Variation of metallothionein-like protein and metal concentrations during the reproductive cycle of the cockle Cerastoderma glaucum from an uncontaminated site: A 1-year study in the Gulf of Gabes area (Tunisia)

Natural variation of metallothionein-like protein (MTLP) concentrations in the cockle Cerastoderma glaucum were analysed over a 1-year period in specimens collected from a relatively uncontaminated site in the Gulf of Gabes (Tunisia). Sampling was carried out from June 2004 to June 2005. At each sampling time, water temperature and salinity were measured. Levels of zinc, cadmium and copper were measured in the whole soft tissues of the cockle C. glaucum. MT-like protein and subcellular metal partitioning were measured in the digestive gland. A histological study was conducted simultaneously to follow the development of the gonad in relation to the reproductive cycle of this bivalve. Results showed a significant fluctuation in MTLP concentrations throughout the whole year, with a maximum value measured in summer and in the late winter. The relationship between MTLP and metal concentrations, abiotic (temperature and salinity) and biotic (mass of target tissues) and also maturity index (MI) factors were examined. Metal levels in the tissues are likely to change during the season, reflecting variability in metabolism and/or a change in tissue weight. However, possible seasonal changes in trace metal concentration are unlikely to entirely explain the observed pattern in the MTLP levels. The physiological changes caused by gonad development and food abundance (change in tissue mass) contribute to a significant variability in MTLP levels in the cockle C. glaucum and can interfere with the estimation of level changes induced by anthropogenic sources.