170
Reef Zoobenthos
factor in the life of reef corals (Gareau and Hartman 1963; Connell 1973;
Evseev 1981; Trudgill 1983). Among the boring molluscs there are six
families of bivalves and one of gastropods (Soliman 1969; Appukutan 1972;
Hadfield 1976; Kleeman 1980). The shape of their shells are shown in Fig.
5.1. The dominating taxa of boring clams are the representatives of mytilids
and belong to the specialized genus Lithophaga, which numbers about ten
species. Lithophaga have a finger-like shell 0.5-3 cm long. Some species
attain lengths of up to 5-9 cm.
The boring molluscs are distinctly specialized in relation to the kind of
preferred substrate (Soliman 1969; Evseev and Silina 1982; Peyrot-Clausade
et al. 1989). Some five to six species of Lithophaga live only in dead colonies
of scleractinian corals (L. laevigata, L. malaccana and L. teres are among
them). Other species prefer the flat rocks or the "nigger heads" (L. obesa).
A group of species live only within colonies of living corals: L. cumingiana
and L. lessepiana in Stylophora, L. hanleyana, L. lima in Cyphastrea and in
Montipora, L. kuehnelti in Acropora and in Stylophora, L. simplex in Favia,
in LobophyJlia and in Symphillia. One of the causes of this fine
specialization could be seen in the diversity of the mechanisms for boring in
lime material, used by different species of borers. They bore mechanically,
chemically or combining these two techniques (Gohar and Soliman 1963).
The most widespread are the latter (Barthel 1981). The veligers of boring
molluscs settle on the surface of living or dead coral colonies, selecting
substrates peculiar to a given species. After their metamorphosis the young
clam bores into the colony lime skeleton or into the rock. It forms then a
pear-like channel with a narrow exit. Through it the clam shows outside its
syphones. The distal end of the channel is gradually cemented by the clam.
If the rate of growth of the host coral exceeds the rate of boring the clam
begins to bore the channel backward to prevent the closing of the exit hole
(Soliman 1969; Barthel 1981). The rate of boring of solid lime material by
Lithophaga ranges within 2 to 4 mm year- 1 in lime rocks of flats.
The clam Fungicava bores his holes into the central part of the corallite of
the solitary coral Fungia, and thrusts out his syphones close to the gastral
cavity of its polyp. Thus the clam filters water enriched with zooxanthellae
excreted by the polyp (Gareau et al. 1970; Hadfield 1976). The boring clam
Gastrochaena lives only in dead coral heads. With the use of X-radiographic
techniques it was established that from the massive corals of Caribbean reefs
the boring bivalves excavated 15-20% of their total skeletal mass
(Highsmith 1980), using only the mechanical mode (Soliman 1969). The
density of populations of boring molluscs is usually very high. For example,
14 colonies of living corals from six genera collected at the edge of the reef
flat on one of the Swane reefs a 20 x 30 cm piece of lime rock and one
tridacna shell alone contained 600 specimens of boring clams. On the reefs
off Barbados the mean number of boring bivalves was three for each coral
head (Highsmith 1980). The degree of the infestation of corals by these
borers seems to be directly dependent on the level of planktonic
Reef Zoobenthos
factor in the life of reef corals (Gareau and Hartman 1963; Connell 1973;
Evseev 1981; Trudgill 1983). Among the boring molluscs there are six
families of bivalves and one of gastropods (Soliman 1969; Appukutan 1972;
Hadfield 1976; Kleeman 1980). The shape of their shells are shown in Fig.
5.1. The dominating taxa of boring clams are the representatives of mytilids
and belong to the specialized genus Lithophaga, which numbers about ten
species. Lithophaga have a finger-like shell 0.5-3 cm long. Some species
attain lengths of up to 5-9 cm.
The boring molluscs are distinctly specialized in relation to the kind of
preferred substrate (Soliman 1969; Evseev and Silina 1982; Peyrot-Clausade
et al. 1989). Some five to six species of Lithophaga live only in dead colonies
of scleractinian corals (L. laevigata, L. malaccana and L. teres are among
them). Other species prefer the flat rocks or the "nigger heads" (L. obesa).
A group of species live only within colonies of living corals: L. cumingiana
and L. lessepiana in Stylophora, L. hanleyana, L. lima in Cyphastrea and in
Montipora, L. kuehnelti in Acropora and in Stylophora, L. simplex in Favia,
in LobophyJlia and in Symphillia. One of the causes of this fine
specialization could be seen in the diversity of the mechanisms for boring in
lime material, used by different species of borers. They bore mechanically,
chemically or combining these two techniques (Gohar and Soliman 1963).
The most widespread are the latter (Barthel 1981). The veligers of boring
molluscs settle on the surface of living or dead coral colonies, selecting
substrates peculiar to a given species. After their metamorphosis the young
clam bores into the colony lime skeleton or into the rock. It forms then a
pear-like channel with a narrow exit. Through it the clam shows outside its
syphones. The distal end of the channel is gradually cemented by the clam.
If the rate of growth of the host coral exceeds the rate of boring the clam
begins to bore the channel backward to prevent the closing of the exit hole
(Soliman 1969; Barthel 1981). The rate of boring of solid lime material by
Lithophaga ranges within 2 to 4 mm year- 1 in lime rocks of flats.
The clam Fungicava bores his holes into the central part of the corallite of
the solitary coral Fungia, and thrusts out his syphones close to the gastral
cavity of its polyp. Thus the clam filters water enriched with zooxanthellae
excreted by the polyp (Gareau et al. 1970; Hadfield 1976). The boring clam
Gastrochaena lives only in dead coral heads. With the use of X-radiographic
techniques it was established that from the massive corals of Caribbean reefs
the boring bivalves excavated 15-20% of their total skeletal mass
(Highsmith 1980), using only the mechanical mode (Soliman 1969). The
density of populations of boring molluscs is usually very high. For example,
14 colonies of living corals from six genera collected at the edge of the reef
flat on one of the Swane reefs a 20 x 30 cm piece of lime rock and one
tridacna shell alone contained 600 specimens of boring clams. On the reefs
off Barbados the mean number of boring bivalves was three for each coral
head (Highsmith 1980). The degree of the infestation of corals by these
borers seems to be directly dependent on the level of planktonic