Methylmercury bioconcentration in muscle tissue of the European eel (Anguilla anguilla) from the Adour estuary (Bay of Biscay, France).

The life history of the European Eel (Anguilla anguilla) begins in the Sargasso Sea in the Atlantic Ocean where the Leptocephalus larvae drift with the gulf stream in order to reach European coastal waters. After their metamorphosis into transparent juveniles "glass" eels and an acclimatising phase in the estuaries, they migrate upstream into the rivers to become yellow eels (sub-adult stage). The yellow eels spend between 2 and 20 years of their lifetime in freshwater until they change into silver eels (adult stage) and finally migrate back to the Atlantic Ocean for spawning (Gomez-Mourelo, 2005). A. anguilla is thus an organism able to tolerate a wide range of environmental conditions, such as variations in oxygen availability, different ranges of salinities and exposure to many anthropogenic compounds. In addition, it is a migratory, benthic and benthivorous species at the top of the food chain and is characterised by a high fat content (>30%). For all these reasons A. anguilla is prone to bioaccumulate a wide range of contaminants and it has been widely employed throughout the last years as a bioindicator of the pollution caused by metals (Batty et al., 1996; Has-Schön et al., 2006) and organic contaminants (Storelli et al., 2007; Yamaguchi et al., 2003). The environmental and toxicological impact of Hg bioaccumulation in fish is related to the methylation of inorganic mercury to form the more toxic methylmercury (MeHg) species. In this way, fish tend to concentrate in their tissues MeHg by a factor of 105-107, leading to dangerous levels even in areas with tolerable Hg concentrations (Mason et al. 1996). It has been reported that about 98% of the Hg present in aquatic systems is immobilised in the sediments (Stein et al., 1996) and that most of the MeHg is produced at the sediment water interface as a result of biotic or abiotic transformations caused by specific redox gradients and bacterial activity (Gilmour and Henry, 1998.). According to this, A. anguilla may be an effective biomagnificator and bioaccumulator of Hg due to its longevity during the continental development phase in freshwaters (where it forages and lives upwards 15 years) and its position at the top of the food chain as a carnivorous species feeding on the benthic fauna (Mancini et al., 2005).