Ectosymbiosis associated with cidaroids (Echinodermata: Echinoidea) promotes benthic colonization of the seafloor in the Larsen Embayments, Western Antarctica.

7 pagesInternational audienceIce-shelf collapses in the Larsen A and B embayments along the Weddell side of the Antarctic Peninsula resulted in new open-water areas that are likely reorganizing benthic communities. It is a natural laboratory to assess colonization of the sea bottom under new conditions. We tested the hypothesis that the epibionts associated to cidaroid echinoids could promote or enhance the colonization of hard surfaces. In fact, being vagile, cidaroids might improve dispersal capabilities of the sessile animals that are attached to their spines, e.g., promoting the colonization of areas where the fauna has been eradicated by iceberg scouring. If this hypothesis is correct, pioneer sessile species present locally on stones might have affinities with ectosymbionts on cidaroids, and the sessile fauna present both on cidaroids and stones should be more similar in the Larsen embayments than in undisturbed areas. We therefore compared sessile species living on cidaroids with those living on stones in three areas: Larsen A and B embayments and two undisturbed and geographically different areas, Atka Bay and Elephant Island. Overall, richness, evaluated as ‘morphotypes' richness, was lower in the Larsen area than in other areas, but levels of species diversity were similar among the three zones. The estimate of similarity between assemblages (using the C22 index of Chao et al., 2008) also suggests that cidaroid epibionts are either species-specific or specific to cidaroids, while the composition of sessile fauna on stones is more variable and probably dependent on local factors. In the two undisturbed areas, sessile fauna are highly different between stones and cidaroids. This contrasts with the Larsen embayments where cidaroids share more than 80% of epibionts with the surrounding stones. These results suggest that ectosymbioses linked to cidaroids strongly contribute to benthic colonization of the seafloor in the Larsen enbayments. With time, secondary successions are expected to occur, increasing the difference between epibiotic communities on cidaroids and those on stones, and lead to the situations observed in unperturbed sites