DO HYDRODYNAMIC FACTORS AFFECT THE RECRUITMENT OF MARINE-INVERTEBRATES IN A MACROTIDAL AREA - THE CASE-STUDY OF PECTINARIA-KORENI (POLYCHAETA) IN THE BAY OF SEINE (ENGLISH-CHANNEL)

For marine benthic invertebrates exhibiting complex life cycles, chang es in populations' distribution and abundance are governed by a large variety of physical, chemical and biological processes. From field obs ervations in the Bay of Seine and laboratory experiments conducted sin ce 1987 on the polychaete Pectinaria koreni, the present study highlig hts the relative importance of hydrodynamical and biological factors w hich affect individuals within both the planktonic and benthic phases at different scales of space and time in a macrotidal area. Pectinaria koreni is one of the main macrofaunal component of the Abi-a alba mud dy fine sand community of the eastern Bay of Seine. Despite a highly a dvective and diffusive environment, a relative larval retention near a dult population was reported due to some local hydrodynamics features (e.g. tidal residual circulation, Seine river plume front) and the int eraction between the vertical current structure and the larval vertica l migration. Although larval retention could be disrupted by wind indu ced currents, multiple spawning events over the reproductive period in crease the likelihood that at least one larval cohort ensures a high r ecruitment during the life-span. Following a massive settlement whatev er the sediment grain size, the newly settled larvae exhibited a high immediate decrease of their densities as a result of postlarval mortal ity and migration. Postlarval drifting was induced by a combination of physical factors (i.e. tidal currents and swell) and postlarval behav iour in response to sediment texture and adult/settlers interactions. According to the hydrodynamics of the bay, this process may generate a postlarval transport from offshore bottoms to coastal suitable habita ts and counteract the demographic effects of larval dispersal. A conce ptual model of factors governing the recruitment and population mainte nance of Pectinaria koreni is proposed and discussed in comparison wit h results obtained on another polychaete, Owenia fusiformis, in the sa me area.