Effects of body size on suspension feeding and energy budgets of the pearl oysters Pinctada margaritifera and P. maxima

This study compared suspension feeding, assimilation efficiency, respiration and excretion, and energy budgets (= scope for growth, SFG) in relation to body size in 2 pearl oysters, Pinctada margaritifera and P. maxima, at a low food concentration (ca 5000 cells m1⁻¹ Tahitian Isochrysis galbana). Clearance rate (CR), respiration rate (R) and ammonia excretion rate (E) were strongly correlated with body size (p < 0.001) in both species, with exponents of 0.60 and 0.61 (CR), 0.44 and 0.56 (R), and 0.64 and 0.78 (E), respectively, for P. margaritifera and P. maxima. CR did not differ significantly between the species, but absorption efficiency, which was unrelated to size, was significantly greater in P. maxima (57.5 vs 51%, p < 0.05). There was, however, no significant difference in absorbed energy (AE) between the species. Respired energy (RE) and excreted energy (EE) as proportions of AE were significantly lower (p < 0.01) in P. maxima of 0.1 g dry soft tissue wt (ca 36 mm shell height, SH). The former was 0.36 compared to 0.58 in P. margaritifera of the same size. Thus, P. maxima of 0.1 g dry soft tissue wt exceeded P. margaritifera of the same size in SFG, which accords with the former species' more rapid early growth. Both species of pearl oysters have a high ability to acquire energy under low phytoplankton conditions. Both species are exceptional bivalves in terms of energy fluxes, with clearance rates of 50 to 100 l h⁻¹ in large oysters of 150+ mm SH. They show among the highest CR, R, E and SFG values recorded for bivalves (using 1 g dry soft tissue wt as a standard size). The largest giant clam, Tridacna gigas, is one tropical bivalve with comparable SFG. It, however, is dependent on energy from autotrophy as well as heterotrophy to achieve its high SFG