ALLEN, J.D.; ZAKAS, C.; PODOLSKY, R.D.; University of North Carolina at Chapel Hill: Testing the consequences of an egg size reduction for a species with facultatively feeding larvae

In free-spawning marine invertebrates, larval development typically proceeds by one of two common modes: planktotrophy (obligate feeding) or lecithotrophy (obligate non-feeding). In a rare third developmental mode, facultative planktotrophy, larvae can feed but do not require particulate food to complete metamorphosis. Facultative planktotrophy is thought to be an intermediate strategy that results evolutionarily from an increase in the energy content of the egg. We tested the hypothesis that a decrease in egg energy content is sufficient to shift development from facultative to obligate planktotrophy by experimentally reducing the egg size of Clypeaster rosaceus, an echinoid echinoderm with facultatively planktotrophic larvae. Blastomeres were left whole (W) or separated at the 2 and 4-cell stage to create half- (H) or quarter-size (Q) “eggs” from which larvae were reared to metamorphosis, both with and without particulate food. Larvae from all treatments metamorphosed at approximately the same time regardless of food treatment or egg size. In contrast, juveniles that developed from W eggs were significantly larger, had higher organic content, and had longer and more numerous spines than juveniles from H or Q eggs. Larvae from W, H, and Q size eggs were able to reach metamorphosis without feeding, suggesting that the evolution of facultative planktotrophy in C. rosaceus has been accompanied by more than a simple increase in egg size. In addition, our results suggest that resources endogenous to the egg have a larger effect on juvenile size and quality than do exogenous resources such as particulate food.