ABSTRACT
Seasonal and annual variations in the distribution and abundance of echinoid and ophiuroid larvae (early to post larval) were determined from a series of plankton net tows taken at 3 stations in the bay of Villefranche and at 750 m intervals along a transect of 28 miles from Villefranche (France) to halfway to Corsica (France) between 1984 and 1988. Spatial patterns of 5 species along the radial transect, and some mechanisms responsible for dispersal, i.e. water currents, hydroclimate and vertical movements, were determined. The echinoderm larvae exhibited weak vertical migration (few meters), remaining in the superficial layer undergoing hydrodynamic constraints which either limit or promote dispersal far away from the nursery beds. Larval concentration decreased with distance from the coast being generally limited to a 17 miles offshore boundary. This distance coincided with the location of a coastal divergence zone where a rise of subsurface waters confines the larvae to the Ligurian current. For the echinoids Paracentrotus lividus (Lamark) and Arbacia lixula (Linnaeus) spawning appeared to be general and limited in time followed by synchronous development of larval cohorts nearshore and offshore. Recruitment for these species is therefore thought to occur at a well-defined time. There is a close correspondence between the the presence of competent larvae in plankton and the benthic recruitment. New settled sea urchin (less than 1 millimeter diameter), were collected in late-spring and fall on the bottom, using experimental substrata. In contrast, larval populations of ophiuroid (Amphiura filiformis, Ophiopluteus bimaculatus and O. compressus) show mixed size classes. This is consistent with continuous spawning of adults, either from the nearby coast or from coastal areas farther away. Recruitment for these ophiuroids species is spread over time.
