Investigating on the occurrence ofParacentrotus lividusin rocky andPosidonia oceanicahabitat

The sea urchinParacentrotus lividus(Lamarck) is the most common grazer in the Mediterranean infralittoral that at high densities overgrazes complex algal assemblages turning them into barren areas. This study has the aim to investigate whether abundance and population structure ofP. lividusis consistent between rocky boulders andPosidonia oceanicahabitat. At this aim, we have sampledP. lividusat six sites in the Gulf of Alghero (North West Sardinia), 3 fished (sea urchin are harvested) and 3 controls (no harvest is allowed) and at each site the two habitats, 6-10 m deep, were considered. For each combination site x habitat 10 replicates were taken. Density ofP. lividuswas assessed using quadrats of 1 x 1 m. The size of 20 individual (test diameter without spines) per site was measured by means of a calliper (1#1 0.1mm). Sea urchins, finally were grouped into size classes to examine frequency distributions. Sampling was performed at the end of a harvesting period (April-May 2006). Analyses of data have highlighted a significant variability for both response variables among sites while inPosidonia oceanicahabitat a lower density and a higher size were found rather than in rocky habitat. Further data collected at three types ofPosidonia oceanica(patches close to rocky habitat, far from rocky habitat and patches bordered by sediment) suggest that the abundance of adults in the seagrass is probably sustained by immigration from the rocky habitat.