Molluscs
171
productivity in reef water, which itself depends on the nutrients input. The
more nutrients, the more borers, and the more damage to corals. Thus, a
high nutrients content is not healthy at all for the reef ecosystem, which has
been established also experimentally (Kinsey and Davies 1979b). Enhanced
planktonic productivity induced in areas of anthropogenic pollution, while
improving trophic conditions for the filtering, boring bivalves, drastically
increases the level of infestation of corals by borers, and so increases their
mortality.
Some clams are commensals of ramulose scleractinian corals, such as the
Seriatopora. They are "nesting" in their ramules. Among them there are
some species of the genera Barbatia, Pedium, and also the tridacnid T.
crocea. The veligers of this clam settle on the dead patch of the colony or
upon its living surface which then dies. The surrounding living colony grows
around the attached clam and finally the clam becomes "nested" into the
colony. This form of commensalism of boring clams with their scleractinian
hosts is profitable mostly to the clams, saving them from predation. As for
the corals, this commensalism at first sight seems to be only harmful for the
host corals. Indeed, the clams bore numerous holes in their branehes
decreasing so their stability against the wave-stress. But in fact the
destruction of branches of ramose corals, by breaking their parts damaged
by the borers, promotes their vegetative multiplication and their
distribution. The filtering activity of boring clams results in excretion of
large amounts of pseudo-feces, which improves the trophic conditions for
their host corals.
5.1.5 Community Structure, Roles in Reef Ecosystems
The communities of coral reef molluscs are extremely diverse. The total
number of their species in one reef system may attain 100 to 550 (Maes
1967; Ricard and Salvat 1972; Zlatarsky and Estalella 1980; Taylor 1984).
Their fauna was described as found on the Aldabra atoll (Taylor 1976), the
Chagos archipelago (Sheppard 1984), the Phantom shallows (Evseev 1982),
the Scott reefs (Sirenko 1980), the New Hebrides reefs (Budin et al. 1980),
the Canton atoll (Kay 1978), the Micronesian reefs (Demond 1957), the
Fanning atoll (Kay 1971; Kay and Switzer 1974), the One Tree atoll
(Hutchings 1974), and on the reefs of the eastern Pacific (Cantera 1989).
The number of gastropod species comprised on an average 50 to 70% of the
total molluscan fauna. This percentage varies in different reef biotopes (Kay
1971). The share of gastropods in benthic communities is usually less in the
flat biotopes and increases in the biotopes of littoral and lagoon. In the
communities of reef molluscs the following trophic groups could be named:
1. Phytophages feeding on thalloms of seaweeds and leaves of sea grasses
(Aplysia, Turbo, Trochus; see Klumpp and Pulfrich 1989).
171
productivity in reef water, which itself depends on the nutrients input. The
more nutrients, the more borers, and the more damage to corals. Thus, a
high nutrients content is not healthy at all for the reef ecosystem, which has
been established also experimentally (Kinsey and Davies 1979b). Enhanced
planktonic productivity induced in areas of anthropogenic pollution, while
improving trophic conditions for the filtering, boring bivalves, drastically
increases the level of infestation of corals by borers, and so increases their
mortality.
Some clams are commensals of ramulose scleractinian corals, such as the
Seriatopora. They are "nesting" in their ramules. Among them there are
some species of the genera Barbatia, Pedium, and also the tridacnid T.
crocea. The veligers of this clam settle on the dead patch of the colony or
upon its living surface which then dies. The surrounding living colony grows
around the attached clam and finally the clam becomes "nested" into the
colony. This form of commensalism of boring clams with their scleractinian
hosts is profitable mostly to the clams, saving them from predation. As for
the corals, this commensalism at first sight seems to be only harmful for the
host corals. Indeed, the clams bore numerous holes in their branehes
decreasing so their stability against the wave-stress. But in fact the
destruction of branches of ramose corals, by breaking their parts damaged
by the borers, promotes their vegetative multiplication and their
distribution. The filtering activity of boring clams results in excretion of
large amounts of pseudo-feces, which improves the trophic conditions for
their host corals.
5.1.5 Community Structure, Roles in Reef Ecosystems
The communities of coral reef molluscs are extremely diverse. The total
number of their species in one reef system may attain 100 to 550 (Maes
1967; Ricard and Salvat 1972; Zlatarsky and Estalella 1980; Taylor 1984).
Their fauna was described as found on the Aldabra atoll (Taylor 1976), the
Chagos archipelago (Sheppard 1984), the Phantom shallows (Evseev 1982),
the Scott reefs (Sirenko 1980), the New Hebrides reefs (Budin et al. 1980),
the Canton atoll (Kay 1978), the Micronesian reefs (Demond 1957), the
Fanning atoll (Kay 1971; Kay and Switzer 1974), the One Tree atoll
(Hutchings 1974), and on the reefs of the eastern Pacific (Cantera 1989).
The number of gastropod species comprised on an average 50 to 70% of the
total molluscan fauna. This percentage varies in different reef biotopes (Kay
1971). The share of gastropods in benthic communities is usually less in the
flat biotopes and increases in the biotopes of littoral and lagoon. In the
communities of reef molluscs the following trophic groups could be named:
1. Phytophages feeding on thalloms of seaweeds and leaves of sea grasses
(Aplysia, Turbo, Trochus; see Klumpp and Pulfrich 1989).