The aim of this study was to link the responses at different levels of biological organisation of the endobenthic bivalve Scrobicularia plana differentially exposed to anthropogenic pressure. Clams were collected in April 2008 from three estuaries along a pollution gradient (Goyen < Loire < Seine). Biomarkers of defence (metallothionein concentration and glutathione-S-transferase activity) were activated in the Loire and the Seine. Biomarkers of damage revealed neurotoxicity (decreased AChE activity) and impairment of digestive enzyme activities (cellulase or amylase) in these estuaries. The highest lactate dehydrogenase activity was registered in the Loire estuary, in parallel with enhanced levels of vanadium (a metal present in petroleum), likely as a consequence of a small oil spill that occurred one month before the sampling collection. Physiological biomarkers (energy reserves as glycogen, lipids and proteins, condition and gonado-somatic indices) showed a few intersite differences. However, the median size was significantly lower in clams exposed to direct (chemicals) or indirect (available food) effects in the most contaminated site. Burrowing behaviour was disturbed in clams from both of the Loire and Seine estuaries, a response probably due to physiological impairment rather than to avoidance of contaminated sediment. The activation of defence mechanisms towards metals (metallothionein) and other classes of contaminants (the biotransformation enzyme glutathione-S-transferase) do not ensure a total protection since a number of impairments were observed at the infra-organismal (AChE and digestive enzyme activities) and individual (burrowing behaviour) levels in relation to the degree of anthropogenic pressure. However, even in the most contaminated estuary (Seine), historical records do not show a consistent decrease of S. plana populations.
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