Trade-offs between defence and competition in gregarious juvenile fluted giant clams (Tridacna squamosa L.)

Bivalves can derive benefits from living in groups, such as reduced predation risk and increased reproductive success, but at the cost of greater competition for resources. Although gregariousness has been observed in giant clams, both experimentally and in the field, the ecological significance of this behaviour has yet to be evaluated. Here we quantified some benefits and costs of aggregation in the fluted giant clam, Tridacna squamosa , through two laboratory experiments that tested (1) growth and ectoparasite (pyramidellid snails) load, and (2) predation rates, of juvenile clams when reared in two configurations: aggregated and dispersed. Aggregated clams showed significantly lowered growth and greater ectoparasite susceptibility. Clams within aggregations were, however, more resistant to predation by the stone crab, Myomenippe hardwickii , as they required more time to handle compared to dispersed clams. As giant clams in the wild are vulnerable to numerous predators, the defensive advantage conferred by aggregation is potentially sufficient to outweigh the growth and ectoparasitism costs. These benefits may only apply to juveniles, as older individuals should have reached a size refuge from most predators and would therefore receive fewer benefits from aggregation.