Diversity of bryozoans in a Mediterranean sublittoral cave with bathyal-like conditions: Role of dispersal processes and local factors

Abstract

Saltatory dispersal through stepping-stone habitats is the most plausible 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-abyssal organisms, mainly sponges, in a large shallow-water cave from the French Mediterranean coast characterized by a cold homothermal regime below a within-cave thermocline. Temperature recordings performed year-round in this cave attest that inputs of water parcels uplifted from 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 these cross-shelf transfers. Only one presumed stenobathic species from the lower shelf and upper slope, Puellina setiformis, was recorded on the cave wails. The patterns of the spatial distribution of bryozoan species richness (strong negative correlation with distance from cave entrance) and abundance (abrupt decline beyond a topographical change) within the cave's homothermal layer contrast with the success of coexisting sponges. A 14 mo colonization experiment indicates that settlement rate is dramatically reduced for the whole sessile fauna below the within-cave thermocline. The present results suggest that the successful colonization of the cold homothermal cave by allochthonous larvae is likely to be dependent on rare pulse fluxes and is strongly limited by local abiotic and presumably biotic factors, and that the regional pool of deepwater bryozoans is not a probable source of settlers for onshore aphotic habitats.