Zonal recovery after experimental displacement in two sea urchins co-occurring in the Mediterranean

The difference in depth distributions of adult sea urchins Arbacia lixula L. and Paracentrotus lividus Lamarck (Echinodermata: Echinoidea) was assessed along a transect on a steeply and exposed stretch of the Tyrrhenian coast (Elba Island, Tuscany) in the four seasons of the year. P. lividus was always deeper zoned than A. lixula throughout the year, producing a consistent difference in the overall zonal pattern between the two sympatric species. The occurrence of a storm during the winter census had a marked effect on the zonation of A. lixula (consistent dislodgement and consequent downward shifting), while the distribution of P. lividus remained fairly unchanged. At a second, similar site along the Tyrrhenian coast (Argentario Promontory, Tuscany) experiments were conducted on individually marked sea urchins, which demonstrated the capacity of both species to move upward after experimental downward displacement. Only P. lividus also moved downward after experimental upward displacement. The study showed that the zonal segregation occurring between the two species along exposed rocky shores where they coexist is dynamically maintained against vertical diffusion and external perturbations related to water turbulence, by active movements allowing different adjustment of zonation pattern. (C) 1997 Elsevier Science B.V.