Molluscs
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bivalves. One of them (Pectinidae) lies on the surface flat on its side and
may move in water by flopping its valves. Others dig into the sediments and
move with the aid of their "foot" (Cardiidae, Corbulidae, Veneridae,
Psammobilidae, Tellinidae). In soft-bottom biotopes some of the sedentary
bivalves are also often abundant, especially on silty coral sands in the
lagoons of barrier reefs. These are mostly Pinna, and also Codakia from the
family Lucinidae. They attach to pieces of corals or of tridacna shells buried
in the sand. The areas with profuse coral growth often contain dense
populations of boring bivalves hidden inside the coral colonies. Their
biomass remains usually uncounted during benthos quantification studies.
More information about this very important group is given below.
5.1.3 Tridacnids and Their Symbiosis with Algae
The tridacnids or the gigant clams are the largest among the modern and
even among the fossil molluscs. The weight of a large specimen of Tridacna
gigas may exceed 300 kg, and its length 1 m. (Yonge 1975). The size of
Tridacna deresa - the second largest tridacnid - is up to 50 cm, of the
Hyppopus hyppopus 45 cm, of T. maxima and T. squamosa 40 cm, T. crocea
20 cm. Below the lock connecting the valves of their shells tridacnids have a
hole. Through this hole a mighty bissus passes which is used by the mollusc
for attachment to the solid substrate. Tridacna crocea, by making rolling
movements around her bissus, can bore into the rocky ground. Old
specimens of the large tridacnids T. gigas, T. deresa and Hyppopus usually
lose their bissus and lay free on the ground, being fixed to their places by
their own weight. In the daytime tridacna opens its valves and exposes to
the light her magnificently colored mantle. The tissues of the mantle contain
a mass of zooxanthellae cells. The products of their photosynthesis these
symbiotic algae translocate to the cells of the host (Muscatine 1967; Goreau
et al. 1973). Also, an alternative mechanism for the use of algal
photosynthesis products by the host exists. The excess of the zooxanthellae
cells, which are permanently accumulating in mantle tissues, are consumed
and digested by the amoebocytes, which travel in the blood stream. The
non digested substance of zooxanthellae is excreted by amoebocytes in the
liver of the mollusc, which is extremely large. Thus, in the blood stream of
tridacna an additional digestive system is readily functioning, which
increases the efficiency of the use of photosynthetic production of algae by
the host. The faeces of tridacna contain a mass of nondigested
zooxanthellae. They comprise in T. maxima up to 80% of their mass (Ricard
and Salvat 1972).
The symbiotic zooxanthellae of tridacnids, as well as main their
physiologial features: light adaptations, translocation of photosynthates to
the host, are practically the same as in zooxanthellae symbiotic to the
hermatypic corals (Taylor 1973a; Goreau et al. 1973; Scott and Jitts 1977).
167
bivalves. One of them (Pectinidae) lies on the surface flat on its side and
may move in water by flopping its valves. Others dig into the sediments and
move with the aid of their "foot" (Cardiidae, Corbulidae, Veneridae,
Psammobilidae, Tellinidae). In soft-bottom biotopes some of the sedentary
bivalves are also often abundant, especially on silty coral sands in the
lagoons of barrier reefs. These are mostly Pinna, and also Codakia from the
family Lucinidae. They attach to pieces of corals or of tridacna shells buried
in the sand. The areas with profuse coral growth often contain dense
populations of boring bivalves hidden inside the coral colonies. Their
biomass remains usually uncounted during benthos quantification studies.
More information about this very important group is given below.
5.1.3 Tridacnids and Their Symbiosis with Algae
The tridacnids or the gigant clams are the largest among the modern and
even among the fossil molluscs. The weight of a large specimen of Tridacna
gigas may exceed 300 kg, and its length 1 m. (Yonge 1975). The size of
Tridacna deresa - the second largest tridacnid - is up to 50 cm, of the
Hyppopus hyppopus 45 cm, of T. maxima and T. squamosa 40 cm, T. crocea
20 cm. Below the lock connecting the valves of their shells tridacnids have a
hole. Through this hole a mighty bissus passes which is used by the mollusc
for attachment to the solid substrate. Tridacna crocea, by making rolling
movements around her bissus, can bore into the rocky ground. Old
specimens of the large tridacnids T. gigas, T. deresa and Hyppopus usually
lose their bissus and lay free on the ground, being fixed to their places by
their own weight. In the daytime tridacna opens its valves and exposes to
the light her magnificently colored mantle. The tissues of the mantle contain
a mass of zooxanthellae cells. The products of their photosynthesis these
symbiotic algae translocate to the cells of the host (Muscatine 1967; Goreau
et al. 1973). Also, an alternative mechanism for the use of algal
photosynthesis products by the host exists. The excess of the zooxanthellae
cells, which are permanently accumulating in mantle tissues, are consumed
and digested by the amoebocytes, which travel in the blood stream. The
non digested substance of zooxanthellae is excreted by amoebocytes in the
liver of the mollusc, which is extremely large. Thus, in the blood stream of
tridacna an additional digestive system is readily functioning, which
increases the efficiency of the use of photosynthetic production of algae by
the host. The faeces of tridacna contain a mass of nondigested
zooxanthellae. They comprise in T. maxima up to 80% of their mass (Ricard
and Salvat 1972).
The symbiotic zooxanthellae of tridacnids, as well as main their
physiologial features: light adaptations, translocation of photosynthates to
the host, are practically the same as in zooxanthellae symbiotic to the
hermatypic corals (Taylor 1973a; Goreau et al. 1973; Scott and Jitts 1977).