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seasonal variation, and temperature, modulate immune system functions in natural popu-
lations (Duchemin et al. 2007; Gagné et al. 2008a, 2009; Pichaud et al. 2009; Greco et al. 2011).
Immunotoxicological assessment in ecotoxicology may therefore require an integrated
approach based on a comprehensive battery of biomarkers. Bivalve studies point out the
need to base this approach on metabolic pathways, allowing the monitoring of molecu-
lar events to be correlated with cellular responses in order to increase the robustness of
the assessment. Genomic tools may therefore represent the ideal tools to fulfill this need
(Douville et al. 2010).
6.3 Case of Marine Mammals
Marine mammals are valuable indicators of the health of the aquatic ecosystem, because
of their “top predator” position in the trophic food chain, their long life span, and their
long biological half-time of pollutant elimination. Although it is difficult to directly assess
the risks associated with the exposure of wildlife to complex environmental mixtures of
chemicals, marine mammals can served as useful indicators of ecosystem contamination
because they can integrate contaminant signals from aquatic food chains and generate a
“state of the environment” report.
In the aquatic environment, persistent toxic substances (PTSs) are accumulated by inver-
tebrates and fish initially. Food ingestion (coupled with metabolism and excretion) is the
dominant pathway for bioaccumulation of chemicals in marine mammals. Transfer within
the food web can lead to biomagnification (the successive increase in concentration of a
chemical with increasing trophic levels). Marine mammals, as homeotherms, maintain
their body temperature and therefore have greater energy requirements for the same body
weight than poikilotherms (e.g., fish) or terrestrial homeotherms. Thus, their metabolic
rate and caloric requirements are higher. As a result, there is a compensatory increase in
consumption of prey by marine mammals, and the largest biomagnification factors for
PTSs are usually observed between fish and marine mammals (Muir et al
.
1999).
6.3.1 Exposure of Marine Mammals to PTSs
Marine mammals accumulate high levels of trace metals (Das et al. 2003) and have exten-
sive fat stores that can serve as depots to accumulate high levels of lipophilic compounds
such as PAHs, OCs including DDT (dichloro-diphenyl-trichloroethane) and PCBs, and
organobromines (OBs) such as PBDEs and other flame retardants (Tanabe et al. 1994; Rayne
et al. 2004; Hall and Thomas 2007; Kajiwara et al. 2008a; Frouin et al. 2011). Proximity
to contaminant sources represents the most evident factor influencing exposure to PTSs,
near-shore feeders inhabiting industrialized coastal areas having higher contaminant lev-
els than pelagic feeders in deeper waters (Meng et al. 2009). However, levels of pollutants
in marine mammals also depend on other biological, physiological, spatial, and tempo-
ral confounding factors. The position in the food chain, the food web structure, and the
nature of the prey also directly influence the levels of exposure to PTSs in marine mam-
mals (Hansen et al. 1990; Ross et al. 2000; Rayne et al. 2004).
The pattern of distribution of contaminants within the organism is tissue and pollut-
ant specific. For example, trace metals are mostly concentrated in the liver and kidney of
marine mammals (Das
et al. 2003). On the contrary, the highest lipophilic contaminant
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