Chapter 21 - Paracentrotus lividus

Abstract

The purple sea urchin Paracentrotus lividus is a Mediterranean and north eastern Atlantic species. It is particularly common in the subtidal, down to depths of 10–20 m and in tidal pools, mainly on solid rocks, boulders and in seagrass meadows. Densities usually range from a few to a dozen individuals per square meter, but very high densities (over 50 to 100 ind·m−2) may occur. In the field, when food resources are not limited, P. lividus is basically herbivorous, with a number of species of MPOs (Multicellular Photosynthetic Organisms) clearly ‘preferred’ or ‘avoided’. Under conditions of limited food resources, it appears to be a very opportunistic generalist, able to exploit any kind of food resource, including drift material, sponges, hydrozoans and even small conspecifics (cannibalism) and particulate organic matter (POM). Competition between P. lividus and other herbivores (e.g., the sea urchin Arbacia lixula and the teleost Sarpa salpa) and predation by teleosts, crustaceans and mollusks may affect its abundance and behavior, and hence its effect on benthic communities. The density of P. lividus shows a general and conspicuous negative correlation with erect MPO coverage and biomass. It is a key species and structuring force, not only in overgrazed habitats with high sea urchin densities (barren grounds), but also in habitats with low sea urchin densities and a dense cover of MPOs (forests). In addition, it may control the shift between these two alternative ‘stable’ states.