DIVERSITY OF BRYOZOANS IN A MEDITERRANEAN SUBLITTORAL CAVE WITH BATHYAL LIKE CONDITIONS - ROLE OF DISPERSAL PROCESSES AND LOCAL FACTORS

Saltatory dispersal through stepping-stone habitats is the most plausi ble model for exchanges of eurythermal species with limited dispersal potential between coastal dark (caves, crevices, etc.) and deep-water habitats. However, direct shoreward advections of propagules from the slope communities may occur, as suggested by the presence of bathyo-ab yssal organisms, mainly sponges, in a large shallow-water cave from th e French Mediterranean coast characterized by a cold homothermal regim e below a within-cave thermocline. Temperature recordings performed ye ar-round in this cave attest that inputs of water parcels uplifted fro m the slope during upwelling events may advect material right through the cave. In contrast to sponges, bryozoans, another dominant group on cave and deep-water substrates, seem to be unable to benefit from the se cross-shelf transfers. Only one presumed stenobathic species from t he lower shelf and upper slope, Puellina setiformis, was recorded on t he cave walls. The patterns of the spatial distribution of bryozoan sp ecies richness (strong negative correlation with distance from cave en trance) and abundance (abrupt decline beyond a topographical change) w ithin the cave's homothermal layer contrast with the success of coexis ting sponges. A 14 mo colonization experiment indicates that settlemen t rate is dramatically reduced for the whole sessile fauna below the w ithin-cave thermocline. The present results suggest that the successfu l colonization of the cold homothermal cave by allochthonous larvae is likely to be dependent on rare pulse fluxes and is strongly limited b y local abiotic and presumably biotic factors, and that the regional p ool of deepwater bryozoans is not a probable source of settlers for on shore aphotic habitats.