Growth of the Japanese carpet shell : Validation of increment deposition periodicity by chemical marking

Arcachon Bay is the first French field of Manila clam (Ruditapes philippinarum) with an annual production comprised between 500 and 1000 tons in the recent years. In order to improve stock management,an in situ experimentation of chemical marking, using calcein, has been undertaken to validate the periodicity of increment deposition and the optimal conditions of chemical marking in Arcachon Bay. Later on, daily growth rates will be analysed regarding to environmental parameters to increase our knowledge on this species.Manila clam were collected and divided into groups of 80 animals (including control group) which size distribution spread over a large range. Identified groups were marked by immersion in a calcein solution (CAS 1461-15-0), using different concentrations (50, 100 and 200 mg.L-1) and exposure times (30 and 60 minutes), then replaced in situ in different compartments. 38 days later, they were removed and sacrified. One valve of each shell was embedded in epoxy resin, sectioned, polished and observed under fluorescence microscopy. Growth increments were analysed using TNPC software (Numerical Treatment of Calcified Pieces, www.tnpc.fr). All marking conditions realized have been successfull, what strengthens others anterior studies (Fujikura et al., 2003 ; Poulain et al., 2011) for exposure time of 3 hours and concentration of 150 mg.L-1 at least. Time of experience has been lasted for 38 days, comprising of 73 tides. The mean striae number observed between the fluorescent band and the ventral margin is about 68. Consequently, microgrowth increments deposition has a tidal rhythmicity.