Habitat preferences of the sea urchin Paracentrotus lividus

Population structure of the sea urchin Paracentrotus lividus seems to vary depending on the habitat, as, on average, urchin density is higher on rocks, and size is larger in Posidonia oceanica seagrass. Here, we tested the hypothesis that P. lividus would choose P. oceanica rather than rocky habitat due to the greater availability of shelter and food in the seagrass. We conducted 2 experiments by using aquaria divided into 2 equal sections, each containing an array of 2 habitats: natural P. oceanica (P) and rocks (R) in the first experiment and natural (NP) and artificial (AP) P. oceanica in the second (to highlight the reason for preference). Each experiment included 2 experimental stages of no-choice (Stage 1) and choice (Stage 2): in the 2 no-choice treatments, sections were filled with the same habitat, and in the 2 choice treatments, sections were filled with P and R in Expt 1 and NP and AP in Expt 2. Under the null hypothesis that preference of habitat is random, the expected numbers of sea urchins that were distributed in the 2 sections of the habitat after 24 h were estimated by using formulae derived from maximal likelihood estimates. The results evidenced a preference by P. lividus for P. oceanica habitat rather than rocky habitat in the first experiment and for natural P. oceanica rather than artificial seagrass in the second experiment, thus suggesting that P. lividus individuals searched for a well-structured habitat where they could find both shelter and food.