Pelagic dispersal of the brittle-star Ophiothrix fragilis larvae in a megatidal area (English Channel, France) examined using an advection/diffusion model

The brittle-star Ophiothrix fragilis (Abildgaard) is widely distributed in the English Channel and particularly on pebble areas which are characterised by strong tidal currents. The main populations are located between 20 and 80m depth. Although larval dispersal and settlement are apparently under an unfavourable high hydrodynamical constraint (a megatidal regime which generates high tidal currents, and a northeastward tidal residual current from the English Channel to the North Sea which advects larvae away from the adult populations), the density of adult populations showed little year-to-year variability. In order to determine the origin of ophioplutei larvae and recruitment patterns, larval dispersal from eight populations in the English Channel was simulated using a 2D hydrodynamical Lagrangian model integrating both the tidal residual, the wind-driven circulation and the eddy-diffusion. Simulations showed different transport patterns of the passive O. fragilis larvae under advection and eddy-diffusion control in the English Channel and highlighted a significant role of wind forcing on larval dispersal. Larval losses by mortality could exceed losses by hydrodynamic processes. For almost all populations, larval retention appeared to be sufficient to ensure local recruitment regardless of meteorological conditions or larval life span, and to explain the inter-annual stability of adult stocks. The degree of connection among populations was highly variable. While some populations were mainly self-seeding, larval supply from neighbouring populations could exceed local retention for other populations depending on wind forcing.