Spatial and temporal distribution of mercury and methylmercury in bivalves from the French coastline.

Marine mercury (Hg) concentrations have been monitored in the French coastline for the last half a century using bivalves. The analyses presented in this study concerned 192 samples of bivalves (mussels: Mytilus edulis and Mytilus galloprovincialis and oysters: Crassostrea gigas and Isognomon alatus) from 77 sampling stations along the French coast and in the French Antilles sea. The goals of this study were to assess MeHg levels in various common bivalves from French coastline, and to identify possible geographic, taxonomic or temporal variations of concentrations. We show that the evolution of methylmercury (MeHg) concentrations covary with total mercury (HgT) concentrations. Moreover, in most of the study sites, HgT concentrations have not decreased since 1987, despite regulations to decrease or ban mercury used for anthropic activities.

Digenean trematodes-marine mollusc relationships: a stable isotope study.

The stable carbon and nitrogen isotopic composition of digenean trematode parasites and their marine mollusc hosts was investigated to describe the potential influence of parasites on their host and its different tissues, and to obtain further insight into their trophic relationships. Four parasite-host systems were studied: Labratrema minimus-Cerastoderma edule, Monorchis parvus-C. edule, Lepocreadiidae parasites-Nassarius reticulatus and Zoogonidae parasites-N. reticulatus. Among the 4 sampling occasions reported here and corresponding to the 4 parasite-host systems, isotopic shifts from pathologic (i.e. linked to disturbances in host metabolism) and mass-balance (i.e. linked to significant differences between host and parasite isotopic signatures) origins were observed only once. Both corresponded to delta 13C measurements of the L. minimus-C. edule system when the infestation load (percentage parasite dry weight compared to total flesh dry weight) was highest (9 to 25%, mean = 16%) over the sampling period. Overall, measurements indicate that digenean trematode parasitism induced low or no shifts in isotopic signatures of C. edule and N. reticulatus tissues. The 2 endoparasites L. minimus and M. parvus appeared to be slightly depleted in 13C compared to C. edule digestive gland and gonads, which were the most parasitized tissues. In contrast, no fractionation or low 15N trophic enrichments occurred in the parasites. These results highly contrast with the classical trophic enrichment reported in prey-predator systems but are in agreement with the scarce literature regarding other parasite-host systems. Our results indicate that (1) digenean trematodes mainly feed on digestive glands (the cockle tissue with which they are mainly associated) with a possible slight preference for lipids, and (2) fractionation due to parasite metabolism should be low due to abbreviated metabolic pathways and/or slight loss of materials through excretion, tegument diffusion and respiration.