Biomarkers of Defense, Tolerance, and Ecological Consequences - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 60

number of species is reduced, reflecting the number of species able to adapt to low and
variable salinity levels and thus survive (McLusky and Elliott 2004). Thus, in estuaries that
are among the most polluted areas worldwide, the extinction of a small number of species
would be sufficient to hamper ecosystem functionality.

3.4.2 Cost of Tolerance

Living organisms have many defense mechanisms against toxicants present in their envi-
ronment. The ensuing metabolic cost and physiological stress that can be observed in indi-
viduals can have subsequent impacts on populations (Mouneyrac et al. in Amiard-Triquet
et al. 2011). This hypothesis of physiological cost also has implications for the evolution of
resistance to chemical stress, whether it is a fixed or an inducible response (Calow 1991).
Since 1991, this reference has been quoted in scores of articles to support many observa-
tions showing an increase in the metabolic rate of organisms exposed to various stress
factors (e.g., Canli 2005; Smolders et al. 2005; Guan and Wang 2006; Muyssen et al. 2006;
Lannig et al. 2006; Alonzo et al. 2006; Wiegand et al. 2007) inducing, for example, the synthe-
sis of MTs, HSPs, biotransformation enzymes, and antioxidant mechanisms. Rowe (1998)
emphasizes that an increase in metabolic rate is similarly observed in species belonging
to taxa widely separated phylogenetically (crustaceans, amphibians, reptiles) in response
to combustion waste rich in metals, suggesting a general response to metals. Literature
data (see subsection 3.2) show that this phenomenon affects other taxa and other types of
chemical contaminants. However, when physiological disturbances (in oxygen consump-
tion, energy reserves, condition index, growth, reproduction, etc.) do occur in organisms
exposed to chemical stress, it is not easy to distinguish precisely the contribution of the
cost of tolerance from the direct costs of the toxic effects of the contaminant.
Interestingly, research on the freshwater fish Heterandria formosa highlights various
aspects of the cost of tolerance (Xie and Klerks 2004 and literature cited therein). The
authors have conducted selective breeding experiments over eight generations, exposing
specimens from a field population to high doses of cadmium (Table 3.3). Third- and fourth-
generation offspring (F3 and F4) from cadmium-adapted lines were born smaller than
control specimens, and size at birth was positively correlated to survival in this species.

TABLE 3.3
Consequences of Selection of Cadmium-Resistant Freshwater Fish Heterandria formosa over Eight
Generations

F2 F3 F4 F5 F6 F7 F8

Resistance to Cd ⇗ ⇗ ⇗ ⇗ ⇗ ⇗ ⇗

Cross-resistance to Cu ⇗ ⇗ ⇗

Heat resistance at 38°C ⇘ ⇘ ⇘

Size at birth ⇘ ⇘

Lifetime fecundity ⇘ (−18%)

Mean brood size ⇘ (−13%)

Female life span ⇘ (−7%)

Time to first brood ⇗ (+6%)

MT induction ⇗

Cd uptake ⇘ ⇘

Source: Xie, L., and Klerks, P.L., Environ. Toxicol. Chem. , 23, 1499-1503, 2004 (and quoted literature therein). With
permission.
Note: Empty cells correspond to investigations that were not carried out in all generations.