Presence of spionid worms and other epibionts in Pacific oysters (Crassostrea gigas) cultured in Normandy, France

The presence of mudworms and other epibionts were studied in three batches of Pacific oyster Crassostrea gigas from February 2000 to August 2003 at a high productive oyster rearing area in Normandy (France). Spatio-temporal variations of mudworm and epibiont presence were analysed and their impacts on oyster mortality, growth (shell and meat) and condition were assessed. The external shells of the oysters were mainly occupied by barnacles. The proportion of epifauna varied seasonally, showing a peak in autumn for the three oyster batches. The heaviest colonization occurred at the highest areas on the foreshore but there was no obvious relationship between distribution and bathymetry. No significant correlation was detected between epifauna biomass and both oyster growth and condition. Inner shell valves of half-grown (second year of life) and marketable (third year of life) oysters were heavily infested by spionid mudworms whose spatial distribution was significantly correlated with intertidal exposure, the oysters at the highest level being almost free of mudworm tubes and/or blisters. Although it remains unclear how spionid mudworms affect summer mortality of oysters, it was demonstrated that they had a significant negative effect on host growth. Abundant mudworm scars on the inner shell were associated with reduced meat and shell weights, thereby revealing a potential decrease in oyster productivity.