EVOLUTIONARY LOSS OF LARVAL FEEDING - DEVELOPMENT, FORM AND FUNCTION IN A FACULTATIVELY FEEDING LARVA, BRISASTER LATIFRONS

Species with large eggs and nonfeeding larvae have evolved many times from ancestors with smaller eggs and feeding larvae in numerous groups of aquatic invertebrates and amphibians. This change in reproductive allocation and larval form is often accompanied by dramatic changes in development. Little is known of this transformation because the inter mediate form (a facultatively feeding larva) is rare. Knowledge of fac ultatively feeding larvae may help explain the conditions under which nonfeeding larvae evolve. Two hypotheses concerning the evolutionary l oss of larval feeding are as follows: (1) large eggs evolve before mod ifications in larval development, and (2) the intermediate form (facul tatively feeding larva) is evolutionarily short-lived. I show that lar vae of a heart urchin, Brisaster latifrons, are capable of feeding but do not require food to complete larval development. Food for larvae a ppears to have little effect on larval growth and development. The dev elopment, form, and suspension feeding mechanism of these larvae are s imilar to those of obligate-feeding larvae of other echinoids. Feeding rates of Brisaster larvae are similar to cooccurring, obligate-feedin g echinoid larvae but are low relative to the large size of Brisaster larvae. The comparison shows that in Brisaster large egg size, indepen dence from larval food, and relatively low feeding rate have evolved b efore the heterochronies and modified developmental mechanisms common in nonfeeding echinoid larvae. If it is general, the result suggests t hat hypotheses concerning the origin of nonfeeding larval development should be based on ecological factors that affect natural selection fo r large eggs, rather than on the evolution of heterochronies and devel opmental novelties in particular clades. I also discuss alternative hy potheses concerning the evolutionary persistence of facultative larval feeding as a reproductive strategy. These hypotheses could be tested against a phylogenetic hypothesis.