FACTORS INVOLVED IN GROWTH PLASTICITY OF COCKLES CERASTODERMA-EDULE (L), IDENTIFIED BY FIELD SURVEY AND TRANSPLANT EXPERIMENTS

In Arcachon Bay, a macrotidal coastal lagoon in southwest France, grow th rates of cockles Cerastoderma edule (L.) differ greatly between loc ations, Sampling of populations at different tidal levels showed that the mean shell length was significantly and positively correlated with immersion time, whereas no correlation was found with population dens ity and microphytobenthos biomass. Transplants of cockles between two intertidal sites were used to examine the relative importance of habit at and site of origin for growth rate and condition index. Artifacts d ue to manipulation were assessed, i.e. the impact of enclosures on gro wth, condition index and mortality, During a 5-mo reciprocal transplan t experiment, growth rates of the transplanted cockles and the cockles already present were similar, whereas the sites of origin did not aff ect growth rate. Condition index, however, displayed significant diffe rences in relation to both transplant and origin sites. During a furth er 6-mo experiment in which cockles were transplanted from one site to four new sites, growth was mainly influenced by tidal level. These re sults indicate that net growth started when cockles were immersed 30% of the time, It is concluded that, on the scale of Arcachon Bay (156 k m(2)), differences in growth and condition between cockle populations are largely phenotypic. Tidal level (Le. food supply and emersion stre ss) accounts for most of these differences, but the existence of genet ically or physiologically different populations cannot be excluded.