Life-history evolution and the costs of small egg size in echinoderms

The evolution of life histories is constrained by trade-offs between traits. One important trade-off occurs between the number and size of offspring that a female produces. The obvious benefit of a reduction in offspring size is that more offspring can be produced. Why, then, do not all organisms maximize their fecundity by producing the smallest size of offspring possible? In my dissertation, I examined the costs of small egg size using echinoderms as a model system.; I tested the effects of experimental reductions in egg size on development time and juvenile size in four species of echinoids that together span a range of egg sizes (80 mum to 270 mum in diameter). In three species with small eggs (< 130 mum), egg size reductions significantly increased development time. In the species with large eggs (270 mum), egg size reductions did not affect development time but significantly reduced juvenile size. Egg size reductions are therefore costly in terms of both development time and juvenile size, depending on the initial size of the egg.; I also tested the effects of egg size reductions on larval mortality rates using the echinoid Dendraster excentricus. At four larval ages, predation rates on offspring from reduced size eggs were tested relative to offspring from eggs of normal size. Four of the five predators tested consumed significantly more offspring from eggs of reduced size. The fifth predator consumed significantly more offspring from eggs of normal size. All predators consumed younger larvae in greater numbers than older larvae. These results suggest that higher larval predation rates are another cost of small egg size.; I also collected comparative developmental data in seven species of brittlestars in the genus Macrophiothrix that varied more than 20-fold in egg volume. Three species developed from small eggs (< 170 mum) as obligately feeding pluteus larvae. Two species developed from large eggs (> 260 mum) as obligately non-feeding larvae with abbreviated development. Two species developed from eggs of intermediate size (230 to 245 mum) and retained features of feeding larvae but did not require food to complete development.