ABUNDANCE OF FOOD AFFECTS RELATIVE SIZE OF LARVAL AND POSTLARVAL STRUCTURES OF A MOLLUSCAN VELIGER

Veliger larvae of mollusks were predicted to develop a larger velum re lative to the larval shell when reared with scarce food. The functiona l consequences of such developmental plasticity would be (1) greater m aximum capacity for capturing particles when food is scarce and (2) gr eater growth of structures retained in the post-larva when food is abu ndant. The hypothesis was tested by rearing veligers of the oyster Cra ssostrea gigas at high (near satiating) and low (growth limiting) conc entrations of food. Veligers at the measured shell lengths (>200 mum) had significantly larger velar lobes and longer prototrochal cilia tha n veligers reared in low concentrations of food. An analogous response to food levels (relatively longer ciliated band when food is scarce) has now been found for larvae as disparate as oyster veligers and sea urchin plutei. These observations suggest that functionally similar ex amples of developmental plasticity in the growth of larval parts have evolved more than once and may be widespread. An alternative interpret ation is that differential mortality or growth in a genetically hetero geneous batch of oyster larvae results in advanced veligers of differe nt forms at different concentrations of food. Both interpretations sug gest an adaptive advantage to growing a larger apparatus for clearing particles from suspension when food is scarce and shifting materials t o growth of postlarval structures (shell and associated structures) wh en food is abundant.