Molluscs
163
size spectrum within 1 to 12 cm. They are mostly detritus-feeders and inhabit
the detritus accumulating zones on the bottom of lagoons and on outer reef
slopes. Another abundant on reef detritus-feeding molluscan family is the
Strombidiidae. They often form dense monospecific populations in shallow
areas of flat and lagoon, and especially in lagoons of the barrier reefs, where
the detritus arrives from land with the terrigenous materrial. The biotopes
of reefs are inhabited numerous species of the family Cypraeidae. They feed
on periphyton, detritus and also on meyobenthos. Their mass species are
Cypraea moneta, C. caputserpentis, C. annularis, C. arabica, C. mauritanica.
Cypreids are nocturnal animals. They spend the daytime in their hiding
places in trenches, under the stones. Being an object of the collection boom
they were eagerly collected by tourists and local inhabitants. This endangers
the very existence of their populations in many reef sites.
The biotopes of the soft bottom are inhabited, together with the micromolluscs mentioned above, also by the small- and average-sized species
of Rissoinidae, Stomatellidae, Columbellidae, Cerithiidae, Pyramidiidae,
Strombidae, Xenophoridae and Hypponicidae. The last two families represent the species which feed directly on sediments. Among the planktovorous gastropods there are several species of filter-feeders, which belong to
the families Calypteridae, Siliquariidae and Vermetidae. The most abundant
among them are the genera Vermetus and Dendropoma. The shells of these
molluscs have the form of a prolonged cylinder. They are abundant in the
flat and in lagoon areas, being attached to the surface of dead, and sometimes of living, corals (Hadfield et al. 1972; Hadfield 1976). The large phytophagous opistobranch gastropods Aplysia feed on seaweeds and use the
intestinal microflora to accomplish the digestion of crude plant material
consumed in the same way as is done by cattle (Carefoot 1981). The phytophagous pulmonate molluscs from the families Siphonariidae and Onchidiidae could often be found in supralittoral areas of coral reefs.
The predatory gastropod fauna in most of the reefs is extremely abundant
(Kohn and Levitien 1976; Taylor 1978, 1984). They comprise 20 to 30% of
the total species number and the same proportion of the biomass in reef
molluscan fauna, including bivalves. The predatory gastropods relate to 27
families, mostly prosobranchs. The predatory opistobranchs are represented
on reefs mainly by other Nudibranchia. Among them three main trophic
groups could be distinguished: the predators on other molluscs, the predators feeding on other benthic animals, and the corallovorous gastropods.
The basic taxonomic groups of predatory gastropods, as classified by their
biomass and by the number of their species, are the families Muricidae,
Thaisidae, Conidae and Mitridae. The first three families are represented
on reefs by 20-30 species, the most abundant of which being the thaisids
Morula uva and M. squamosa, and the conids Conus sponsalis and C.
ebraens. For example, on the reefs off Guam conids, murcids and thaisids
comprised around 80% of the total number of predatory gastropods (Taylor
163
size spectrum within 1 to 12 cm. They are mostly detritus-feeders and inhabit
the detritus accumulating zones on the bottom of lagoons and on outer reef
slopes. Another abundant on reef detritus-feeding molluscan family is the
Strombidiidae. They often form dense monospecific populations in shallow
areas of flat and lagoon, and especially in lagoons of the barrier reefs, where
the detritus arrives from land with the terrigenous materrial. The biotopes
of reefs are inhabited numerous species of the family Cypraeidae. They feed
on periphyton, detritus and also on meyobenthos. Their mass species are
Cypraea moneta, C. caputserpentis, C. annularis, C. arabica, C. mauritanica.
Cypreids are nocturnal animals. They spend the daytime in their hiding
places in trenches, under the stones. Being an object of the collection boom
they were eagerly collected by tourists and local inhabitants. This endangers
the very existence of their populations in many reef sites.
The biotopes of the soft bottom are inhabited, together with the micromolluscs mentioned above, also by the small- and average-sized species
of Rissoinidae, Stomatellidae, Columbellidae, Cerithiidae, Pyramidiidae,
Strombidae, Xenophoridae and Hypponicidae. The last two families represent the species which feed directly on sediments. Among the planktovorous gastropods there are several species of filter-feeders, which belong to
the families Calypteridae, Siliquariidae and Vermetidae. The most abundant
among them are the genera Vermetus and Dendropoma. The shells of these
molluscs have the form of a prolonged cylinder. They are abundant in the
flat and in lagoon areas, being attached to the surface of dead, and sometimes of living, corals (Hadfield et al. 1972; Hadfield 1976). The large phytophagous opistobranch gastropods Aplysia feed on seaweeds and use the
intestinal microflora to accomplish the digestion of crude plant material
consumed in the same way as is done by cattle (Carefoot 1981). The phytophagous pulmonate molluscs from the families Siphonariidae and Onchidiidae could often be found in supralittoral areas of coral reefs.
The predatory gastropod fauna in most of the reefs is extremely abundant
(Kohn and Levitien 1976; Taylor 1978, 1984). They comprise 20 to 30% of
the total species number and the same proportion of the biomass in reef
molluscan fauna, including bivalves. The predatory gastropods relate to 27
families, mostly prosobranchs. The predatory opistobranchs are represented
on reefs mainly by other Nudibranchia. Among them three main trophic
groups could be distinguished: the predators on other molluscs, the predators feeding on other benthic animals, and the corallovorous gastropods.
The basic taxonomic groups of predatory gastropods, as classified by their
biomass and by the number of their species, are the families Muricidae,
Thaisidae, Conidae and Mitridae. The first three families are represented
on reefs by 20-30 species, the most abundant of which being the thaisids
Morula uva and M. squamosa, and the conids Conus sponsalis and C.
ebraens. For example, on the reefs off Guam conids, murcids and thaisids
comprised around 80% of the total number of predatory gastropods (Taylor