ABSTRACT
Tolerance may be defined as the ability of organisms to cope with stress to which they are exposed in their environment (Amiard-Triquet et al. 2009). The stressor may be a natural factor, such as a large change in salinity or temperature, or anthropogenic disturbance, including the raised bioavailability of a toxic metal. There are two methods by which organisms can become resistant to a toxic contaminant (Klerks and Weis 1987; Amiard-Triquet et al. 2009): (1) Resistance may be gained by physiological acclimation during exposure of the individual organism to a sublethal bioavailability of the pollutant at some period; this resistance is not transferable to future generations. (2) Tolerance may also be acquired as a consequence of genetic adaptation in populations exposed over generations to the toxic contaminant, through the action of natural selection on genetically based individual variation in resistance; this tolerance is transferable to future generations, as usually confirmed by laboratory breeding of at least the F1 generation (e.g., Grant et al. 1989; Levinton et al. 2003). Klerks and Weis (1987) referred to the former as acclimation and the latter as adaptation. This adaptation may be lost in the absence of continuing exposure to the contaminant, again by natural selection, if, as appears to be usual, the genetically based tolerance has a metabolic cost bringing a selective disadvantage in the absence of a threshold bioavailability of the contaminant (e.g., Grant et al. 1989). The presence of such a tolerant population is direct evidence that the bioavailability of the toxic contaminant in the local environment is sufficient to be ecotoxicologically relevant (Luoma 1977).
