Seasonal Variations of Metallothionein Concentrations in the Asiatic Clam ( Corbicula fluminea)

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Natural variations of metallothionein (MT) concentrations in the Asiatic clam Corbicula fluminea were analyzed over a 1-year period in specimens collected from an unpolluted site (Cazaux-Sanguinet lake, southwest France). Sampling was carried out from November 1994 to December 1995, one to three times per month, according to the season. At each sampling time, lake temperature was measured and concentrations of MTs, Cd, Hg, Zn and Cu were determined in the whole soft body and in four organs or tissue samples. A histological study was conducted simultaneously to follow the development of the gonads in relation to the reproductive cycle of this bivalve. Results showed very high fluctuations in MT concentrations over the whole year, with a maximum value measured in the middle of May and ratios of around 4 at the whole organism level between extreme MT values. The tissue compartment presenting the most important variations was the visceral mass, which contains the gonads. Metal accumulation in the organisms did not seem to be involved among the factors likely to account for these variations in MT concentrations; metal concentrations remained at low and relatively constant levels throughout the whole year. The histological study revealed one spawning period from late May to the middle of June, appearing just after the MT “peak,” with maximum incubation of the embryos in the gills in late June. Because MT biosynthesis can be induced by hormonal secretions implicated during reproductive phenomena, the variations in MT concentrations appeared to be directly related to the biological cycle of this freshwater mollusc rather than to the direct or indirect effects of metal bioaccumulation.