Depth regulation in parenchymella larvae of a demosponge: Relative roles of skeletogenesis, biochemical changes and behavior

Abstract

To assess factors that influence depth regulation of sponge larvae, we documented ontogenetic changes in larval size and shape, lipid and protein content, skeletal development, and photo-response in Sigmadocia caerulea, a shallow-water demosponge in the order Haplosclerida. We also measured size and biochemical differences among larvae from different parents to determine how depth regulation might vary across the population. Larvae were photonegative during the entire free-swimming period. Younger larvae swam faster than older larvae, but older larvae swam away from light for greater time and distances. Sinking rates of anesthetized 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 larval life, but protein content increased abruptly just after settlement. Minor differences in length and protein content among offspring from different parents had no apparent effect on depth regulation. Both active movement and passive sinking play roles in moving late-stage larvae towards the sea floor, but increase in larval spicular mass appears to be the most important factor.