Rissoa violacea, Gibbula ardens, Petalifera petalifera, Flexopecten hyalinus) or
present in very low numbers (e.g. Steromphala umbilicaris, Rissoa auriscalpium), a
possible explanation being that the meadows are too patchy to sustain viable
populations of these species (Gofas et al. 2011) or because they are strict Mediterranean species that become rarer towards the Strait of Gibraltar.
The P. oceanica rhizome stratum is very complex, with distinct micro-habitats
(García Raso et al. 1992; Luque and Templado 2004). While the leaf stratum has
characteristic species adapted with small and flattened forms, the stratum formed by
the rhizomes has no exclusive fauna since it is composed of species that are also
found in other adjacent hard substrates. Nevertheless, this stratum of rhizomes
deserves special attention, since it is densely populated by organisms. For example,
50 different decapod species and more than 900 specimens have been recorded in a
30 Â 30 cm frame (0.09 m
2 ) in P. oceanica meadows from Almería (García Raso
1990). Small specimens and juveniles dominate in this stratum, due to its role as
protection and nursery for many species, as observed in a much lesser extent in the
small seagrasses Z. marina and C. nodosa. Frequent macrofauna which inhabits the
rhizomes are the sponges Chondrosia reniformis, Phorbas fictitius and Crambe
crambe; the cnidarians Pachycerianthus sp. and Aiptasia mutabilis; the annelids
Salmacina spp., Protula intestinum, Bispira volutacornis and Sabella spallanzanii;
the bivalves Striarca lactea and Barbatia barbata; the chiton Lepidopleurus
cajetanus; the gastropod Chauvetia mamillata; and the bryozoans Schizobrachiella
sanguinea, Schizoporella errata, Reteporella grimaldii and Schizomavella
mamillata. The echinoderms Holothuria tubulosa, Paracentrotus lividus,
Sphaerechinus granularis, Echinaster sepositus, Ophioderma longicauda and
Antedon mediterranea; the tunicates Didemnum fulgens, Pseudodistoma obscurum,
Phallusia mammillata and Pycnoclavella communis; and fish such as Gobius spp.
are also common in Posidonia meadows. In the rhizomes, two species of hermit
crabs are dominant, Cestopagurus timidus (in shallow water, especially in 3–4.5 m)
and Calcinus tubularis (in deeper water, ca. 9 m), and also show (especially the first
one) day-night movements from the rhizomes to the leaves (García Raso 1990;
Manjón-Cabeza and García Raso 1994, 1995). Other dominant decapods of this
stratum are also common in sciaphilous enclaves of rocky bottoms and include
Athanas nitescens, Alpheus dentipes, Pisidia longimana (also mentioned as
P. longicornis) and Pilumnus hirtellus among others.
In the westernmost part, Posidonia meadows in Calahonda (Málaga) usually have
a sediment-filled rhizome stratum, and the molluscan species found there as dominant, e.g. Tritia incrassata, Bittium reticulatum, Steromphala racketti, Nodulus
contortus, Tricolia pullus and Gibberula miliaria (Urra et al. 2013b), are not specific
to the Posidonia habitat but shared with neighbouring hard substrates with
photophilous algae. In this area, some tropical Atlantic species such as the bivalve
Ungulina rubra inhabit the rhizome stratum, having there the only known
populations for the Mediterranean and the European coasts (Urra et al. 2013b).
Possibly one of the most characteristic species in Posidonia meadows is the
mollusc Pinna nobilis, one of the largest bivalves in the world. Unfortunately,
since 2016 this species suffered a massive mortality that has affected almost 100%
9 Benthic Fauna of Littoral and Deep-Sea Habitats of the Alboran Sea: A. . .
307
present in very low numbers (e.g. Steromphala umbilicaris, Rissoa auriscalpium), a
possible explanation being that the meadows are too patchy to sustain viable
populations of these species (Gofas et al. 2011) or because they are strict Mediterranean species that become rarer towards the Strait of Gibraltar.
The P. oceanica rhizome stratum is very complex, with distinct micro-habitats
(García Raso et al. 1992; Luque and Templado 2004). While the leaf stratum has
characteristic species adapted with small and flattened forms, the stratum formed by
the rhizomes has no exclusive fauna since it is composed of species that are also
found in other adjacent hard substrates. Nevertheless, this stratum of rhizomes
deserves special attention, since it is densely populated by organisms. For example,
50 different decapod species and more than 900 specimens have been recorded in a
30 Â 30 cm frame (0.09 m
2 ) in P. oceanica meadows from Almería (García Raso
1990). Small specimens and juveniles dominate in this stratum, due to its role as
protection and nursery for many species, as observed in a much lesser extent in the
small seagrasses Z. marina and C. nodosa. Frequent macrofauna which inhabits the
rhizomes are the sponges Chondrosia reniformis, Phorbas fictitius and Crambe
crambe; the cnidarians Pachycerianthus sp. and Aiptasia mutabilis; the annelids
Salmacina spp., Protula intestinum, Bispira volutacornis and Sabella spallanzanii;
the bivalves Striarca lactea and Barbatia barbata; the chiton Lepidopleurus
cajetanus; the gastropod Chauvetia mamillata; and the bryozoans Schizobrachiella
sanguinea, Schizoporella errata, Reteporella grimaldii and Schizomavella
mamillata. The echinoderms Holothuria tubulosa, Paracentrotus lividus,
Sphaerechinus granularis, Echinaster sepositus, Ophioderma longicauda and
Antedon mediterranea; the tunicates Didemnum fulgens, Pseudodistoma obscurum,
Phallusia mammillata and Pycnoclavella communis; and fish such as Gobius spp.
are also common in Posidonia meadows. In the rhizomes, two species of hermit
crabs are dominant, Cestopagurus timidus (in shallow water, especially in 3–4.5 m)
and Calcinus tubularis (in deeper water, ca. 9 m), and also show (especially the first
one) day-night movements from the rhizomes to the leaves (García Raso 1990;
Manjón-Cabeza and García Raso 1994, 1995). Other dominant decapods of this
stratum are also common in sciaphilous enclaves of rocky bottoms and include
Athanas nitescens, Alpheus dentipes, Pisidia longimana (also mentioned as
P. longicornis) and Pilumnus hirtellus among others.
In the westernmost part, Posidonia meadows in Calahonda (Málaga) usually have
a sediment-filled rhizome stratum, and the molluscan species found there as dominant, e.g. Tritia incrassata, Bittium reticulatum, Steromphala racketti, Nodulus
contortus, Tricolia pullus and Gibberula miliaria (Urra et al. 2013b), are not specific
to the Posidonia habitat but shared with neighbouring hard substrates with
photophilous algae. In this area, some tropical Atlantic species such as the bivalve
Ungulina rubra inhabit the rhizome stratum, having there the only known
populations for the Mediterranean and the European coasts (Urra et al. 2013b).
Possibly one of the most characteristic species in Posidonia meadows is the
mollusc Pinna nobilis, one of the largest bivalves in the world. Unfortunately,
since 2016 this species suffered a massive mortality that has affected almost 100%
9 Benthic Fauna of Littoral and Deep-Sea Habitats of the Alboran Sea: A. . .
307