162
Reef Zoobenthos
spectra, and display numerous ecological and morphological adaptations,
which make for their extremely high density in most of the reef biotopes and
for a high degree of use of trophic resources. Their sizes vary between
several millimetres in micro molluscs and 1 m, and weights of up to 100 kg
(tridacnides). Among them we may find species which represent all trophic
levels ranging from symbiotic primary photosynthetic producers to plant
feeders, omnivores, detritovores, filter-feeders, predators, coprophages,
commensals, and parasites. Thus their communities are able to form
themselves into a complete food-web (Taylor 1984). Molluscan species may
occupy all the main bottom econiches: depths and the surfaces of sediments
and rocks, dead coral heads, living corals or seaweed's thalloms.
5.1.1 Gastropods
The molluscan communities are represented on reefs mainly by bivalves and
gastropods plus several species of scaphopods and chitons. Among the
gastropods the prosobranchs are most numerous. The Opistobranchia are
mostly represented by various nudibranchs and Aplysia species. Several
species of Pulmonata also could be found in supralittoral areas.
The reef chitons enlist only a few species, some of which may reach 10 cm
length (Acanthopleura gemmata). They are nocturnal animals feeding on
periphytonic algae. At the beach rocks the density of their populations
might be of 10-15 sp. m- 2 .
The reef omnivorous prosobranchs feeding on seaweeds, on periphytons
and on microphytobenthos of soft sediments comprise 16 basic families.
Some of these families include the micromolluscs with a shell size of less
than 0.7 cm. Among them are Triphoridae, Littorinidae, and Rissoinidae. In
biotopes of coarse coral sands and of rubble, which usually contain a
significant amount of detritus, the micromolluscs from these families may
compose a main portion of the total benthic biomass (Kay 1971; Kay and
Switzer 1974). For example, at the Fanning atoll in such biotopes the
micromolluscs of 1-3 mm size included 19 genera, like Obtortio, Odostomia,
Tricolia, Enchelus, Merelina, an Diala. The small- and medium-sized gastropods from the families Fissurellidae, Littorinidae, Patellidae, Trochidae,
Rissoinidae, and Planaxidae inhabit rocky and stony littoral or sublittoral
areas with medium to strong wave stress. The density of communities, which
these gastropods form there, may reach several hundred sp. m- 2 .
On the rocks of the reef flat and on outer slopes gastropods from the
families Haliotidae and Stomatellidae are common. On reef flats and on
patch reefs the Turbinidae, which have a beautiful nacreous shell, could
often be observed. In the trenches of the rocks at the reef-flat edge and at
the beach rocks the cerithiids are often abundant. The family of Cerithiidae
is among the most widespread ones in reef biotopes. Their species have a
Reef Zoobenthos
spectra, and display numerous ecological and morphological adaptations,
which make for their extremely high density in most of the reef biotopes and
for a high degree of use of trophic resources. Their sizes vary between
several millimetres in micro molluscs and 1 m, and weights of up to 100 kg
(tridacnides). Among them we may find species which represent all trophic
levels ranging from symbiotic primary photosynthetic producers to plant
feeders, omnivores, detritovores, filter-feeders, predators, coprophages,
commensals, and parasites. Thus their communities are able to form
themselves into a complete food-web (Taylor 1984). Molluscan species may
occupy all the main bottom econiches: depths and the surfaces of sediments
and rocks, dead coral heads, living corals or seaweed's thalloms.
5.1.1 Gastropods
The molluscan communities are represented on reefs mainly by bivalves and
gastropods plus several species of scaphopods and chitons. Among the
gastropods the prosobranchs are most numerous. The Opistobranchia are
mostly represented by various nudibranchs and Aplysia species. Several
species of Pulmonata also could be found in supralittoral areas.
The reef chitons enlist only a few species, some of which may reach 10 cm
length (Acanthopleura gemmata). They are nocturnal animals feeding on
periphytonic algae. At the beach rocks the density of their populations
might be of 10-15 sp. m- 2 .
The reef omnivorous prosobranchs feeding on seaweeds, on periphytons
and on microphytobenthos of soft sediments comprise 16 basic families.
Some of these families include the micromolluscs with a shell size of less
than 0.7 cm. Among them are Triphoridae, Littorinidae, and Rissoinidae. In
biotopes of coarse coral sands and of rubble, which usually contain a
significant amount of detritus, the micromolluscs from these families may
compose a main portion of the total benthic biomass (Kay 1971; Kay and
Switzer 1974). For example, at the Fanning atoll in such biotopes the
micromolluscs of 1-3 mm size included 19 genera, like Obtortio, Odostomia,
Tricolia, Enchelus, Merelina, an Diala. The small- and medium-sized gastropods from the families Fissurellidae, Littorinidae, Patellidae, Trochidae,
Rissoinidae, and Planaxidae inhabit rocky and stony littoral or sublittoral
areas with medium to strong wave stress. The density of communities, which
these gastropods form there, may reach several hundred sp. m- 2 .
On the rocks of the reef flat and on outer slopes gastropods from the
families Haliotidae and Stomatellidae are common. On reef flats and on
patch reefs the Turbinidae, which have a beautiful nacreous shell, could
often be observed. In the trenches of the rocks at the reef-flat edge and at
the beach rocks the cerithiids are often abundant. The family of Cerithiidae
is among the most widespread ones in reef biotopes. Their species have a