RELATIONSHIPS BETWEEN SIZE, MANTLE AREA AND ZOOXANTHELLAE NUMBERS IN 5 SPECIES OF GIANT CLAM (TRIDACNIDAE)

Relationships between body size and both projected mantle area and num bers 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 marked ly between clam species. Mantle areas were consistently lowest in Trid acna derasa. T. crocea and Hippopus hippopus had the largest mantles a t small body size (2 cm), but because of slow length-related increment s were soon overtaken by T. gigas and T. squamosa, the latter developi ng 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. hip popus, T. crocea or especially T. derasa; but by 30 cm length T. squam osa and T. gigas had the largest zooxanthellae populations. When expre ssed per unit body mass zooxanthellae numbers declined rapidly with si ze in all species. The rate of decline was most marked in T. crocea, t his being a function of its rapid length-related increment in flesh ma ss. This is probably the main factor restricting T. crocea to small te rminal body size. By contrast, rapid length-related increments in mant le area and zooxanthellae numbers in both T. gigas and T. squamosa app ear to favour the large body size and rapid growth observed in these s pecies. The reason why T. squamosa is unable to realize the rapid grow th and enormous terminal size observed in T. gigas is obscure, but may be a function of the relative reproductive output of these species, w hich remains unquantified.