Settlement and recruitment of the sea urchin Paracentrotus lividus in two contrasting habitats in the Mediterranean

We measured settlement and recruitment of one of the main invertebrate herbivores in the Mediterranean, the sea urchin Paracentrotus lividus, in 2 neighbouring and contrasting habitats: a seagrass meadow and a vertical rock wall. We quantified and compared temporal and bathymetrical variability in settlement and compared settlement with recruitment over a 4 yr period. Two settlement peaks were observed each year, a main peak in spring-early summer and a second peak in autumn-early winter. Interannual variability in settlement was very high (ca. 1 order of magnitude). Settlement was generally higher on the rock wall than in the seagrass meadow. No appreciable successful recruitment was observed in the meadow during the 4 yr studied despite the arrival of settlers. The few juvenile urchins found in the meadow were encountered at the shallow part and were completely hidden between the rhizomes. However, no differential settlement on the rhizome stratum was observed. In contrast to the seagrass meadow, recruitment occurred every year on the rock wall. Despite the arrival of settlers in both habitats, the population in the seagrass meadow appears to be recruitment-limited and probably mainly sustained by immigration, while settlement and recruitment appear to determine, to a certain extent, adult population structure and dynamics on the rock wall.