EFFECTS OF EGG SIZE ON POSTLARVAL PERFORMANCE: EXPERIMENTAL EVIDENCE FROM A SEA URCHIN

Many life-history and developmental studies of marine invertebrates assume that eggs of species with nonfeeding larvae are large because they provide materials for rapid development. Using the sea urchin Heliocidaris erythrogramma which has 400 μm eggs and nonfeeding larvae, we removed an acellular, lipid-rich component from the blastula equivalent to ca. 40% of the egg volume and ca. 50% of the organic mass. Experimentally manipulated, reduced-lipid larvae survived well, developed in the usual time (3.5 d), and high percentages of the original numbers metamorphosed into anatomically normal juveniles. Control juveniles were larger at metamorphosis, grew more, and survived longer than siblings that lacked this lipid-rich material. Moderate increases in egg size in species with nonfeeding larvae may enhance postlarval performance significantly and therefore may reflect selection on early juvenile traits. The contrasts of our results and comparable experiments with feeding larvae suggests that egg size may play fundamentally different roles in species with feeding and nonfeeding larvae. The accommodation of materials reserved for the juvenile stage should be considered among hypotheses on evolutionary modification of developmental patterns.