Ultrastructure and dispersal potential of sponge larvae: tufted versus evenly ciliated parenchymellae

In this work we analyse the ultrastructure and behaviour of two representative types of parenchymella sponge larvae: the evenly ciliated larva of Scopalina lophyropoda , and the tufted larva of Cacospongia mollior . We sought to find correlates of the dispersal abilities and distribution patterns of these species. Laboratory experiments were performed under two different conditions of water movement (still water and a slight oscillatory movement), and at different times of the larval life-span. Larvae of S. lophyropoda were motionless most of the time, maintaining a vertical posture and rotating on their main axis, and they did not respond to directional light. Larvae of C. mollior were active swimmers that responded negatively to light stimuli. In both cases, movement decreased as the larval age increased. Ultrastructurally, C. mollior larvae were more compact, with more reserves and abundant collagen bundles. Desmosome-like structures were seen connecting the apical parts of the ciliated surface in both species. Two different types of putative light-sensitive structures were found. Both structural and behavioural characteristics favour a greater dispersal capability for C. mollior and a philopatric dispersal mode for S. lophyropoda . These predictions are in agreement with the observed ecological distribution of the species.