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.