MEPS 137:139-147 (1996)  -  doi:10.3354/meps137139

Relationships between body size and both projected mantle area and numbers of symbiotic zooxanthellae were calculated for 5 species of giant clam from the Great Barrier Reef. Parameters were closely correlated in all species, but the allometry of the relationships differed markedly between clam species. Mantle areas were consistently lowest in Tridacna derasa. T. crocea and Hippopus hippopus had the largest mantles at small body size (2 cm), but because of slow length-related increments were soon overtaken by T. gigas and T. squamosa, the latter developing a mantle area double that of other species at 30 cm length. Similar allometric variations were evident in zooxanthellae numbers. At small size these were much lower in T. squamosa and T. gigas than in H. hippopus,T. crocea or especially T. derasa; but by 30 cm length T. squamosa and T. gigas had the largest zooxanthellae populations. When expressed per unit body mass zooxanthellae numbers declined rapidly with size in all species. The rate of decline was most marked in T. crocea, this being a function of its rapid length-related increment in flesh mass. This is probably the main factor restricting T. crocea to small terminal body size. By contrast, rapid length-related increments in mantle area and zooxanthellae numbers in both T. gigas and T. squamosa appear to favour the large body size and rapid growth observed in these species. The reason why T. squamosa is unable to realize the rapid growth and enormous terminal size observed in T. gigas is obscure, but may be a function of the relative reproductive output of these species, which remains unquantified.

Giant clam . Tridacna . Hippopus . Symbiosis . Zooxanthellae