Effects of digenean trematodes and heterotrophic bacteria on mortality and burying capability of the common cockle Cerastoderma edule (L.)

Population dynamics of marine invertebrates is controlled by a variety of abiotic and biotic factors. Among these, some have received lesser attention from marine ecologists because of their ‘discrete’ nature. This is the case of parasitism and bacterial load. In the present study, we focused on the role that both digenean trematodes and heterotrophic aerobic bacteria might play in the mortality and burying behaviour of cockles, Cerastoderma edule.The bivalves were sampled monthly during 1 year from two sites in Arcachon Bay (French Atlantic coast). Mortality rates were assessed after transferring in the laboratory normally buried and unburied (i.e. found lying at the sediment surface at low tide) cockles. Their digenean and bacterial loads were determined for both positions (normally buried and unburied). Mortality rate was significantly higher for cockles found out of the sediment at low tide, suggesting that this abnormal position was a prelude to cockles' death. Comparison of digenean load of cockles showed no significant difference between buried and unburied bivalves. In contrast, bacterial load was significantly higher in unburied cockles than in normally buried animals. The effect of high concentration of a marine bacterial strain (Pseudomonas fluorescens) on cockles' burying behaviour and mortality was tested in the laboratory. Results showed that these bacteria could trigger the emergence of animals from the sediment but did not cause cockles' death. These field observations and laboratory experiments suggest that bacteria, rather than digenean trematodes, could play a role in the emergence of cockles and, hence, affect their survival in the wild.