Sponge inventory of the French Mediterranean waters, with an emphasis on cave-dwelling species.

Mediterranean sponges represent about 10 % of the world sponge biodiversity, with these sessile organisms dominating in terms of diversity and biomass in most of the rocky bottoms shaded from light. After 60 years of intensive study of the sponge diversity along the French coast, we present the first comprehensive reference-list for this biogeographic area. A total of 389 sponge species was recorded, of which 222 known in the Marseille region. In this area, special attention was paid to species from underwater caves. Although this particular habitat appeared as one of the richest, a wealth of hidden diversity still requires description. About 37 % of the sponge diversity can be found in underwater caves, most of these species being also distributed in other habitats. However, 23 % of this sponge diversity is cave-exclusive. An easy and rapid assessment method was developed with a selection of 65 representative sponge species, for the monitoring of semi-dark cave communities. This method, based on data acquisition with photoquadrats and their processing using a DataBase built with ACCESS, was deployed in 13 studied sites. Altogether, this study represents a useful contribution for marine environment managers who might refer to this French reference list and apply the rapid and easy assessment method in the framework of several European Directives and international Conventions.