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 photoresponse
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 freeswimming 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
This study was supported by a Fulbright
Fellowship awarded to M.M. (FU93-0227057). This is contribution
number 1149 of Harbor Branch Oceanographic Institution
Peer reviewed