The difference between available 2-APB reserves and energy consumption has been proposed to be indicative of an organism\'s overall condition (De Coen and Janssen, 2003), and a decrease in the available energy reserves can be used as a biomarker of metal stress (Scott-Fordsmand and Weeks, 2000). In this context, the energy status of mussels was studied to gain insight into the costs of living in an environment with metal pollution. We noted a negative significant correlation between the Cd concentration in the tissue and the body composition reserve in P. perna ( Table 7). Moreover, the lowest energy reserve for the same species at sites PC, CP and CS corresponds with highest tissue Cd load. The elevated level of internal Cd causes harmful biochemical shifts such as diseases, emphysemas, hypertension, chronic and acute effects on the metabolism, and usually disruption at all functional levels ( Fergusson, 1990). Moreover, metal resistance often involves an increased production of ligands, proteins, and enzymes involved in detoxification, which are energy demanding biological process. Therefore, a decline in an energy reserves or augmentations in the metabolic rates have been associated with metal toxicity in animals ( Moolman et al., 2007). Furthermore, corresponding to the finding of metal accumulation in mussels, energy reserve was also dependent (r = 0.3860, P < 0.05) on the salinity ( Fig. 5A). Low energy content was noted in mussels collected from fresh water site and exactly at the same sites a reverse pattern (i.e. elevation) was revealed for body burden for many metals (Cd, Cu, Fe and Mn); suggesting that the salinity gradient not only influences the metal uptake, but to certain extent is also associated with the dynamics of the energy budget and the osmoregulatory costs at low salinity.