Experimental assessment of the combined exposure to methylmercury and nutritional stress on multiple reproductive endpoints on female zebrafish (Danio rerio).

Abstract

Wildlife is exposed to contaminants in their environment and also to other stressors such as cold or low food availability. The consequences of exposure to some contaminants and stressors have been studied to some detail in laboratory conditions but mostly exposing wildlife to a single stressor, while the consequences of combined exposure to contaminants and other stressor remain mostly unexplored. We exposed 228 adult zebrafish to a sublethal dose of methylmercury (MeHg), a globally distributed contaminant, and food restriction, probably the most common environmental stressor, in four treatment groups: control, MeHg (5 ppm through diet), food reduction (25 % of control food mass) and the combination of MeHg and food reduction. Our hypothesis was that combined exposure will produce synergistic (interactive) effects different and more severe than the mere addition of the individual effects. We exposed fish for 43-day sublethal exposure, and allowed them three reproductive attempts, whose success we measured using seven reproductive endpoints. We assessed each endpoint with five competitive models: No effect; MeHg effect; food restriction effect; MeHg and food restriction additive effect; and MeHg and food restriction interactive effect. In six of the seven endpoints the evidence contained in the results supported the interactive effect, strongly supporting our hypothesis. In the remaining one, no effects was observed in any group. However, contrary to our expectation, the combined exposure treatment group produced more viable offspring than any other. We currently have no explanation for that result and argue that combined exposure might have triggered life history trade-offs. Further research on the longer term and assessing carry-over and inter-generational effects could improve our understanding of the consequences of combined exposure to MeHg and food restriction.