general and Posidonia meadows in
particular, on whose leaves they
usually develop. Vagile fauna includes
Antedon mediterranea on Neptune grass
a large group of organisms belonging
to different phyla and with very different morphological and dimensional
characteristics, ecological requirements and functional roles. However, many
of these fauna living on seagrasses may also be found in other habitats, like
photophilic macro-algae and the coralligenous, and are therefore highly
cryptic organisms of herbivorous and herbivorous-detrivorous habits.
The first research on vagile fauna associated with Neptune grass and other
seagrasses, which we call “small” (seahorse grass, common eelgrass, dwarf
eelgrass, halophila seagrass) was carried out by the French school in the mid1960s. This research is now flanked by more studies along the Italian and
Spanish coasts. Although the fauna related to these systems in the eastern
Mediterranean is still little known, some new data are now available for the
coasts of Greece, Turkey and Tunisia. From the methodological viewpoint,
sampling of vagile fauna is very complex, at times poorly selective, and is
carried out in various ways by researchers. As regards leaf layers, the most
commonly used method by scuba divers is hitting the base of rhizomes hard
with a net which then grazes the leaf surface. This is a semi-quantitative
method which, once standardised, enables researchers to carry out reliable
Purple sea urchin (Sphaerechinus granularis) and sciophilous algae on rhizomes of Posidonia oceanica
Larger, or extremely vagile fauna (e.g.,
large decapods, mysidiaceans) is
captured with beam trawls (called
chalut by the French and gangamo by
the Italians) and epibenthic dredgenets. Underwater aspirators are the
most commonly used and least
destructive means of collecting fauna
associated with rhizomes and the
seabed. In addition, when this method
is used on fixed surfaces, larger
numbers of samples can be collected.
Other techniques, like core-boring, the
Snake star (Ophidiaster ophidianus)
use of grabs and removing matte
sections, with saws, cutting blades or shovels, are clearly very destructive.
Generally speaking, a single method of sampling vagile fauna does not exist
and therefore, according to the type of organisms analysed, their mobility,
cryptic characteristics, behaviour and ecology, a sampling method should be
devised especially for them.
From the taxonomical viewpoint, the main groups of vagile fauna associated
with seagrass systems are molluscs, crustaceans and polychaete annelids
(segmented marine worms) and, to a lesser extent, nematodes (roundworms),
platyhelminthes (flatworms) and echinoderms (like starfish). Flatworms and
roundworms are generally small, and roundworms are actually part of what is
called meiofauna (animals not longer than 0.5 mm) and are highly cryptic in
seagrass systems, although very diversified. Segmented worms are highly
diversified and well represented in fauna associated with Neptune grass,
especially at rhizome level and in mattes, where they are the dominating
invertebrate group. They have a metameric body structure, i.e., made up of a
series of equal segments along the longitudinal axis - a structure they share with
arthropods and one also frequently observed in the evolution of vertebrates.
Molluscs, which are a very large phylum, including terrestrial and freshwater
species, generally have a calcareous exterior shell to protect them. The shell
may be in single spiral, like that of gastropods, or divided into two or more
pieces (as in bivalves and chitons). Among gastropods, nudibranchs are
shell-less, just like cephalopods (cuttlefish, octopus and squid), many of
which have an interior supporing structure (the well-known cuttlebone).
Seagrass systems host particularly diversified gastropods, including several
nudibranchs and some cephalopods.
Crustaceans (arthropods) are among the most diversified group of vagile
fauna and, from many viewpoints, play a similar role to that of insects, their
close relatives, in freshwater and terrestrial systems.
Crustaceans associated with seagrasses are very small peracarids
(amphipods, isopods, tanaidaceans, cumaceans or hooded shrimps, and
mysidaceans) with very particular mouthparts. All these species develop
directly, their eggs being contained in brood pouches, from which the young
Decapod crustaceans are also very abundant and widespread. They are larger
than crustaceans and have 5 pairs of legs (hence the name decapod = tenlegged) with swimming forms (prawns and shrimps) and creeping forms like
crabs and the common hermit crabs.
Harpacticoid copepod crustaceans are also part of vagile fauna. They belong
to meiofauna and are the only ones that lead benthic lives, as opposed to the
majority of copepods, which are typically planktonic.
Echinoderms are frequently found in seagrass systems, with a few, very
characteristic and well-known species, including starfish, sea urchins and
brittle stars (ophiuroids), sea cucumbers (holothuroids) and feather stars
(crinoids). Generally, echinoderms exhibit fivefold radial symmetry, with
particular external structures - the outer shell of urchins - which become very
small plates or sclerites in starfish and sea cucumbers.
The gastropod Gibbula ardens, typically grazing Posidonia leaves
Vagile invertebrate fauna associated
with Posidonia oceanica canopies.
The mobile fauna associated with
Neptune grass canopies is generally
very small, mostly herbivorous or
herbivorous-detrivorous, and finds
shelter and nutrition by exploiting the
varied, complex epiphytic felt covering
the leaf surfaces. Organisms that have
this particular life-style are called
mesoherbivores, typical forms of many
other vegetated coastal systems. It is
precisely in seagrass canopies that we
Starfish (Astropecten spinulosus)
find the most characteristic species
associated with these plants, because leaf covers have unique micro-climatic
conditions that select living forms which are specially adapted to them. Most
of the vagile fauna associated with seagrass canopies (crustaceans, molluscs,
echinoderms) carry out daily migrations along the vertical axis of meadows,
indicating that species and numbers vary considerably between day and
night, and it is also very difficult to determine whether some species rightly
belong to the canopy or to rhizomes.
Vagile fauna, which is variously associated with the dynamics of seagrass
canopies and their animal and plant epiphytes, has very different temporal and
spatial composition and population structures, due to the highly dynamic life
habitat. The limited research carried out in the Mediterranean on the
relationship between meadow structure and vagile fauna suggests a positive
relationship between leaf bundle density, and therefore leaf cover, and the
diversity and/or abundance of fauna, at least during the seasonal development
of the canopies. However, many of the main groups of vagile fauna develop in
patches, and the spatial variability scale is still unclear, as are the abiotic and
biotic environmental factors causing it.
From the functional viewpoint, vagile fauna plays an important although often
underestimated role in Posidonia meadows, in which greater attention has
been given to the debris chain or route, which seems to prevail in energy and
biomass terms. The group of organisms that make up vagile fauna are the
‘grazing chain’, which exploits plant epiphytes of leaves to transfer matter and
energy to higher trophic levels, secondary consumers and large predators
(large decapods, octopuses, fish). Although in biomass terms vagile fauna has
clearly lower values than the large detrivorous invertebrates of Posidonia
systems (urchins, sea cucumbers, many large decapods) and rhizome
epibiotic filterers (sponges, sea squirts, bryozoans, sabellid polychaetes), the
higher turnover of mesoherbivores - due to their small size and short lifecycles - compensates their net production. In this way, the two main routes of
energy transportation - debris and grazing - probably converge.
Posidonia canopies host few polychaetes (even at night), both as number of
species and especially as individuals. Although this group is highly diversified
in rhizomes and mattes (250 species counted in some meadows of the
Tyrrhenian and Spanish coasts), only 5% of the species are estimated to live in
the canopies. Among the most frequently found are the nereidid Platynereis
dumerilii, a herbivorous species feeding on epiphytic macro-algae, the ophelid
Polyophthalmus pictus, and several species of small, interstitial syllids like
Sphaerosyllis spp. and Exogone spp., which feed on the felts of epiphytic
diatoms, and macro-benthic syllids like Syllis, most of which are carnivores
and live on animal epiphytes, especially hydrozoan colonies. Although there
are few polychaetes in canopies, they become more frequent and diversified in
the deeper sections of the meadows.
Molluscs, especially prosobranch gastropods, make up one of the prevailing
groups on canopies. Many of them are grazing mesoherbivores whose
mouthparts (radulae) are specialised in scraping epiphytic layers. Although
almost all of them are herbivores, each species is specialised for a certain
A hermit crab and hydrozoans on the tip of a Posidonia leaf
group of plant epiphytes, in order to
restrict competition and better exploit
food availability within the system. The
most typical and frequent are the
globular rissoids (minute sea snails)
with species like Rissoa variabilis, R.
ventricosa and R. violacea. Other
common rissoids belong to the genus
Alvania (A. discors, A. lineata) and to
the genus Pusillina.
Other gastropods typical of surface
meadows are Gibbula ardens and G.
umbilicalis, the trochid (top snail)
Jujubinus striatus and J. exasperatus
and the turbinids (turban snails) Tricolia
pulla, T. speciosa and T. tenuis. More
ubiquitous species, which migrate
Turban snail (Tricolia tenuis) grazing on a
vertically between leaves and
rhizomes, are also noteworthy, like
Bittium reticulatum, B. latreilli and Columbella rustica.
There are also opisthobranchs (gastropods with small shells) and nudibranchs
(shell-less) occasionally found between leaves. These are quite specialised
carnivorous organisms that feed on sessile epiphytic animals. Among them
are the genera Doto, Eubranchus and Cuthona, which prey on hydrozoans, the
genera Polycera and Janolus, which feed on epiphytic bryozoans, and
Goniodoris and Berthella, preferring colonial tunicates (sea squirts). Other
organisms like Chauvetia mamillata and Favorinus branchialis are specialised
in preying on the eggs of other invertebrates. Cephalopod molluscs like
cuttlefish (Sepia officinalis) and bobtail squids (Sepiola sp.) sometimes swim
between the leaves of seagrass meadows looking for food and shelter from
Molluscs are the group that best shows population zoning along meadow
height, generally accompanied by particular morpho-functional adaptations
of the species (shell and foot shape, locomotion, size, type of reproduction,
nutrition) associated with the overall environmental gradient. Generally
speaking, the upper sections of meadows (0-5 m) are less populated,
although the species are larger and more characteristic (Gibbula, Jujubinus).
Middle sections (10-15 m) host larger numbers of more diversified species,
and deep sections (15-20 m) contain more ubiquitous species as well as
those coming from nearby environments (soft seabeds, coastal debris, the
These organisms exhibit great biodiversity and considerable variability in
population composition and structure, according to geographic area, season,
depth, soil characteristics, and the circadian rhythm that some of them have.
However, the genera and some of the species listed above make up a
constant nucleus often found in many environments and geographical areas of
the Mediterranean, with some substitutions. Most mollusc species usually
have short life-cycles (1-2 years) and reproduce directly by laying small
clusters of eggs. Larval stages occur inside the eggs, which hatch to reveal
already formed juveniles. This type of reproduction is associated with the
small size of these animals in seagrass meadows.
Crustaceans colonise canopies: most of them are peracarids, especially
amphipods whose large numbers make them the favourite prey of many
cephalopods and fish. They are therefore an essential link in the food-chain of
Posidonia meadows, linking primary producers (plant-vegetal epiphytes) to
higher trophic levels. Posidonia meadows host 80 amphipod species, which
are those that carry out the most evident and greater daily migration: their
numbers in canopies are large by day and even larger by night.
Although there are no amphipod communities with species and structure
exclusive to Posidonia meadows and constantly found in meadows, the species
Cuttlefish (Sepia officinalis)
most frequently found in canopies are
chiereghinii, Aora spinicornis, Ampithoe
helleri, Caprella acanthifera, Hyale
schmidtii, Phtisica marina, Eusiroides
dellavallei, Ampelisca pseudospinimana
and Maera inaequipes.
Most of them are herbivorous or
herbivorous-detrivorous and can feed
on several species of plant epiphytes,
from diatoms to filamentous macroalgae, which they remove and brush
with their antennae equipped with
thin filaments acting as combs.
Amphipods, like all peracarids,
develop directly, and adult females
incubate their eggs.
The decapod Hippolyte inermis camouflaged
The great diversity of this crustacean
on a Posidonia leaf
group is thought to be favoured by this
type of reproduction, which limits spatial dispersion and increases the
possibility of reproductive isolation and adaptations to particular local
Although isopods are another group of peracarid crustaceans less diversified
than amphipods, their retinue species are more markedly adapted to
Posidonia meadows and their canopies in particular. This is the case of Idotea
hectica, one of the few directly herbivorous species, i.e., capable of feeding on
the living tissues of Neptune grass leaves. Other typical species are Astacilla
mediterranea and a few species of the genera Gnathia, Cymodocea and
Cleantis. Large numbers of isopods also migrate daily, especially at night.
Among peracarids floating near Posidonia leaves, there is a large group of
mysidaceans - micro-shrimps - which form dense, fast-moving swarms, the
favourite food of several fish. The species associated with seagrasses are
Siriella clausii and Mysidopsis gibbosa, some species of the genus
Leptomysis, with L. posidoniae and L. buergii, and a species recently
described in Italian meadows, Heteromysis riedli. This was named in memory
of one of the most important biologists of the Mediterranean, the Viennese
Rupert Riedl, a pioneer in marine biology research and examination of sea
caves by scuba-diving, as well as the inventor of an original benthos zoning
chart based on the dynamics of shore water movement.
Other, smaller groups of peracarid
crustaceans are tanaidaceans and
cumaceans, with a retinue of quite
ubiquitous species, like Leptochelia
savignyi, a species linked with plant
debris generally found in many
vegetated systems along the coast.
Decapod crustaceans are well
represented both by forms floating
near leaves and by creeping ones. The
most abundant and diversified family of
floating animals are hippolytid shrimps
of the genus Hippolyte. In particular, H.
inermis is brilliant at camouflage: its
original bright-green livery can change
colour very rapidly, and a pinkish shade
is adopted when imitating the colour of
Creeping hermit crab (Calcinus tubularis)
epiphytes, especially those of
encrusting corallinaceous algae. In situ
and laboratory studies have also revealed the particular life-cycle and
reproductive biology of this species. The diatom-based diet of post-larvae
living in leaves favours the precocious sexual transformation of males (all
hippolytid shrimps are born males) into females, thus balancing the gender
ratio in the population.
Other decapod species typically living on leaves are Thoralus cranchii,
Palaemon xiphias and species of the genus Processa. These last species are
carnivores that migrate to the leaves at night to feed on other small
invertebrates. Other, very numerous species, particularly at night, are the
creeping hermit crabs Cestopagurus timidus and Calcinus tubularis, and the
galatheids (squat lobsters) Galathea bolivari and G. squamifera.
Among echinoderms, the only species truly typical of Posidonia canopies and
very similar to the more common larger, pinkish starfish Asterina gibbosa, is the
asteroid Asterina pancerii, whose greenish colour gives it effective camouflage
in its habitat. A. pancerii is a typical Mediterranean endemic, strictly nocturnal
and a carnivore, feeding, like many other starfish, on small molluscs. This
species incubates eggs, a rather unusual characteristic among Mediterranean
echinoderms, which may represent an adaptation to life in seagrass systems.
Among other leaf-loving echinoderms is the common edible sea urchin,
Paracentrotus lividus, whose numerous populations live in Posidonia meadows
where they find protection between rhizomes by day. At night, especially their
young migrate to the canopy to graze on epiphytic felts. The marks of their
grazing are typically seen on the tips of the oldest leaves. Crinoids, or featherstars, are frequently seen on the leaves, like Antedon mediterranea, a species
exhibiting different colours and which incubates its eggs.
The vagile fauna associated with the canopies of meadows formed of small
seagrasses (common eelgrass, seahorse grass, dwarf eelgrass) has been the
least well examined in the Mediterranean. More information is available on
seahorse grass, which is the most widely distributed species after Neptune
grass, and may even be found at considerable depth (30 m).
The mobile fauna on seahorse grass canopies is equal to that living on
Neptune grass, although with fewer species for each characteristic group
(amphipods, isopods, molluscs, polychaetes), i.e., an impoverished fauna
compared with that living on Posidonia. Echinoderms, for instance, are almost
absent on small seagrasses, except for the minute green sea urchin
Psammechinus microturbeculatus which, precisely because of its tiny size,
can graze on the leaves of seahorse grass and is not found on Neptune grass.
Although their total biodiversity is reduced, some species may actually be more
numerous in these habitats. For example, some interstitial syllid polychaetes of
the genera Sphaerosyllis spp. and Exogone spp., the molluscs Bittium
reticulatum and Jujubinus gravinae, which appear to replace J. exasperatus and
J. striatus ecologically, are typically found on Neptune grass, and some
peracarids (like the tanaidacean Leptochelia savignyi, and amphipods
Synchelidium haplocheles and Pariambus typicus) also appear. Generally, the
limited complexity of these canopies, due to the small size of leaves and faster
temporal dynamics, favour small species with interstitial habits.
The numbers of mobile animals on seahorse grass undergo greater seasonal
variations than those living on Posidonia, due to the greater variations of leaf
bundle density in meadows and the morphology of canopies over the year.
The extent of faunal colonisation therefore depends on meadow density and
general environmental conditions. When exposed to strong wave action, for
example, Cymodocea meadows disappear in winter because tufts are
uprooted by waves: this explains the extreme dynamics of these systems and
the consequences for their associated communities.
One of the few manipulative experimental studies available for the
Mediterranean shows how reduction and gradual, complete removal of
Cymodocea canopies have dramatic effects on the structure of fauna, with
plummeting numbers of some groups, which may disappear completely.
When wave action is very strong or leaf bundles are not very dense, as in
meadows at 15-20 m, or in winter, Cymodocea systems are very similar to
bare, soft seabeds. This explains why the benthic bionomy of the French
school defines these systems as “epiflora facies” of fine sand.
Asterina pancerii, the typical asteroid echinoderm of Posidonia meadows
Green sea urchin (Psammechinus microturbeculatus)
Vagile invertebrates associated with rhizomes and the seabed. The animals
associated with Posidonia rhizomes and the seabed include species from
several phyla, many of which have been mentioned above as living on
Posidonia canopies. Vagile animals associated with rhizomes are generally
larger, less specialised and more ubiquitous, and may be found in other
vegetated habitats, even in softbed biotopes, because they are associated with
the type of sediment in which Neptune grass grows. In addition to groups
typical of canopies which, as mentioned before, carry out conspicuous daily
migrations from rhizomes to leaves (amphipods, isopods, tanaidaceans,
molluscs), the most frequently found forms in the rhizome-seabed area are
polychaetes, decapod crustaceans, molluscs and echinoderms. Very important
and numerous are also smaller groups of platyhelminthes, and meiofauna like
nematodes and harpacticoid copepods. Similarly to sessile fauna, also for
mobile fauna living on rhizomes and the seabed, there are fewer species
associated with Posidonia systems than those living on this plant’s leaves or
migrating to it at night. Near rhizomes and on the sea bottom, wave action and
light are restricted and decrease with depth, and the accumulation of
suspended particles increases as they are caught in seagrass canopies.
Zoning of fauna associated with rhizomes is less clear, since it sometimes
overlaps or may even be completely absent, both at different depths and in
different meadows. In addition, the type of substrate on which meadows grow
Hairy crab (Pilumnus hirtellus)
(rock, debris, coarse sand, silty sand)
and the different rates of local
sedimentation cause the associated
fauna to become richer in species
related to hard substrates or typical of
very different types of sediments. The
numbers of animals living in these
areas of meadows are also influenced
by the density of bundles, i.e., by
substrate availability for colonisation
and meadow type in general.
Continuous or patchy distribution of
meadows, and the presence of
clearings, channels and other
interruptions to meadow continuity
give rise to mosaic patterns that favour
colonisation by other species, thus
An isopod on a leaf of Neptune grass
increasing overall biodiversity.
Polychaetes are highly diversified precisely at rhizome and seabed levels,
although no community is precisely typical of Neptune grass meadows.
Polychaete populations associated with seagrasses are composed of a
mixture of species with different ecology coming from vegetated
environments, habitats with soft or hard beds, and none of them are exclusive
to Posidonia systems. Exceptions are some species that appear to be more
closely related to Neptune grass, like Pontogenia chrysocoma, Pholoe minuta,
Kefersteinia cirrata and the sedentary species Polyophthalmus pictus.
Generally, about one-third of polychaete species living in this area of
meadows belong to the syllid family, with both macrobenthic organisms like
Syllis garciai, S. columbretensis and S. gerlachi, and interstitial ones such as
Sphaerosyllis spp., Exogone spp. and Salvatoria spp.
Other very diversified families living at rhizome level are phyllodocids (paddle
worms), polynoids (scale worms), nereidids (ragworms), hesionids, and some
large species like the aphroditid Laetmonice hystrix (fireworm), a predator that
migrates to meadows from nearby silty beds in search of food. Noteworthy are
some species of eunicid worms (bobbit worms) living in Posidonia fascicles
(the bases of fallen leaves that remain attached to rhizomes to form muffshaped covers), in which they burrow characteristic winding tunnels. These
animals belong to the special category of borers, and are among the few
which can chew into the horny fascicles of Neptune grass, use and move this
type of Posidonia debris, which is
usually considered inedible and is only
attacked by fungi and bacteria.
Examples of borers are Lysidice
ninetta, L. collaris, Nematonereis
unicornis and Marphysa fallax. Boring
polychaetes were first described in
meadows along the Italian coastline,
and were later found in other
Lysidice ninetta, a polychaete boring into
Mediterranean areas (Spain, France,
Croatia, Turkey and Greece). They
colonise Posidonia rhizomes along all
the distribution area of the plant,
particularly in intermediate and deep
meadows, and are usually found in
fascicles between 2 and 4 years old
(lepidochronological years). These
species live in the fascicles throughout
the year, become sexually mature in
summer, and produce pelagic larvae, a
Nematonereis unicornis, a polychaete which
fact which favours their large-scale
also bores into Posidonia fascicles
Among molluscs associated with rhizomes, many species also live on leaves,
like the genera Alvania, Gibberula, Jujubinus, Pusillina and Bittium which, as
mentioned above, move between leaves and rhizomes in the daytime. There
are also larger species, such as cerithids - Cerithiopsis tubercularis, C.
minima, and Cerithium vulgatum - muricids like Hexaplex trunculus (banded
dye-murex) and Bolinus brandaris (purple dye-murex), and members of other
families like Conus mediterraneaus and Calliostoma laugeri.
In meadows growing on rock, such as most of those along the Sicilian
coasts, rhizomes are colonised by several species living on hard substrates,
like green ormer (Haliotis tuberculata) and the cypraeids Mediterranean
cowry (Erosaria spurca) and Luria lurida, whose empty shells are often found
in clearings and channels near mattes. Attached to rhizomes and to the
pebbles scattered on the bottom are sedentary chitons (sea cradles, like
Lepidopleurus cajetanus). Clearings in between mattes often host gaudy
species like the opistobranch (sea slug) Umbraculum mediterraneum and
the knobbed triton Charonia lampas, which is a threatened species listed in
the Habitats Directive.
Among cephalopods, there is the common octopus (Octopus vulgaris) and the
white-spotted octopus (Octopus macropus), whose typical lairs are found at
the margins of meadows, and in clearings and channels between mattes.
These cephalopods are the most active predators living in meadows where,
especially at night, they feed on decapod crustaceans and other molluscs, like
green ormer and many bivalves.
Among peracarid crustaceans which, as previously described, are those
carrying out the greatest vertical migrations from rhizomes to leaves, some
isopods are noteworthy because they are associated with the rhizome layer,
like Cleantis prismatica. This animal inserts part of its body inside a piece of
Posidonia root, which it carries around and uses as a shelter when needed.
Another peracarid associated with fascicles is Limnoria mazzellae, a species
dedicated to the Posidonia botanist Lucia Mazzella. This species is a fascicle
borer like the eunicid polychaetes mentioned above. It burrows complex
tunnels in recent fascicles (0-1 years old), starting from the fascicle tip.
Especially in summer, the tunnels host entire families made up of two or more
adults and several juveniles. The species, which is particularly abundant in
summer in the superficial areas of meadows subjected to stronger wave
action, is an example of speciation associated with Posidonia. This species is
different from other Limnoria species - usually woodborers - probably
precisely in order to adapt to such a particular micro-environment like that
offered by the fascicles of this seagrass. Direct development, which is
common to all isopods, is the adaptation that undoubtedly favoured this
process of speciation.
Peracarids dominate very particular micro-environments which seasonally
form in meadows, i.e, debris mounds,
which are accumulations of Posidonia
leaves, bundles and propagules
collecting in clearings and channels or
along the upper and lower margins of
meadows. These microhabitats are
quite ephemeral, as they are linked
with extensive leaf shedding in autumn
and with local wave action, which
favours their formation and sudden
destruction (storms, bottom currents).
The little research available on debris
mounds shows that they host dense
Common octopus (Octopus vulgaris)
populations of gammarid amphipods
of the genus Gammarus, with species that are not found on leaves (G.
aequicauda, G. subtypicus, G. crinicornis), presumably specifically
associated with this type of habitat. Other typical species are the amphipods
Atylus spp. and Melita hergensis, and the isopods Idotea hectica and I.
baltica. These small detrivorous crustaceans play an important role in the
fragmentation of leaf debris, which is essential for recycling into the system a
source of carbon which would be wasted if unused. By reducing the size of
debris, these organisms favour its further degradation by bacteria and fungi,
and make it available to other detrivores like sea cucumbers, as we shall see.
Most crustaceans associated with the rhizome and bottom layers are
decapods, especially creeping forms, which become more abundant and
diversified here than in other areas of canopies. Once again, the prevailing
species of hermit crabs at rhizome levels are Cestopagurus timidus and
Clibanarius erythropus, which use the empty shells of dead gastropods
associated with Posidonia.
Other frequent species are Athanas nitescens, Pisidia longimana, Alpheus
dentipes, Processa edulis and Galathea spp. Among crabs, there are several
species of portunids, xanthids (mud crabs) and majids (spider crabs), which
may sometimes be quite large (Macropipus spp. and Maja spp.). Creeping
decapods are generally detrivorous and feed on leaf debris and its epibionts,
and actively move about on the bottom, between rhizomes and leaves. There
are also species preying on other invertebrates (Processa spp., Galathea spp.),
some of which, like Dromia personata and Scyllarus arctos, come from nearby
habitats. These animals are in turn preyed on by fish (mullet, scorpionfish) and
cephalopods (common octopus, white-spotted octopus) and play an
important role in the food-chain of Posidonia meadows.
Among the most typical vagile animals found in rhizomes, the most abundant
and frequent are echinoderms, especially sea urchins and sea cucumbers. The
most typical sea urchin species are the common edible sea urchin
(Paracentrotus lividus) and the purple sea urchin (Sphaerechinus granularis),
whose spines exhibit colours ranging from deep purple to white. Both species
graze on rhizomes (although the young of Paracentrotus may actually climb
leaves in order to chew on them), removing debris and plant epiphytes and
leaving their typical bite-marks on fascicles.
Sea cucumbers are typically found along the margins and boudaries of
meadows, clearings, intermatte channels and in debris mounds. They
relentlessly devour sediment and fine leaf debris, from which they first obtain
the energy required to survive, and then expel them in the form of
characteristic sediment cordons compacted by their intestinal mucus. Their
implacable feeding makes sea cucumbers the greatest bio-disturbers of
meadows, as they dislocate great quantities of sediment daily. The most
common species on Neptune grass are Holothuria polii and H. tubulosa. As
The decapod Processa sp. on a meadow of Cymodocea nodosa
Sea cucumber (Holothuria tubulosa)
adults, these species do not have
natural predators and live in a peculiar
type of symbiosis with the pearlfish
Carapus acus, which lives in their
Other occasional visitors to Posidonia
meadows are the red starfish
Echinaster sepositus, the spiny starfish
Marthasterias glacialis, and the snake
star Ophidiaster ophidianus, which are
usually found on hard substrates
associated with photophilic algae.
Red starfish (Echinaster sepositus)
Also the sand-burrowing brittle star
Acrocnida brachiata and the small
brittle star Amphipholis squamata find
the ideal habitats for their cryptic life in
the rhizomes of Posidonia meadows.
All these organisms actively prey on
bivalves and sea urchins associated
with rhizomes and sediments.
In systems formed by other small
seagrasses, rhizomes are always
hypogeal (growing under the surface),
and therefore specific layers cannot be
identified. Small seagrasses colonise
Banded dye-murex (Hexaplex trunculus)
sediments (lagoons, estuaries and
ports), and host many species typical of these environments.
Epifauna associated with the seabed is therefore restricted, as is often the case
in soft beds. There are few decapod crustaceans - except for some hermit
crabs - and echinoderms - apart from the green sea urchin Psammechinus
microturbeculatus and a few brittle stars. The gastropods Bolinus brandaris and
Hexaplex trunculus are the most numerous mollusc species living on seahorse
grass and common eelgrass.
Limited research has also been carried out on the fauna associated with
another small seagrass, Halophila stipulacea. Its fauna is very similar to that
living on seahorse grass, which is influenced by local sedimentary and
ecological conditions. The most abundant species are gastropods Bittium
reticulatum and amphipods Caprella acanthifera and Gammarella fucicola.
Infauna of mattes, clearings and swards. As described in the previous
chapters, mattes are bioconstructions typical of Posidonia systems. They
form as a result of the horizontal and vertical growth of this plant’s rhizomes,
of sediments and organic debris accumulating on the bottom, and of
entwining roots and rhizomes.
Mattes are therefore particular substrates which have the characteristics
of both hard substrates (roots, hypogeal portion of rhizomes, calcareous
remains of organisms, pebbles, etc.) and soft ones. Their compactness
and penetrability change according to the climatic conditions affecting
the meadow in terms of sedimentation and wave action. Mattes may
develop from a few centimetres to up to a few metres and depend on type
of sediment; their degree of compactness may favour colonisation by
The complexity and compactness of mattes deeply affect sampling and study
of this habitat, which are particularly difficult. This is one of the reasons why
matte fauna is the least studied and known of all areas of Posidonia.
Fauna living in mattes belongs to infauna, i.e., benthic fauna living in the
substrate. Mattes persist long after bundles and entire portions of
meadows have died, and are thus known as dead mattes. Due to fine
sediments and the compacting action of rhizomes, mattes are oxygenated
only in the upper sediment layers, generally the top 5-10 cm: below this
Soft venus (Callista chione), a bivalve mollusc typical of Posidonia mattes
level, conditions gradually become anoxic (lacking oxygen). This influences
the fauna, which lives in the upper portion of mattes and dramatically
decreases with depth. The only exceptions are some tube-dwellers and
borers, which may penetrate the deepest layers and re-emerge outside, i.e.,
on the sediment surface.
Matte infauna is mainly composed of polychaetes and a few other groups,
like molluscs - especially bivalves - and a few decapods and echinoderms.
There are also large meiofauna groups, like nematodes and harpacticoid
crustaceans, especially in the top 1-2 cm of sediment.
The polychaete species living in mattes are typical of sandy-muddy
sediments, with burrowing detrivores feeding on sediment, like capitellids
(lugworms) and maldanids (bamboo worms), and superficial detrivores like
spionids, paraonids, cirratulids (fringe worms), lumbrinerids and nereids.
More than 180 polychaete species live in mattes.
Among molluscs, the most numerous are bivalves, which are typical of soft
bottoms, and scaphods (tusk shells). Other animals that prefer to live in these
environments are some edible species like warty venus (Venus verrucosa) and
soft venus (Callista chione), which are harvested by means of fishing methods
that destroy mattes and meadows. Ubiquitous species associated with
mattes also include Plagiocardium papillosum, Tellina balaustina, Lucinella
divaricata, Glans trapezia, Venericardia antiquata, and the tusk shell Antalis
Although gastropods are less frequent, some carnivorous species live partially
burrowed in sediments and feed on bivalves, like Tectonatica filosa, Lunatia
poliana and Nassarius (Hinia) incrassata.
There is only one decapod species typical of mattes, Upogebia deltaura, a
mud lobster that burrows deep, winding tunnels inside sediments. Although
matte populations are not very different from those living in dead mattes, it
would be worth improving the scanty research that has so far been carried
out on them.
Particular environments in Posidonia meadows are pockets of debris found in
clearings and channels, discontinuous meadow structures generally
surrounded by exposed, well-developed mattes. In these habitats, sediments
are rather coarse and mainly composed of organic debris, i.e., deriving from
the calcareous shells of organisms living in meadows themselves, like
molluscs, echinoderms, bryozoans and corals. These are the erosion areas of
meadows, with the strongest bottom currents and greatest wave action,
which produce the typical ripple marks on the sea floor. Due to the particular
dynamic conditions, debris accumulates in clearings and channels. Infauna
living in these pockets of coarse sediment is made up of species generally
found in coastal debris, the most impressive of which are bivalves of the
genus Glycymeris and Tellina, the groove burrowing sea urchin Brissus
unicolor and the purple heart sea urchin Spatangus purpureus, whose empty
shells often lie on sediment.
Fauna associated with the sediments of meadows formed of small
seagrasses is composed of species characteristic of incoherent seabeds
whose composition generally depends on the particle size of the
sediments themselves. However, the presence of plants and hypogeal
roots and rhizomes do influence the characteristics of sediments to a
For instance, surface meadows of mixed seahorse grass and dwarf eelgrass,
with tufts of 2000 bundles/m2 may produce a thick, compact step of
sediment, roots and rhizomes, called a sward which, despite the shallow
water, contains high percentages of mud. A few centimetres deep in swards
(0-5 cm) conditions become anoxic. In this environment, fauna, mostly
composed of polychaetes, lives in the superficial layer of sediment, and only
a few species adapted to very low oxygen levels can survive at greater
depths, like capitellids (Heteromastus filiformis, Capitella sp.) and the bivalve
Lucinella divaricata, whose mantle tissues contain symbiontic
Purple heart sea urchin (Spatangus purpureus), typically found in intermatte clearings
■ Sessile fauna
Sessile animals are those living permanently attached to the substrate. In the
sea, they are quite numerous on rocky bottoms and, more generally, on any
substrate hard enough to enable them to adhere. On sandy and muddy
seabeds, sessile organisms may be found only on the very small hard
substrates (stones, shells, etc.) scattered on sediments. Seagrass leaves and
rhizomes offer sessile fauna very particular substrates, which often require
special adaptations. In particular, seagrass leaves fluctuate and continually
renew themselves and, due to their small size, can only be colonised by
minute organisms. Although rhizomes are slightly stabler, they are more
selective than the rocky floors which sessile fauna usually inhabits. Due to
their adaptations, these organisms living on seagrasses are called epiphytes,
but are different from those found on other substrates. This is particularly true
of Neptune grass leaves, less for its rhizomes and for other seagrasses, whose
fauna is less characteristic.
Seagrasses therefore play an important role in the ecology and evolution of
sessile fauna. Hard, rocky substrates make up only a small portion of littoral
seabeds, which are generally composed of sedimentary rocks, inhospitable
habitats for these types of animals. Sessile organisms can colonise these
environments only in two ways: by “jumping” from one small island of hard
Electra posidoniae, an epiphytic bryozoan exclusive to Posidonia leaves
substrate to another – avoiding direct
contact with sediments - or by putting
up with the hostile habitat and
developing adaptations that prevent
them from sinking into the sediment.
The latter solution is used by large
animals, like burrowing sea anemones
and sea pens, and the former has been
adopted by small, opportunistic
species with short life-cycles. This,
however, has led to ecological
speciation for epiphytes of living
Epiphytic hydrozoans on Posidonia leaves
organisms, like molluscs (some
hydroids, for instance, live exclusively
on bivalves, gastropods, or hermit crabs in gastropod shells) or seagrasses. In
the Mediterranean, Posidonia oceanica is the seagrass that forms the most
extensive and stable meadows, thus offering good opportunities for avoiding
sedimentary environments. It is not surprising then, that the only true
specialised epiphytes live on Neptune grass. Generally, sessile species living
on seagrasses are colonial: colonies are created by budding from a founding
individual developing from a single, minute larva colonising the substrate.
Colonies represent efficient strategies to monopolise suitable substrates, once
they have been located. Seagrass epiphytes usually belong to several
taxonomic groups, from the simplest, like protozoans, to the most highly
evolved, like chordates.
Protozoans are unicellular organisms that were classified as animals in the
past and are now ascribed to other kingdoms: they are generally microscopic,
although some species are visible to the naked eye. Most of those living on
seagrasses belong to the class of forams (foraminiferans).
Porifers or sponges are the most primitive animals, almost exclusively sessile
and of various sizes and shapes. There are two types living on seagrasses:
calcisponges, with a few, tiny species, and demosponges, which include most
Cnidarians are a large group of animals with stinging cells (cnidocysts) and
two different body forms: polyp and medusa. Polyps are generally sessile, and
medusae, such as jellyfish, are free-swimming. Polyps may live alone or in
colonies, according to species. On seagrasses, hydrozoans are the most
numerous species (with polyps and, less frequently, with medusae), and there
are also some anthozoans (corals).
Although most polychaete annelids are vagile, some are sessile and live in
tubes that they secrete themselves and which secure them to the substrate.
Tubes may have mucus, mud, parchment and rubber textures, like those of
feather duster worms (sabellids), and may even be calcareous, like those of
serpulids and spirorbids. Some of these families are occasionally found on
Surprisingly, arthropods, which in the sea include mainly crustaceans, are
represented here by sessile species, like barnacles (cirripeds) and the wellknown Poli’s stellate barnacle. They may also be found on seagrasses.
Almost all bryozoans (also known as moss animals) are colonial, and each
colony houses many tiny individuals called zooids. Colonies are sometimes
very large and are typically sessile, encrusting or erect. Seagrasses host
species of three orders: cyclostomes, with calcitic, tubular zooids;
cheilostomes, with box-shaped calcitic zooids; and ctenostomes, with noncalcified sac-shaped zooids.
Even chordates include sessile species: tunicates belonging to the ascidians
(sea squirts). There are both solitary or “simple” and colonial species. The
minuscule individuals that form their colonies are called ascidiozooids. If they
are joined at the base by a single stolon, they are called social ascidians; if
they are all enclosed in one tunic, they are known as compound ascidians.
Some species colonise seagrasses.
Aglaophenia harpago, an epiphytic hydrozoan, on Posidonia leaves
Epiphytic fauna on Posidonia oceanica. Although several animals are
epiphytic on the leaves of Neptune grass, only cnidarians and bryozoans include
species which are so highly specialised that they cannot be found on any other
substrate. They are called exclusive characteristic species. Their level of
specialisation is such that they dominate over the many other leaf colonisers,
and sometimes even make up 95% of the entire epiphytic population. As
Neptune grass is a Mediterranean endemic, these species are also endemic,
although their distribution is similar throughout the Mediterranean so that, apart
from a few exceptions, epiphytic fauna exclusive to Neptune grass is always the
same, independently of the geographic area within the Mediterranean.
Bryozoans are generally the most numerous animals on the surface of leaves,
with the exclusive species Electra posidoniae. This is a cheilostome forming
ribbon-shaped, slightly calcitic, encrusting colonies.
Other bryozoans exclusive to Neptune grass leaves are cheilostomes that
produce encrusting colonies which are, however, round instead of ribbonshaped, like Collarina balzaci, Fenestrulina joannae and Ramphostomellina
posidoniae. The last species is so far known to inhabit only the Aegean Sea,
and may therefore be an exception to the general rule that epiphytic fauna on
Posidonia is found throughout the Mediterranean.
Many other bryozoans are found on leaves, although not exclusive to them.
Among ctenostomes there is Mimosella verticillata, M. gracilis and Pherusella
tubulosa. Cheilostomes are represented by species of the genus Aetea (A.
anguina, A. lepadiformis, A. sica, A. truncata), Celleporina caliciformis,
Chorizopora brongniartii, Fenestrulina malusii, Haplopoma impressum and
Microporella ciliata. Cyclostomes are Disporella hispida, Patinella radiata and
Hydroids may also be quantitatively and qualitatively very important. Four
species are typical and exclusive: Aglaophenia harpago, with feather-like
colonies about 15 mm high, Orthopyxis asymmetrica (= Campanularia a.), with
stolon-shaped colonies a few millimetres high, Pachycordyle pusilla, which
also has stolon-like colonies with naked polyps, and Sertularia perpusilla, with
erect colonies up to 7 mm high. A. harpago and P. pusilla are not precisely
exclusive, as they have also been found on seahorse grass. There is also a
Sertularella species that may be exclusive to Posidonia leaves, but experts still
need to describe it formally.
From the ecological-evolutionary viewpoint, an interesting case is afforded by
Monotheca obliqua. This species has pinnate colonies as high as 40 mm, and is
found on various substrates and on Posidonia leaves. Some morphological
differences have led experts to distinguish a typica form - ecologically
distributed on hard sea floors between 5 and 30 m in depth - and a posidoniae
form, exclusive to Posidonia leaves. Habitat specialisation may lead to genetic
isolation of the two forms, and Monotheca posidoniae, as it is sometimes called,
may be an instance of initial speciation.
Several other species of hydroids are frequently found on Posidonia leaves,
although these are not their favourite habitats. Among the most commonly listed
species in literature there are Aglaophenia picardi, Antennella secundaria,
Campanularia hincksi, Clytia hemisphaerica, Dynamena disticha, Eudendrium
simplex, Halecium pusillum, Obelia dichotoma and O. geniculata. Hydroids
typically found on Posidonia do not have a medusa phase, and this is viewed as
an adaptation enabling them to stay inside the habitat for which they are
specialised: medusae could easily float away with the current and end up in
areas with no Posidonia.
However, whenever dealing with biodiversity, there are odd exceptions, and one
of these is the root-arm medusa Cladonema radiatum. Its polyps are joined in
small, simple, slightly ramified colonies rising from a creeping hydrorhiza.
Although they are not exclusive to Posidonia leaves, large numbers of these
medusae are frequently found in these environments. Umbrella-shaped
medusae about 3-4 mm wide are not free-swimming, and creep on leaves by
adhering with special stalked buttons placed on their tentacles. Olindias
phosphorica and Scolionema suvaense have similar adaptations.
Anthozoans also have a typical species exclusive to Posidonia leaves: the small
sea anemone Paractinia striata, whose polyp is brownish with longitudinal
stripes. For the sake of precision, sea anemones should not be included in
sessile fauna because, although their basal disc does adhere to the substrate,
it is not secured in one place, thus enabling the animal to move ever so slightly.
P. striata’s specialisation to life on Posidonia leaves is due to its large basal disc
and flattened body, characteristics this animal shares with other typical sessile
Other species are sessile forams like Cibicides lobatulus, Iridia serialis and
Rosalina globularis. Although other animals colonise Posidonia leaves,
sometimes even in large numbers, they are not characteristic of them. The most
frequent are spirorbids (calcareous tubeworms) and sea squirts.
The former include both clockwise (Pileolaria militaris, Simplaria pseudomilitaris)
and anti-clockwise spiralling tubes (Janua pagenstecheri, Neodexiospira
pseudocorrugata), and are generally smaller than 2 mm in diameter.
Sea squirts always have representative compound species on leaves: the most
common is Botryllus schlosseri, a colourful, jelly-like animal. Botrylloides leachi,
which is more frequent on other seagrasses, is easily identified, because its
ascidiozooids are patterned in a linear instead of rosette-shaped assemblage.
Epiphytes on Posidonia leaves live on an ever-changing substrate, because
although the leaves grow from their base, their tips break off continually, due
both to mechanical actions and grazing by herbivores. New leaves grow inside
the bundles, while the older, outer ones are gradually shed, and this influences
the structure and dynamics of the epiphytic community.
Typical exclusive species usually colonise internal and therefore the youngest
leaves, to avoid competition with more aggressive species colonising the outer
leaves, which are the oldest and also the most highly populated. This
phenomenon has been observed for the bryozoan Electra posidoniae and for
hydroids Aglaophenia harpago, Monotheca obliqua, Orthopyxis asymmetrica
and Sertularia perpusilla. Experimental studies reveal that the planulae (very
young larvae) of these hydroids colonise only green leaves, i.e., those which are
not already inhabited by other organisms and are therefore young.
Similarly, typical exclusive species colonise leaves starting from their base, so
as to occupy new areas of the leaf as they form. This is true of the bryozoan
Fenestrulina joannae and hydroids Monotheca obliqua, Orthopyxis
asymmetrica and Sertularia perpusilla. However, the exclusive hydroid
Aglaophenia harpago is generally found on leaf tips, like the non-exclusive
bryozoan Aetea truncata. Central leaf parts are colonised by the exclusive
bryozoan Electra posidoniae and by the non-exclusive hydroid Antennella
Root-arm medusa (Cladonema radiatum)
secundaria. Leaf tip colonisation by
non-specialised species is an example
of acrophilia, i.e., the tendency of
passive filterers to colonise positions
that give them easy access to food
suspended in the water column. To
monopolise the available surface more
rapidly, the colonies of many species
develop parallel to the leaf margins, so
that they can expand along the length
of the leaves. This is clearly visible in
the case of Electra posidoniae and Sertularia perpusilla.
There are also differences in colonisation between the two sides of the leaf
blades, that on the outer side generally being slower. Perhaps this is due to the
fact that the outer sides are less protected and more exposed to abrasion, which
may jeopardise the adherence and development of colonising organisms.
Many hydroids, both exclusive (Monotheca obliqua, Orthopyxis asymmetrica)
and non-exclusive (Antennella secundaria), prefer the inner side, like some nonexclusive bryozoans (Aetea, Mimosella). The exclusive hydroid Sertularia
perpusilla apparently prefers the outer side. When colonising flexible and
ephemeral substrates like those offered by Posidonia leaves, some adaptations
are important, and are usually found in exclusive species.
Substrate flexibility inevitably requires epiphytic colonies - generally long and
extensive - to be just as flexible. And this is undoubtedly a problem for
cheilostomes: it is no surprise that the zooids of Electra posidoniae are only
slightly calcified, and the delicate peduncles of the erect zooids of Aetea anguina
and A. sica have special rings that make them flexible. Fenestrulina joannae is an
encrusting species with calcitic zooids that develops better in deep, calm
waters, where wave action is restricted. Conversely, continual leaf fluctuation
requires epiphytes to be robust, as they may otherwise become easily worn by
continuous mechanical action, or scratched by leaf rubbing, and therefore
hydroids have particular thickened or reinforced structures, especially when
living in environments affected by strong wave action.
Due to leaf shedding, the lives of hydroids colonising Neptune grass are shorter
than those living on other substrates. The outer leaves become brown and fall
away, and the hydroids living on them are short-lived too, because shed leaves
roll and accumulate on the bottom, where environmental conditions are very
hostile. Precocious reproduction is the only means for maintaining the species.
Sexual reproduction is unusual, and species rely on vegetative reproduction.
One of the most abundant and
strictly epiphytic species, Orthopyxis
asymmetrica, has a very short sexual
reproduction period in summer, with
the production of a few gonothecae
(sexual organs). Conversely, like other
hydroids, it has well-developed stolons:
a suitably modified part of the colony
can grasp a nearby leaf and adhere to
Epiphytic hydroids on Posidonia leaves
it, detaching itself from its original
colony. These propagatory stolons are
produced in great quantities in all seasons. In Monotheca obliqua, O.
asymmetrica and Sertularia perpusilla, the stolons look like flexible elongations
with rounded tips; in Aglaophenia harpago and in the still undescribed
Sertularella species, they have more complex shapes, hooked and claw-like,
respectively. Widespread stolonisation gives rise to concentration, and species
may be abundant in one area and totally absent in another nearby.
Persistence and restricted reproduction enable many Posidonia epiphytes to be
found all year round. This is the case of some hydroids (Aglaophenia harpago,
Monotheca obliqua, Sertularia perpusilla) and bryozoans (Mimosella verticillata,
Chorizopora brongnartii, Electra posidoniae). Just as many, however, are present
only in certain seasons: spring (Clytia hemisphaerica, Disporella hispida,
Dynamena disticha, Pachycordyle pusilla), summer (Eudendrium simplex,
Halecium pusillum, Paractinia striata) and autumn (Orthopyxis asymmetrica),
although there is no apparent correlation between seasons and specialisation
levels for Posidonia leaves. We must also emphasise the fact that the very few
studies on this subject do not allow us to draw final conclusions.
Leaf areas vary with depth, and are larger near the surface. Similarly, epiphytic
communities also vary quantitatively and qualitatively with depth. Epiphytes are
more abundant in shallow water, although the number of species is smaller.
Larger numbers of species in deep water are mostly composed of ones with
great ecological tolerance, and species typical and exclusive to Posidonia are
more numerous at low and intermediate depths. This observation supports the
theory that P. oceanica originated in shallow water and is today found at greater
depths only because the sea level fell during the last glaciation. More
specifically, Aglaophenia harpago, Collarina balzaci, Electra posidoniae,
Orthopyxis asymmetrica and Sertularia perpusilla prefer depths of less than 15
m. Monotheca obliqua has wide bathymetric distribution, and species that live
at greater depths are Antennella secundaria, Clytia hemisphaerica, Halecium
pusillum and Pachycordyle pusilla. However, these general tendencies may be
contradicted locally, if affected, for instance, by strong wave action. The
bathymetric distribution of epiphytes is mainly due to wave action, and this is
why many hydroids colonise the protected base of leaves when in shallow
water and the more exposed leaf tips in deep water.
Study of epiphytic fauna on P. oceanica is associated with very interesting
scientific aspects concerning issues of adaptation and evolution, which are
important in applied ecology. Epiphytes as indicators are sensitive to natural
and anthropic disturbances, and are affected by environmental alterations due
to the deterioration of water quality sooner than the plant hosting them.
Epiphyte fauna of the rhizomes of Posidonia oceanica. The epiphyte fauna
living on the rhizomes is more heterogeneous and generalist than that on the
leaves, being composed of species which also populate the hard littoral and
circumlittoral substrates. It is thus a community with high specific richness but
generally lower abundance. With rare exceptions, the species differ from those
on the leaves. Nonetheless, hydroids and bryozoans are also the dominant
groups on the rhizomes.
The most common hydroid is Sertularella ellisii (= S. gaudichaudi). Its colonies,
which may reach a height of 50 mm, have erect simple or branched
hydrocauli, with a characteristic zigzag pattern. S. ellisii is also found in a
The bryozoan Margaretta ceroides
variety of other environments, from algal beds to coral and underwater caves,
and from the surface to a depth of about 100 m. Depending on habitat, it has
an extremely varied appearance - to the extent that several different varieties
were described in the past. The many other species of hydroids found on the
rhizomes include Cladocoryne floccosa, Kirchenpaueria pinnata, Sertularia
distans and Aglaophenia picardi - the last two species being among the few
examples of hydroids which colonise both rhizomes and the basal part of
As well as in rocky environments, many species of bryozoans are found on
detritus beds with small hard substrates scattered in the sediment. Their
abundance on the seagrass rhizomes should therefore come as no surprise,
as they are generally affected by sedimentation to a varying degree. If the
sedimentation level is high, some ctenostomes may be abundant, such as
Nolela stipata, N. dilatata, Bowerbankia imbricata, Amathia lendigera and
Pherusella tubulosa - the last also being common on the leaves. The small
cheilostomatan Aetea truncata may be found on both leaves and rhizomes,
but it is mainly the large cheilostomatans which characterise the rhizomes,
especially where sedimentation is not excessive - these are again species
more often found on coral and/or detritus beds. One of the most significant
examples is Margaretta cereoides, with its pinkish or beige tree-like colonies
up to 5 cm tall, extensively branched and weakly attached to the substrate;
the branches are stick-like and are connected by a short narrow horny
peduncle that makes them supple. As well as living at the base of seagrass
rhizomes this species is also found on detritus beds, mostly at depths of
between 10 and 50 m.
Reteporella grimaldii prefers beds of coral. Its colonies, up to 10 cm in height,
are composed of erect laminar expansions, wavy and folded in varying ways;
the laminae are reticulated like nets and are exceedingly fragile. They are
brilliant pink or orange in colour, but quickly fade when dry. Turbicellepora
magnicostata and Calpensia nobilis have encrusting colonies which may form
a thick cover around the rhizomes; the latter species is more abundant in the
Miniacina miniacea is a small (about 1 cm) colony-forming species which at
first sight may be mistaken for a bryozoan instead of a foraminiferan. Its pinkcoloured colonies are irregularly branched in shape. They are found on coral
and in caves and other rocky environments, but on seagrass rhizomes they
may be so abundant that the calcareous skeletons of dead colonies
occasionally pile up in large quantities on the beaches facing large seagrass
meadows, forming eye-catching pink bands on the foreshore.
The tubicolous polychaete Sabella spallanzanii
Among the other epiphyte invertebrates
on Posidonia oceanica rhizomes, the
poriferans, or sponges, have the
largest number of species. Despite
this, the association living in seagrass
meadows is heterogeneous, and
represented by an impoverished
selection of those of the hard littoral
beds: for example, the insinuating and
endolithic species are missing. There is
therefore no typical poriferan species of
seagrass meadows. Among the more
The sea anemone Alicia mirabilis
frequently found species are some
calcisponges, such as Leucosolenia
botryoides and L. variabilis, which have the appearance of branched and whitish
tubular nodules, and the barrel-shaped Sycon raphanus. Thanks to their small
size (1-2 cm) they may also sometimes settle on leaves, especially in waters rich
in suspended solids. Among the numerous demisponges, Mycale contarenii
appears to prefer shallow depths and has also been found on leaves.
Hymeniacidon perlevis is also an encrusting species, and is reddish-orange in
colour. A common yet curious species is Chondrilla nucula, with its appearance
of spherical or long cushions, united in groups that may be over 20 cm long.
These cushions have a smooth surface and vary in colour between purple and
greenish-brown, because of the presence of photosynthetic cyanobacteria
symbionts (zoocyanelles). Lastly, Calyx nicaeensis is worth mentioning: it has a
typical cup shape, is 5-20 cm tall, of fibrous consistency and brown in colour.
Very common until the 1960s and 1970s, it has now become extremely rare, The
reasons for this are unknown, but are perhaps linked to a disease.
The anthozoans living on Neptune grass rhizomes include three species which
prefer deeper meadows. The first is the sea anemone Alicia mirabilis, with its
powerful sting: the contracted polyp has the appearance of a small nodule,
but when it is expanded it may reach more than 50 cm in height, exhibiting a
yellowish-green column with clusters of yellow or orange vescicles and
numerous tentacles. The small specimens may also adhere to the leaves. The
second is the gorgonid Eunicella singularis, with its erect colonies, up to 50 cm
tall, and with narrow, parallel branches which give it its characteristic
candelabra shape; it is dirty-white or yellowish-grey in colour, due to the
presence of micro-algal symbionts (zooxanthellae). The third and last species
is the colony-forming madrepore Cladocora caespitosa. The colonies form
large, globular, encrusting cushions around 20 cm in diameter on average; the
polyps are greenish-brown in colour, again in this case due to zooxanthellae.
While the vast majority of sessile animals feed by filtering or capturing
particles of organic matter suspended in the water, the latter two species, like
the previously-mentioned Chondrilla nucula, are able to integrate their diet
with the products of their micro-symbionts, which mainly provide
carbohydrates, while filtering supplies proteins and lipids.
The sessile polychaetes that are more easily found on seagrass rhizomes
belong to the sabellid and serpulid families. The former are represented by a
species that is well-known to all scuba-divers and aquarium lovers: Sabella
spallanzanii, with its characteristic plumose gills coiled in spiralling bright
colours - commonly yellowish-brown, striped with purple, brownish-orange,
blue, and sometimes white. Two other common species are Bispira mariae,
with its two gill lobes coiled in a spiral, and Sabella pavonina, with funnelshaped gill tufts, not spiralled. Various serpulids, none of them characteristic,
may settle on rhizomes: among the most eye-catching species are Serpula
vermicularis, with its rugose or often carenated pink or yellowish tube, around
5 cm long; Protula tubularia, with its smooth cylindrical tube, pure white and
up to more than 20 cm in length; and Salmacina dysteri, which usually
appears as friable clusters of tiny smooth white tubes, each one a maximum
of 6-7 mm long.
A cirripede which may occasionally be found on seagrass rhizomes is Verruca
spengleri (in the past confused with V. stroemia, a similar species from outside
the Mediterranean). It is identified by its irregularly shaped greyish or brownish
shell about 5 mm in diameter.
Various species of ascidians may colonise seagrass rhizomes. Compound
ascidians are represented both by massive forms like Aplidium conicum or,
more commonly, by encrusting forms such as Diplosoma listerianum,
Didemnum fulgens and D. coccineum. The social ascidian Clavelina
lepadiformis may occasionally be found, and is extremely widespread in
various other environments. Among the simple ascidians, Phallusia
mammillata and Halocynthia papillosa are quite common. The former,
commonly called sea squirt, has a thick cartilaginous tunica of a pale greyishwhite colour, and is up to more than 15 cm in length. The latter, called sea
potato, is slightly smaller and is carmine red above and yellow-orange below.
The colonial tunicate Aplidium conicum
The colonial tunicate Didemnum fulgens
Epiphyte fauna of other phanerogams. The epiphyte fauna of the other
phanerogams has many fewer and rarer species than that of Posidonia
oceanica: it is almost impossible to find a belt of Neptune grass without
epiphytes, whereas this situation is common for the other phanerogams.
Perhaps also for this reason, knowledge of their epiphytes is scarce: most
studies have been carried out on Cymodocea nodosa, something is known
about Nanozostera noltii, very little
about Zostera marina, and nothing at
all on the Mediterranean specimens of
Contrary to what has been described
for P. oceanica, there are no truly
exclusive species on the leaves of the
other marine phanerogams, but at most
preferential ones. It is significant that
The hydroid Laomedea angulata
even when P. oceanica forms mixed
meadows with other phanerogams, the
relative epiphyte populations remain separate, composed of various species,
demonstrating stringent parallelism in their morpho-functional adaptations to
the environmental gradients, first and foremost hydrodynamics. The rhizomes of
other phanerogams are generally rather inconspicuous compared with those of
Neptune grass, and are often completely or partly buried in the sediment. They
are therefore scantily colonised, often by the same species as those found on
the leaves. In these cases, the phenomenon of dwarfism may occur, with a
reduction in the sizes of the colonies on the leaves compared with those on the
rhizomes: the best examples are found among the hydroids, particularly in
colonies of Eudendrium, which maintain their erect bearing on rhizomes,
whereas on leaves they are prostrate and lack vertical branching.
Another important ecological aspect is the fact that other phanerogams,
especially Z. marina and N. noltii, are more euryhaline than P. oceanica and
can therefore penetrate into brackish lagoons: the epiphyte fauna may thus be
composed of species typical of these environments, already adapted to living
in severe environments and capable of colonising various types of substrate.
Despite all these differences, the principal groups of epiphytes are once again
cnidarians and bryozoans. The hydroids are without doubt the best studied
group. The species counted on various phanerogams are relatively numerous,
so that a few general comments are possible.
The largest number of epiphyte species have been found on Cymodocea
nodosa (which is the most frequently studied species). Also worthy of mention
are Aglaophenia harpago and Pachycordyle pusilla, two species which live
mainly on the leaves of P. oceanica and only occasionally on C. nodosa. The
majority of other species have a wider ecological range: many are common
fouling constituents, such as Clytia hemisphaerica and Obelia dichotoma, or
are epibionts of various organisms, like Campanularia volubilis. The largest
number of species, and also the most abundant, have been observed in
autumn; a change in the dominant
species with season has also been
found: Clytia hemisphaerica in winter
and spring, Campanularia volubilis in
summer, and Laomedea angulata in
autumn. L. angulata is the only hydroid
which shows a certain preference for
seagrass leaves, while the closely
related L. calceolifera prefers rocky
The sea anemone Paranemonia cinerea
beds: however, the two species have
occasionally been found together on
phanerogams. L. angulata is the only one found on all studied phanerogams,
but it is particularly frequent on C. nodosa. It has unbranched colonies, around
1 cm in height, sub-transparent to white in colour, and exhibiting a stolon
regenerating capacity comparable to that of the species exclusive to P.
oceanica. Another species found on various phanerogams, although more
common on other substrates, is Ventromma halecioides, with its pinnate and
erect colonies which may be more than 10 cm in height, but which remain
smaller and more delicate on seagrasses.
When phanerogams penetrate brackish waters, the above-mentioned species
may be joined by Pachycordyle napolitana, the colonies of which, smaller than
1 cm, can also be recognised on the typical lagoon phanerogams of the genus
Ruppia. If salinity is particularly low, the only epiphyte hydroid found (for
example, on N. noltii) is Cordylophora caspia, which is similar to the previous
species but able to tolerate almost fresh water, where it colonises the leaves of
Potamogeton with unbranched colonies, 0.5-1 cm tall. As well as hydroids,
another cnidarian may be found on lagoonal phanerogams which offers a cue
for illustrating a case of ecological vicariance: the anthozoan Paranemonia
cinerea. This is a small sea anemone (around 1 cm in diameter and 6 mm in
height), whose olive-green colour with lighter longitudinal stripes allows it to
achieve excellent camouflage on the leaves of Zostera and Ruppia on which it
settles. On these phanerogams, P. cinerea assumes the same role that
Paractinia striata has on Posidonia.
The case of the bryozoan Electra pilosa is even more interesting, as it replaces the
closely related E. posidoniae on Cymodocea and Zostera, occasionally also
penetrating into lagoons. Another encrusting cheilostomatan, Tendra zostericola,
which prefers to colonise the leaves of Zostera, is more strictly lagoonal.
Spirorbids and ascidians also occasionally colonise other phanerogams, often
with the same species as those found on Neptune grass leaves.
Tackling the subject of vertebrates
associated with marine phanerogams
essentially means discussing the fish
fauna associated with the submerged
meadows that these plants form in
There are no large herbivorous
mammals or reptiles (such as dugongs
or turtles) living in close association
with and feeding on seagrass
Black seabream (Spondyliosoma cantharus)
meadows in the Mediterranean, as
happens in other parts of the world.
The fish fauna particularly associated with Posidonia oceanica has been
studied by many marine biologists, mainly along the French, Italian and
Spanish coasts. But the fish fauna associated with other marine phanerogams
has received little attention, although interest has been growing in recent
As mentioned in the previous chapters, P. oceanica differs from the other
Mediterranean marine phanerogams - the most common are Cymodocea
nodosa and Nanozostera noltii, which may form mixed meadows at shallow
depths. It has greater structural complexity (e.g., in terms of three-dimensional
structure and height of leaf fronds, the system of rhizomes and mattes, variety
of substrates), the wider bathymetric range within which the meadows grow,
the type of environments colonised (typically marine for P. oceanica, and
sheltered ones, including coastal lagoons with brackish waters, for the other
phanerogams). Also, P. oceanica has different time dynamics on an annual
scale: it substantially maintains its architecture, whereas the leaf cover and
density of C. nodosa and N. noltii are reduced during winter, sometimes
drastically. As may be easily imagined, this all influences the composition and
annual dynamics of the associated fish population, as well as the role these
Salema (Sarpa salpa)
seagrass systems may play during
some crucial stages in the life cycle of
many species of fish (e.g., hosting the
Before describing the fish fauna
associated with Mediterranean marine
phanerogams, it should be noted that
the term “associated” is used here to
define the fish species that may be
found, in higher or lower numbers, in
seagrass systems and which may
frequent them for different purposes.
Blotched picarel (Spicara maena)
The species of fish defined as
“associated” choose between different
habitats in some way, selecting
seagrass meadows for specific uses
during their juvenile and/or adult
stages, e.g., as a habitat in which to
prey or graze, hide from predators,
breed, etc. The wide variety of “uses”
these fish make of the seagrass
systems explains the large number of
species that may be found there. This
means that seagrass systems make an
Two-banded seabream (Diplodus vulgaris)
maintenance of fish biodiversity in the
Mediterranean coastal environment.
The wide variety of fish species associated with Mediterranean marine
phanerogams is due to the level of dependence on seagrass meadows and
the exploitation of space: there are species which populate the water column
above the leaf cover, species which swim just above or among the leaves, yet
others which frequent the base of the plants, on the rhizomes or live on the
substrate on which the plants grow. From the trophic point of view, seagrass
meadows host herbivores, detrivores (or rather, fish which also feed on
detritus) and carnivores of different orders and types (e.g., piscivores, and
those which feed on zooplankton), which may stably frequent seagrass
meadows or just visit them for brief excursions in search of prey. Others follow
a more precise nychthemeral (day/night) cycle, and so may be observed in the
P. oceanica meadows mainly or exclusively at night or during the day.
■ Fishes associated with seagrass meadows
The species of fish which commonly occupy the column of water above the
meadows of P. oceanica includes damselfish (Chromis chromis), a small
planktivorous fish which may form huge shoals by day. At night, the adults
seek refuge among the seagrass leaves.
Although the most suitable environment for nocturnal hiding-places are the
rocky infralittoral beds, with their many crevices, damselfish (juveniles are an
electric-blue colour) may also find adequate shelter among the meadows of
P. oceanica. As well as the damselfish, picarel (Spicara smaris), blotched
picarel (S. maena), bogue (Boops boops) and saddled seabream (Oblada
melanura), all essentially planktivorous fish, are also frequently observed in
the water column above P. oceanica meadows. In sites particularly sheltered
from waves, it is also possible to observe shoals of whitebait (Atherina sp.)
above Posidonia leaves.
The fishes that may be found on seagrass meadows also include the mullets
(such as Liza aurata), which have a diet at least partially composed of
detritus. Some researchers believe that mullets mainly frequent P. oceanica
meadows in summer and then abandon them in winter, but this does not
appear to be true everywhere. Mullets may swim in open water, sometimes
just slightly below the surface, but they can also be observed on the seabed,
Seabream (Dentex dentex)
especially at points where the detritus of P. oceanica and its associated
invertebrates tend to accumulate (e.g., in channels or sandy hollows close to
The common dentex (Dentex dentex), one of the largest and most efficient
predators of the Mediterranean coastal systems, is one of the biggest fishes
that it is possible to find above the mantle of P. oceanica, and it is sometimes
found in shoals of dozens of individuals. Seabream may reach 1 m in length,
are very gregarious, and essentially piscivorous (i.e., they feed mainly on other
fish). The sudden arrival of a shoal of seabream above the meadows - as
occurs with other large predators - is often heralded by a mass dash for safety
by the small fish (e.g., damselfish, bogue) swimming in the water column.
It is also not rare to observe other species of predators in P. oceanica
meadows, such as the barracuda (Sphyraena viridensis and S. sphyraena),
greater amberjack (Seriola dumerili) and sea bass (Dicentrarchus labrax).
Barracuda usually pause in a shoal above the seagrass leaves, they are
considered to have an affinity for warmer waters and their increase in many
areas of the Mediterranean is imputed by many scientists to warming of the
seas. The two species of barracuda differ, as has recently been shown by
researchers at the University of Genoa, by both the maximum size they reach
and their colours. Sphyraena viridensis may grow to more than a metre in
length, whereas S. sphyraena barely reach more than half a metre. The former
Barracuda (Sphyraena viridensis)
also has a series of unmistakable dark
transversal stripes on its body; the
latter has a dark back and silvercoloured underbelly.
Greater amberjack are in constant
motion, almost always in shoals,
especially when they are small. Sea
bass are also always on the move in
search of prey above the leaf mantle of
Rainbow wrasse (Coris julis)
Posidonia. They frequent not only
seagrass habitats, but are also found in
rocky environments, on sandy/muddy
substrates, in areas exposed to the
movement of waves, and equally in
sheltered areas, brackish waters, and
even inside harbours.
Many species of fish live more closely
associated with the leaf mantle of P.
oceanica. These are species which
swim slightly above and/or among the
Ornate wrasse (Thalassoma pavo)
leaves, such as many of the labrid
family. These include the brown wrasse (Labrus merula) and green wrasse (L.
viridis), among the largest of the Mediterranean wrasses, which may assume a
wide variety of livery, but always tend to have a greenish livery when they live
associated with P. oceanica. They grow to a maximum length of around 45-50
cm, and have predatory habits (they feed on invertebrates and, to a lesser
extent, small fish). There are also species like the peacock wrasse
(Symphodus tinca), Mediterranean rainbow wrasse (Coris julis), Symphodus
ocellatus and, especially in the more southerly parts of the Mediterranean,
ornate wrasse (Thalassoma pavo). These are smaller than those of the genus
Labrus (from 40 cm maximum length in the peacock wrasse to 12 cm in S.
ocellatus) and characterised by pronounced differences in livery, size and
morphology of the body in relation to sex (sexual dimorphism). Males are
generally larger and brighter in colour, especially during the breeding season.
A quite common labrid on P. oceanica is also the blacktailed wrasse
(Symphodus melanocercus). This small fish (maximum length 14 cm) is a
characteristic “cleaner” which interacts with many other species of fish,
including the peacock wrasse, various sea breams (sparids of the genus
Diplodus) and small serranids like the comber (Serranus cabrilla) and painted
comber (S. scriba). The blacktailed
wrasse, like all cleaner fish, feeds on
ectoparasites, mucus, scales, infected
tissue and food residues, which it
removes from the body of another fish.
A particular spot where these cleaning
labrids are “invited” to work by other
fishes is called a “cleaning station”.
Specific rituals are followed: a fish
which wants to be cleaned invites the
cleaner by assuming a particular
posture, e.g., placing itself semiSalema (Sarpa salpa)
vertically or vertically, with its head
down or up, immobile, often with its
mouth wide open and all fins and gills open.
Other smaller labrids (between 12 and 18 cm in length, depending on species)
that may commonly be found on P. oceanica, again belong to the genus
Symphodus and are essentially carnivorous fishes which feed on small vagile
invertebrates (e.g., echinoderms, molluscs, polychaetes and crustaceans),
which they find among the leaves, on the rhizomes and in the sediment. These
include axillary wrasse (Symphodus mediterraneus), five-spotted wrasse (S.
roissali), S. doderleini and S. rostratus. Instead, the grey wrasse (S. cinereus) is
commonly found on the stretches of sandy bottom next to P. oceanica or
where leaf litter accumulates close to the meadows. Both S. rostratus and grey
wrasse may assume various liveries, but are often a pale green colour when
they live in seagrass meadows.
Many species of sparids are associated with the leaf mantle of P. oceanica.
First and foremost is the salema (Sarpa salpa), which is the most important
herbivore (at least as an adult) in the Mediterranean littoral system. The
salema, which may reach a maximum length of 50 cm, often form shoals
composed of hundreds of individuals. This gregariousness is to be found
both in juveniles (which have an omnivorous diet) and in adults. Like many
terrestrial herbivorous vertebrates, the salema move around in groups, and
the moment an individual detects the possible presence of a predator,
including a human being, the entire shoal quickly flees in a coordinated
movement. Although the salema are also found in rocky environments (where
they feed on microalgae), it is common to observe them grazing on P.
oceanica, leaving the classical half-moon sign of their bite on the leaves (see
figure on page 41).
It is still not entirely clear, as the salema
often graze on the epiphyte-bearing
tips of the leaves of P. oceanica, if their
diet is essentially herbivorous - taking
into account that the living plant
tissues of P. oceanica do not have a
high energy value - or if it also includes
a reasonable proportion of epiphytes,
i.e., the plants (algae) and animals of
the sessile benthos that colonise, grow
and live on P. oceanica leaves. These
epiphytes are particularly abundant on
Annular seabream (Diplodus annularis)
the tips of the oldest leaves. There is no
doubt, however, that salema grazing
can be intense enough to determine differences in the average height of the
leaves of P. oceanica among the meadows where they are abundant (and large)
and those where they are more or less absent.
Among the sparids of the genus Diplodus (which includes fish commonly
known as seabream), the one which is associated more closely to P. oceanica
than all the others, is definitely the annular seabream (D. annularis). This is the
smallest of the seabreams (with a maximum length of 25 cm) and is silverycoloured with greenish-yellow tinges, which camouflages it very well amongst
the P. oceanica leaves. It feeds on small vagile and sessile invertebrates that it
finds on the leaves. The other seabream, i.e., white seabream (D. sargus),
sharpsnout seabream (D. puntazzo) and two-banded seabream (D. vulgaris)
are often found in association with P. oceanica, although they are also very
frequent on rocky sea-beds. These three species have a much higher
commercial value than the annular seabream and grow much larger (the white
and common two-banded types reach 45 cm in length, the sharpsnout may
even reach 60 cm). Although their diets are not entirely the same (especially for
sharpsnout), all three feed on invertebrates - in some cases also relatively
large ones, such as adult sea urchins.
Another sparid which is found typically associated with P. oceanica is the black
seabream (Spondyliosoma cantharus), which may reach 60 cm in length, and
which feeds on invertebrates, including jellyfish. Although the largest specimens
are mainly found in rocky areas, juveniles and medium-sized adults may often
be observed swimming in the open waters above Posidonia meadows. The
sparids which may be found on P. oceanica once again include the gilthead
seabream (Sparus auratus) and common pandora (Pagellus erythrinus).
Female parrotfish (Sparisoma cretense)
Another species commonly associated
with P. oceanica is the brown meagre
(Sciaena umbra). This fish, which may
become very large (around 70 cm in
total length), is often gregarious and
feeds mainly on small invertebrates.
Usually not very mobile because it
stays in groups near shelter, if
disturbed, it hides in rocky crevices or
among the seagrass leaves, and even
In the more southern parts of the
Brown meagre (Sciaena umbra)
Mediterranean, it is possible to observe
swimming alone or in small groups in the vicinity of the leaf mantle of P.
oceanica. This fish, which has a robust beak, has a diet which often includes
leaves of P. oceanica with epiphytes on the tips. As in the case of the salema, in
energy terms the importance of epiphytes with respect to plant tissues is still
not well understood, as in rocky environments Mediterranean parrotfish graze
on calcareous algae (such as Halimeda tuna, which often also bear epiphytes)
and on small sessile invertebrates with calcareous structures (e.g., bryozoans).
The dusky grouper (Epinephelus marginatus) is a large sedentary and territorial
predator, which can grow to well over a metre in length and live for 50 years. It
may be observed on P. oceanica, especially when the meadows are
interspersed with rocky substrates which offer adequate refuge. The dusky
grouper and brown meagre are much sought-after by spearfishermen. Their
tendency not to escape quickly into open water, but to take refuge in hidingplaces close to the areas where they have been spotted mean that they are at
particular risk of local extinction in areas where underwater fishing is
particularly intense. This situation has prompted countries like France to
declare a moratorium on these fish, which involves a ban on their capture by
As well as sea bass, P. oceanica meadows are frequented by another two much
smaller serranids (a maximum of around 35 cm), already mentioned above
because of their frequent association with the cleaner labrids. These are the
painted comber and comber, which are also territorial piscivorous species (given
their smaller size, their attention turns to small fish species or, in most cases, to
juvenile forms). The brown comber (Serranus hepatus) is only reported in some P.
oceanica meadows of the central-northern Adriatic, along the Croatian coasts.
The striped red mullet (Mullus surmuletus) may often be found in P. oceanica
habitats as well as on rocky bottoms mixed with sand. Striped red mullet may
be observed swimming above the leaf mantle of Posidonia, but it is certainly
easier to find them intent on searching for the small invertebrates on which
they feed in the sandy patches inside Posidonia meadows. They are often
followed by a retinue of other species, like wrasse and small seabream, which
exploit the powdery drifts raised by the red mullet to snatch any small
invertebrate that becomes available. The red mullet (Mullus barbatus), instead,
is more commonly associated with sandy/muddy substrates, although some
authors report it as being present (but not abundantly, and mainly in juvenile
stages) in the deeper meadows of Posidonia.
The conger eel (Conger conger) and moray eel (Murena helena) are typically
associated with rocky substrates with abundant crevices. At night, these
voracious predators move towards the meadows of P. oceanica situated near
the rocky seabed in search of prey. They swim at the base of the leaf fronds,
close to the substrate.
Other species which are to be found mainly at night in Posidonia meadows are
the small gadids, such as the shore rockling (Gaidropsarus mediterraneus) and
three-bearded rockling (G. vulgaris), ophidiids such as Ophidion rochei and
Parophidion vassali and, lastly, the dogfish or smaller-spotted catshark
Some fish species have evolved on the basis of their association with P.
oceanica. More specifically, these are the pipefish (Syngnathus acus, with a
more pointed snout, and S. typhle, with its higher snout and more vertical
profile), which feed mainly on small vagile invertebrates. These fish may be
either pale green or brown, in this way imitating the colours of the leaves of
living Posidonia (green) and dead (which become brown in colour). In order to
camouflage themselves even better, the pipefish often assumes a vertical
position, parallel to the Posidonia leaves, and undulates with the leaves set in
motion by the waves or currents. Other famous representatives of the
Syngnathidae which may be observed in Posidonia meadows, often attached
by their tails to the rhizomes or leaves, are the seahorses (Hippocampus spp.).
Various species belonging to the gobiesocids (known as cling fish) are
reported as being associated with P. oceanica. These include the Connemara
cling fish (Lepadogaster candollei) and Opeatogenys gracilis. The latter is a
small green-coloured fish that adheres to Posidonia leaves, perfectly
The black scorpionfish (Scorpaena porcus) is often to be found resting at the
base of the leaf fronds or directly on the seabed where P. oceanica is growing
and, less often, the small red scorpionfish (S. notata) and largescaled
scorpionfish (S. scrofa). All scorpionfish are predators which lurk on the
bottom, suddenly opening their large mouths to suck in their prey, and then
Syngnathus typhle, an example of perfect camouflage
Black scorpionfish (Scorpaena porcus)
gripping them tightly between their
teeth before swallowing them.
Gobies may also be observed resting
on the seabed beneath the leaf
mantle, including the slender goby
(Gobius geniporus), red-mouthed
goby (G. cruentatus) and rock goby
(G. paganellus), some blenniids
(blennies) such as the tompot blenny
(Parablennius gattorugine), or flatfish
like the turbot (Bothus podas).
Among the gobies, the quagga goby
(Pomatoschistus quagga) should also
be mentioned. This small fish typically
rises from the bottom and stays in
open water, not too far (about 1 metre)
from the substrate. More rarely,
Dusky spinefoot (Siganus luridus)
triglids like the streaked gurnard
(Chelidonichthys lastoviza) have also been observed.
In the areas where seagrass meadows have tall mattes, which at a certain
moment start to become eroded because of the displacement of sediment, a
particular microhabitat may be created, with small semi-dark recesses in the
free spaces between the rhizomes. In these microhabitats, as well as
damselfish fry, lives the cardinal fish (Apogon imberbis), a small markedly
sciaphilous fish (attracted by shady environments) typically associated with
rocky substrates with plenty of crevices or underwater grottoes. In the sand
beneath or close to the P. oceanica meadows, are other species of fish, such
as the weever-fish (Trachinus spp.), stargazer (Uranoscopus scaber) or Atlantic
lizardfish (Synodus saurus), which spend most of their time buried, waiting to
ambush their prey as they pass close by.
It should be remembered that the Mediterranean fish fauna is in continuous
evolution and that some of the changes are due to humans, as in the case of
the species of fish that entered the Mediterranean Sea following the opening
of the Suez Canal and which are expanding rapidly as a result of gradual
colonisation of new areas. The non-indigenous species which may be found
associated with P. oceanica in some areas of the southern Mediterranean
include the dusky spinefoot (Siganus luridus). This is a herbivorous fish and
may be observed as it swims or grazes on the leaves of P. oceanica,
occasionally in mixed shoals with salema or parrotfish.
■ Temporal variations
A description of the fish fauna requires further elements in order to understand
its dynamics over time. Indeed, the fish fauna of P. oceanica may change on a
seasonal scale, but also on the briefer daily scale. During the year, changes are
linked to the life-cycles of many of the species hosted in Posidonia meadows.
For example, some species which exploit P. oceanica as a habitat for the
installation or enlistment of juveniles, may demonstrate higher total
abundances (often due to many small individuals) only in specific periods of
the year - in Posidonia, this happens mainly in spring and early summer.
Concerning the species composition of the fish populations as well as their
relative abundance, large differences have not been noted between winter and
summer at Port Cros (France), considering only the adult portion of the
populations. Instead, off Ischia (Italy), a reduction in both number of species
and abundance of fish, excluding those more closely associated with the
water column, has been observed in winter. However, it is not known if these
differences are to be attributed to effective migration from P. oceanica
meadows in winter, or to the reduced mobility of the fish during the cold
The changes that may occur between day and night are a great deal more
marked. This is due mainly to the active presence at night of conger eels,
gadids and ophidiids, all nocturnal predators which abandon their daytime
refuges in the mattes, in sediment, or among the rocks in Posidonia meadows.
Instead, damselfish, blotched picarel and picarel tend to sink between the
Posidonia leaves during the night. The scorpionfish show their peak of feeding
activity during the night and tend to rise on the leaves of Posidonia (especially
the smaller individuals). During the day, the adults stay on the bottom or
among the rhizomes, whereas the young slip inside the mattes or other natural
crevices. At night, labrids are almost entirely inactive, sinking towards the
bottom among the Posidonia fronds and remaining practically immobile. In
general, several studies report a quite variable number of fish species
associated with P. oceanica - in the range of 30-50. This variability is due not
only to differences in the specific composition of the populations between
different sectors of the Mediterranean, but is also partly attributable to the
study methods used.
From the point of view of overall populations, the labrids certainly dominate in
terms of number of species, followed by the sparids. Instead, in terms of
number of individuals, the gregarious and planktivorous species which occupy
the water column (damselfish, blotched picarel, bogue) are dominant (they
Vertical distribution of main fish families in Neptune grass meadows, by day and night
may represent up to 90% of the
Concerning the trophic organisation
of the fish fauna, as previously
mentioned, the majority of fish species
most closely associated with P.
oceanica feed on small invertebrates
and are thus substantially linked to the
detritus chain rather than that of the
herbivores. The proportion of primary
production that is used by herbivores
and transferred higher up the foodPipefish (Syngnathus acus)
chain is therefore negligible.
Many invertebrates are the prey of a
large number of fish living in P. oceanica meadows. These fish, in turn, are the
prey of large piscivores.
As regards juveniles, it is known that in many temperate regions seagrass
meadows play a crucially important role, i.e., that of constituting nurseries in
which the juvenile stages of many fish species, including many of commercial
importance, pass the earliest periods of their life-cycle. After fecundation, the
eggs of most coastal fish species remain in the open sea, although there are
some (like those of the gobies) which have benthic eggs. The eggs eventually
hatch and the larvae emerge, and, after a variable period (generally a few
weeks), head towards the coast. The pelagic larval stage is followed by
metamorphosis, during which the post-larvae slowly adapt to a more
pronounced benthic stage. Depending on species, these early stages
demonstrate clear preferences for different habitats. Being small (at times less
than 2 cm), the juveniles are often subjected to intense predation, so they
need habitats where they can find refuge as well as food.
For clarification, it should be mentioned that many fish biologists define the
transition between the pelagic and benthic stages as “settlement”.
“Recruitment” is the stage during which the individuals of a new generation of
a given species join the adult population. These two stages are distinct for
species whose juveniles live in different habitats from those of the adults, but
overlap for species whose post-larvae choose the same habitat as that of the
adults. The above terms should not be confused with “colonisation”, the
process by means of which a species occupies a geographical area where it
was not previously present (e.g., species entering the Mediterranean Sea
through the Suez Canal).
At a world level, seagrass meadows appear to be particularly well adapted to
playing the role of nurseries, especially where they grow in shallow waters, or in
brackish environments such as coastal lagoons or river mouths. In the
Mediterranean, many have transposed this role automatically to P. oceanica, but
data are only available for a small number of locations in the north-western
Mediterranean. The list of species which use P. oceanica meadows as nurseries
includes many of little commercial interest, like various wrasses of the genus
Symphodus (such as S. ocellatus or the axillary wrasse) and Labrus (brown
wrasse). Among the sparids, the annular seabream and black seabream (and, to a
lesser extent, the common seabream, Pagrus pagrus) appear to prefer Posidonia
oceanica meadows during their juvenile stage. During this stage, the colours of
many species are particularly appropriate for camouflage among the leaves.
Instead, the striped red mullet is a species of commercial importance, which
also exploits Posidonia oceanica meadows during its juvenile stages, as do
black scorpionfish and blotched picarel. Many of the young of these species
are present in the late spring or summer, while there are few species whose
young are observed during colder seasons. As well as juveniles, i.e., in a
particularly early stage of the life-cycle, many species frequent the Posidonia
oceanica meadows as sub-adults. These are often the same species as those
whose adults live in association with the meadows (such as salema, comber,
axillary wrasse and ornate wrasse).
The fish fauna associated with other Mediterranean marine phanerogams has
certainly received less attention than the populations associated with P. oceanica.
There is very little known about the fish fauna associated with Zostera marina. A
study conducted some years ago on meadows of Z. marina and Nanozostera
noltii off Rovinj (Croatia) listed more than 30 fish taxa associated with these
phanerogams. The majority of them have already been mentioned for P.
oceanica, while the presence of others, such as some gobiids (gobies like the
black goby, Gobius niger) and soleids (soles), reveal the sandy or sandy-muddy
nature of the substrates where the meadows of these lesser phanerogams grow.
In shallow sheltered environments - both typically marine, like sheltered bays, and
in the brackish waters of coastal lagoons - there may be mixed meadows of C.
nodosa and N. noltii. The fish fauna associated with them has only received some
attention in recent years and currently available studies are few and localised.
A series of studies conducted in bays of the Gulf of Olbia (Sardinia), which have
slightly less salty waters than those of typical marine conditions, has
demonstrated the importance of the cover of C. nodosa and N. noltii
(henceforth called “small phanerogams”) for the associated fish fauna. In the
studied meadows, 23 taxa of fish fauna were counted, by monthly visual
sampling over a full year’s cycle. Whitebait were the most frequent and
abundant throughout the year, followed by some labrids, such as grey wrasse,
peacock wrasse and green wrasse, and some sparids, such as gilthead
Two-banded seabream (Diplodus vulgaris)
Green wrasse (Labrus viridis)
seabream, two-banded seabream, annular seabream and salema. Most of the
other species were either less abundant or occasional. Medium-sized
specimens of garfish (Belone belone) and painted comber were relatively
frequent. These are both piscivores: the needlefish sometimes chased and
captured whitebait; the painted combers were observed lying in wait to
ambush the juveniles of other species.
For the fish associated with the meadows of small phanerogams, no distinction
can be made between the species that live on the bottom or between the
rhizomes and those which swim among the leaves or just above the leaf mantle,
as described for the fish fauna associated with P. oceanica. The small
phanerogams have a less complex structure, with lower and more delicate leaves
and a looser system of rhizomes that does not form mattes. There is therefore no
shadier environment at the base of the leaves, so there are no habitats suitable for
particularly sciaphilous species (e.g., cardinal fish). Moreover, unlike P. oceanica,
small phanerogams only grow on sandy/muddy substrates. This means that,
among the fish which may be found associated with meadows of small
phanerogams, especially when they are growing in very sheltered coastal
environments and with an influx of freshwater, there are also species typically
associated with sandy or muddy-sand substrates, such as striped seabream
(Lithognathus mormyrus), black goby and Bucchich’s goby (Gobius bucchichi) the latter species often associated with sea anemones (Anemonia viridis).
Striped seabream (Lithognathus mormyrus)
Concerning the temporal dynamics of
the fish fauna, meadows of small
phanerogams have a more variable
structure during the year than those of P.
oceanica. For example, the small
phanerogam meadows studied in the
Gulf of Olbia showed a clearcut annual
cycle in the density of leaf fronds, which
fell from almost 2000 fronds/m2 in
Saddled seabream (Oblada melanura)
summer to less than 1000 in winter. This
involved a notable reduction in the
complexity of the habitat and fewer hiding-places for the fish. There was also a
change in water temperature, between 25 °C in summer and 13-14 °C in winter.
These alterations in environmental conditions and leaf density were correlated to
important variations in the fish fauna during the year. Species richness between
summer and winter dropped from 15-18 to 3-4. The total number of fish does not
change much overall, but only because the vast majority of individuals are
whitebait, for which the protection against predators offered by shallower depths
(where in any case they become more vulnerable to other predators, such as
seabirds) is perhaps more important than the plant cover. Instead, if the
abundance of fish is studied without the contribution of planktivorous fish, the
picture is very different. The average number of fish counted on the standard
surface area of 150 m2 varies from about 20-100 specimens in summer-autumn
(when leaf density is high), to a few individuals (even less than 5) between
December and April (when leaf density is at its minimum). The species whose
abundance increases or diminishes in relation to the increase or reduction of leaf
density are the peacock wrasse, grey wrasse, green wrasse, striped red mullet,
annular seabream, two-banded seabream, salema and gilthead seabream.
An important fact is that the vast majority of specimens observed were
juveniles or sub-adults. This suggests that the small phanerogams may play a
more specific role as nurseries than P. oceanica. All the individuals observed
belonging to commercially important species such as common seabream,
gilthead seabream, striped seabream and saddled bream, plus a relatively
significant proportion of striped red mullet, two-banded seabream and white
seabream, were juveniles. On the whole, striped red mullet and annular
seabream appear to use these meadows of small phanerogams, like the P.
oceanica meadows, for installing the juveniles and initial sub-adult stages.
The shallow depths and the leaf mantle render these habitats particularly
suitable for juvenile fish.
Among fish species associated with
the small phanerogam meadows,
those which also maintain a
conspicuous proportion of the adult
population are the green wrasse (20%
of adults), grey wrasse (12%), striped
red mullet (7%) and annular seabream
(6%). Grey wrasse males, in particular,
have been observed building their
Peacock wrasse (Symphodus tinca)
nests in the meadows, using drifting
fronds of macroalgae, small pebbles
and leaves of small phanerogams. Many of these fishes can be observed
beside heaps of small pebbles inside the meadows. Juveniles and adults of
striped red mullet were almost always observed while searching for prey by
probing the sand with their barbels at the base of the leaf fronds or in the
patches of sand within or at the edges of the small phanerogam meadows.
Studies conducted on the fish populations associated with the meadows of
C. nodosa, N. noltii and Zostera marina in the Lagoon of Venice have
demonstrated a very different situation to that of the Gulf of Olbia. Species
richness was of the order of 30-40 associated taxa. The dominant fish
species were the black-striped pipefish (Syngnathus abaster), pipefish,
whitebait (Atherina boyeri) and grass goby (Zosterisessor ophiocephalus). It
should be noted that most of the species counted were almost always adults.
From this viewpoint, the phanerogam meadows of the Lagoon of Venice
would seem to play an important role in terms of habitat where reproduction
takes place (especially for the pipefish and grass goby), whereas the role of
nursery appears to be marginal. In general, the reduction in mortality due to
predation of juveniles is often attributed to the nursery function of the
phanerogams, thanks to the protection given by the leaf mantle. However,
limitations to the architectural complexity and homogeneity of habitat do
exist, beyond which there is no longer an optimal balance between reduction
of the risk of predation and optimisation of searching for one’s own prey,
often ensured in habitats of a different type in proximity to the one which
offers the best protection from predators.
Compared with other habitats, the homogeneity of the phanerogam meadows
in the Lagoon of Venice may be the reason for the scarcity of juvenile fish,
which are more frequent in areas where the phanerogams are laid out in
patches on loose unvegetated sea-beds. It should also be noted that these
phanerogam meadows host abundant adults that prey on juveniles, such as
grass goby and pipefish, which can
feed on both vagile invertebrates and
fish larvae and juveniles.
As well as the native phanerogams of
the Mediterranean Sea, there has been
the addition of another species which
arrived through the Suez Canal Halophila stipulacea. The only study
conducted on the fish fauna associated
Grey wrasse (Symphodus cinereus)
with this species was carried out in
eastern Sicily, in a site where the
meadow grows at a depth of about 20 m, where 30 fish species have been
counted. The species most frequently observed were ornate wrasse,
saddledbream, comber and painted comber. In terms of abundance, although
the frequency of observation was not very high, salema was the most abundant
species on average, followed by rainbow wrasse, saddled bream and striped
red mullet. The size structure of the fish population did not reveal many large
individuals, while for 8 species, more than 25% of the individuals recorded
were small specimens or juveniles, particularly black scorpionfish, saddled
bream and common two-banded seabream. Overall, as also observed for the
fish fauna of the small phanerogams in the Gulf of Olbia, the highest species
richness was observed in summer and autumn, and the lowest in winter. The
total density followed a similar seasonal trend, with maximum values in
summer-autumn and minimum in winter. This fact is interesting because, as H.
stipulacea maintains a highly structured and therefore stable architecture
throughout the year, the observed seasonal differences may be attributed to
other factors (e.g., water temperature).
The fish fauna associated with H. stipulacea sampled in eastern Sicily
revealed similarities and differences with respect to the assemblages
associated with the other phanerogam species. In all phanerogam systems,
the dominance of fishes belonging to the labrid and sparid families is
obvious. Conversely, H. stipulacea had a fish fauna characterised by a
scarcity of planktivorous fish and the presence of species such as the
blackbelly rosefish (Helicolenus dactylopterus) and butterfly blenny (Blennius
ocellaris), which are species typical of deeper loose seabed environments.
However, this being the only available study, we cannot be sure whether
these differences are really due to the presence of H. stipulacea or to the local
characteristics of the studied site, which may be influenced by the special
conditions of the nearby Strait of Messina.