Sentinel Species - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 163

analysis of caged and wild mussels may improve the characterization of the studied area:
wild mussels express higher genetic damage (micronuclei frequency) as a result of cumu-
lative exposure, whereas, in contrast, caged mussels show higher DNA strand breaks as a
result of recent exposure.
Chronic exposure to contaminants can lead to the acquisition of a certain tolerance in
individuals (Amiard-Triquet et al. 2011). Natural selection tends to favor these individuals
to the detriment of the ones that are more sensitive to the pollutants, and leads to an increase
of the representation of resistance genes over successive generations (see Chapter 14). So,
the environmental history of the parents must also be taken into account, and not only that
of the animals intended for transplantation. This aspect can be illustrated by the work of
Damiens et al. (2006), who studied the effects of copper contamination on some biomark-
ers [concentrations of metallothioneins (MT) and thiobarbituric acid reactive substances,
catalase (CAT) activity] in oyster larvae (larvae D of Crassostrea gigas ) obtained from par-
ents that lived in areas differentially contaminated by this metal. They concluded that
larvae of oysters from a strongly copper-contaminated estuary (Bidassoa) are better pro-
tected against the toxicity of the metal than the larvae of oysters living in an environment
with low copper exposure (Bassin d'Arcachon). Collin et al. (2010) exposed to pesticides
(lindane, metolachlor, carbofuran), at environmentally relevant concentrations, two popu-
lations of oysters from sites differing in pollution levels, particularly pesticide exposure:
Arcouest (low pollution) and the Vilaine estuary (high pollution). They observed signifi-
cant differential gene and protein expression between the two populations, and concluded
that—as gene expression is often heritable within populations and supposing that the
Arcouest and Vilaine oyster populations have different genetic backgrounds—these dif-
ferences could be explained by a combination of environmental and genetic components.
Consequently, whenever it is possible, it is recommended, for bivalves in particular, to
use farmed (mariculture) animals for transplantation. For species for which such breeding
sources are unavailable, particular attention must be paid to the absence of pollution in the
site of origin of the transplants.
If exposure history in terms of exposure to pollutants influences future responses of the
organisms, it is also important to pay attention to history of exposure to other ecological
parameters that may act as confounding factors. Camus et al. (2004) showed, for example,
that according to their bathymetric position intertidally or subtidally, mussels are more or
less accustomed to periods of emersion and immersion, and then react differently to the
oxidizing stress generated by the metabolization of pollutants, or simply resulting from
collection and transport during the transplantation process. More generally, Honkoop et
al. (2003) have tested the impact on their physiology (scope for growth) of the stress of
handling animals (i.e., mussels), moving them out of their habitat to a new habitat, and
caging them. They have used combinations of treatment of mussels including reciprocal
transplants, but also false ones consisting of moving mussels but putting them back into
their place of origin or in zones of the same contaminant exposure. They concluded that
to allow for the confounding effects of moving and disturbance on the physiology of the
animals, correct control treatments should be included in the study.
7.1.3.2.3 Period of Latency
The use of transplanted sentinel organisms involves a period of latency between the
arrival of the individuals in the environment and the beginning of sampling. According to
Couillard et al. (1995a) or Geffard et al. (2002), several months are necessary for the organ-
isms (bivalves in both cases) to reach a state of equilibrium with their environment in terms
of their total accumulated metal concentration, and changes over time are also observed in