DEPTH REGULATION IN PARENCHYMELLA LARVAE OF A DEMOSPONGE - RELATIVE ROLES OF SKELETOGENESIS, BIOCHEMICAL-CHANGES AND BEHAVIOR

To assess factors that influence depth regulation of sponge larvae, we documented ontogenetic changes in larval size and shape, lipid and pr otein content, skeletal development, and photoresponse in Sigmadocia c aerulea, a shallow-water demosponge in the order Haplosclerida. We als o measured size and biochemical differences among larvae from differen t parents to determine how depth regulation might vary across the popu lation. Larvae were photonegative during the entire free-swimming peri od. Younger larvae swam faster than older larvae, but older larvae swa m away from light for greater time and distances. Sinking rates of ane sthetized larvae increased as a function of age, not because of lipid depletion or shape changes, but because addition of spicules increased density. Neither lipid nor protein changed significantly during larva l life, but protein content increased abruptly just after settlement. Minor differences in length and protein content among offspring from d ifferent parents had no apparent effect on depth regulation. Both acti ve movement and passive sinking play roles in moving late-stage larvae towards the sea floor, but increase in larval spicular mass appears t o be the most important factor.