LIVE UNDER TIDAL REGIME: THE ROLE OF THE BRITTLE-STAR OPHIOTHRIX FRAGILIS BEDS FROM THE EASTERN BAY OF SEINE IN THE FINE PARTICLE DEPOSIT-SUSPENSION MECHANISMS

In the English Channel, the brittle-star Ophiothrix fragilis is a common epifauna species mainly found in strong tidal current characterized by pebbles benthic habitats (Holme, 1984). In the Bay of Seine, O. fragilis is however living on gravel and coarse sandy sediments and more locally, it occurs in areas with unexpected amount of fine particles for such high hydrodynamic areas (Mear et al., 2006; Lozach et al., 2011). This species forms dense aggregation supporting high density populations (1,500 to 7,000 ind.m-2) and both ophiuroid aggregation morphology and behaviour of juveniles play an important role in formation of relatively large patches in term of surface area on the seafloor. Moreover, living in dense aggregations may reduce displacement by strong currents (Warner and Woodley, 1975). Adults, although mobile, are not highly active, but O. fragilis can be a crawling epibenthic species; individuals will crawl back and forth across water currents until a conspecific was found (Broom, 1975). Some migration of adults from nearby populations may be possible. Where dense Ophiothrix aggregations are found on bedrock surfaces they may monopolize the substratum, virtually to the exclusion of other epifauna. In contrasts, beds on soft bottom may contain a rich associated fauna, with a dominance of large suspension-feeders. In addition, O. fragilis plays a major role in pelago-benthic transfer of particles from the water column to the benthic habitats due to its suspension feeding activity (Davoult and Gounin, 1995).