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Reef Zoobenthos
plackard. Many of them are extremely beautiful in their coloration and
forms, being one of the remarkable decorations on the reef. In the soft
sediments of lagoons Scaphopoda could be found. They have curious toothform shells and easily move inside the sediments. Their size is 3-10 cm. In
bottom reef biotopes also various cephalopod molluscs could be met with -
squids and small octopuses.
5.1.2 Bivalves
The communities of Bivalvia are also abundant on reefs both faunistically
and by their biomass. They comprise 10-30% of species of the reef's
malacofauna and about the same part of its total biomass. Reefs have vast
spaces of rocky hard substrate for the settling of sessile bivalves and plenty
of soft-bottom areas for their vagile species. Being filterers they have a good
food supply from reef waters rich in microplankton and detritus. Some reef
bivalves have symbiotic zooxanthellae and feed autotrophically. The rocky
littoral biotopes exposed to wave-stress, as well as the surfaces of dead
massive coral colonies at the windward reef-flat edges, are inhabited by
oysters (family Ostrea). The basal valves of their shell is attached to the
rock surface, thus firmly fixing the molluscs onto the substrate surface. Some
oysters, like Ostrea cucullata, O. sandwichiensis, Crassostrea gigas, and
Saccostrea amasca form on reefs of the Indo-Pacific region extremely dense
monospecific populations counting hundreds of specimens per square meter.
The bivalves spondilus ("thorny" oysters) and some of Chamidae prefer
temperate wave-stressed places, as channels and caves.
Most of the bivalves attach their shells to the rock surface with the aid of
bissus. Among them are the families Mytilidae, Pteriidae, Pinnidae, and
Malleidae. They also inhabit the solid substrates in areas of temperate to
weak wave stress: rocky surfaces of reef-flat, dead coral heads, walls of
trenches and channels. Many of these bivalves are collected for food or for
pearls and nacre. In biotopes of reef-flats, on patch reefs, and in dead coral
colonies the bivalves of the families Limidae, Isognomidae, Arcidae,
Nucleidae are also usual. Some species of arcids, nucleids and lucinids
inhabit also the coarse sands and rubble substrates on the flat and in the
littoral areas. One of the most important elements of fauna in the reef-flat
and patch-reef areas are the large bivalves of the family Tridacnidae, which
have symbiotic zooxanthellae. Less abundant is another relative to
tridacnids is the bivalve Corculum cardissa. It has a fine transparent shell.
The tissues of mantle under these shells also contain symbiotic
zooxanthellae. Corculum lives in soft sediments. But it digs into the
sediment so that the upper part of its shell remains exposed to sunlight,
necessary for his symbionts. The ecology of tridachids is described below.
The sandy bottom of channels crossing the reef-flat and its outer slopes,
as well as the bottom of the lagoon, are inhabited mainly by free-living
Reef Zoobenthos
plackard. Many of them are extremely beautiful in their coloration and
forms, being one of the remarkable decorations on the reef. In the soft
sediments of lagoons Scaphopoda could be found. They have curious toothform shells and easily move inside the sediments. Their size is 3-10 cm. In
bottom reef biotopes also various cephalopod molluscs could be met with -
squids and small octopuses.
5.1.2 Bivalves
The communities of Bivalvia are also abundant on reefs both faunistically
and by their biomass. They comprise 10-30% of species of the reef's
malacofauna and about the same part of its total biomass. Reefs have vast
spaces of rocky hard substrate for the settling of sessile bivalves and plenty
of soft-bottom areas for their vagile species. Being filterers they have a good
food supply from reef waters rich in microplankton and detritus. Some reef
bivalves have symbiotic zooxanthellae and feed autotrophically. The rocky
littoral biotopes exposed to wave-stress, as well as the surfaces of dead
massive coral colonies at the windward reef-flat edges, are inhabited by
oysters (family Ostrea). The basal valves of their shell is attached to the
rock surface, thus firmly fixing the molluscs onto the substrate surface. Some
oysters, like Ostrea cucullata, O. sandwichiensis, Crassostrea gigas, and
Saccostrea amasca form on reefs of the Indo-Pacific region extremely dense
monospecific populations counting hundreds of specimens per square meter.
The bivalves spondilus ("thorny" oysters) and some of Chamidae prefer
temperate wave-stressed places, as channels and caves.
Most of the bivalves attach their shells to the rock surface with the aid of
bissus. Among them are the families Mytilidae, Pteriidae, Pinnidae, and
Malleidae. They also inhabit the solid substrates in areas of temperate to
weak wave stress: rocky surfaces of reef-flat, dead coral heads, walls of
trenches and channels. Many of these bivalves are collected for food or for
pearls and nacre. In biotopes of reef-flats, on patch reefs, and in dead coral
colonies the bivalves of the families Limidae, Isognomidae, Arcidae,
Nucleidae are also usual. Some species of arcids, nucleids and lucinids
inhabit also the coarse sands and rubble substrates on the flat and in the
littoral areas. One of the most important elements of fauna in the reef-flat
and patch-reef areas are the large bivalves of the family Tridacnidae, which
have symbiotic zooxanthellae. Less abundant is another relative to
tridacnids is the bivalve Corculum cardissa. It has a fine transparent shell.
The tissues of mantle under these shells also contain symbiotic
zooxanthellae. Corculum lives in soft sediments. But it digs into the
sediment so that the upper part of its shell remains exposed to sunlight,
necessary for his symbionts. The ecology of tridachids is described below.
The sandy bottom of channels crossing the reef-flat and its outer slopes,
as well as the bottom of the lagoon, are inhabited mainly by free-living