ALLEN, J.D.; University of North Carolina at Chapel Hill: Is bigger better? Testing the consequences of changes in egg size on larval predation rates.

Empirical data and theoretical models suggest that egg size is an important life history parameter in marine invertebrates. In echinoderms that develop indirectly from small eggs, separating blastomeres at the two-cell stage yields two viable offspring that grow and develop through metamorphosis, although at a smaller size. This observation has led researchers to ask the question: why don�t echinoderms produce the smallest eggs capable of development? In laboratory experiments I addressed one potential cost of small egg size: that larvae developing from small eggs suffer increased rates of predation. Blastomeres of the Pacific sand dollar Dendraster excentricus were separated at the two-cell stage and reared to produce halved and full-size sibling larvae. The two larval types were presented simultaneously as potential prey items to five common predators: (1) crab zoea, (2) crab megalopae, (3) chaetognaths, (4) solitary tunicates and (5) postlarval fish. Larvae were tested at several developmental stages ranging from prism larvae through eight-arm plutei. Crab zoea, megalopa, chaetognath and tunicate predators consumed significantly more halved larvae than full-size larvae. Postlarval fish showed the opposite trend, consuming significantly more full-size larvae than halved larvae. For each predator tested, differences in consumption rate were greatest at early developmental stages, when size differences between full and halved larvae were also greatest. These results suggest that the direction of selection on egg size will vary with the presence and abundance of particular predators. Large larvae may reach a refuge from specialists on small prey items but larvae developing from small eggs will reach an equivalent size and be subjected to increased predation rates by specialists on larger prey.