Population structure of the sea urchins Paracentrotus lividus and Arbacia lixula across a depth gradient

Sea urchins are considered to have a paramount role in structuring rocky shallow communities. Under certain conditions, grazing by sea-urchins has been reported to produce a shift from macroalgae-dominated communities to barren habitat dominated by encrusting algae. However, this process is not recorded at all depths. The effect of depth on the structure of the sea-urchin populations may be thus a key feature to understand the origin and maintenance of the barren habitat within certain depths. In the present study, the structure of Paracentrotus lividus (Lam.) and Arbacia lixula (L.) populations are analyzed across a depth gradient in the Ustica Island MPA (NW Mediterranean), where the barren habitat extend from about -3 to -7 m. The densities of three size classes of the two species were scored at five depth ranges, in coincidence with changes in habitat type. Counts were made in four sites to control for spatial variability and to allow proper spatial replication. The results indicate that the population structure of the two sea-urchins change across depth, but patterns also reflect a high spatial variability among sites. Our results suggest that spatial- and depth-dependent process, like recruitment and predation, could be involved in the origin and maintenance of the barren habitat.