Do Contaminants Influence the Spatial Distribution of Aquatic Species? How New Perspectives on Ecotoxicological Assays Might Answer This Question

In ecotoxicology, although the search for the most sensitive species to be used in assays has almost been abandoned (as the concept of “the most sensitive species” has no physiological foundation), the development of methods seeking more sensitive and ecologically relevant endpoints is a continuous challenge. The ecological relevance of the approach used is of concern to prevent providing information that is unrealistic. The majority of the ecotoxicity assays present a particular characteristic (forced exposure) regarding how the organisms are exposed to contaminants. Forced exposure supposes a mandatory exposure to the contamination on the part of the organisms, and no possibility of escape is provided. Because of this, the information that such assays provide is based on how toxic a contaminant is for the organisms. This toxicity is measured by different endpoints that go from subcellular (e.g., biochemical disruption, DNA damage) to population (e.g., reproduction rate, sex ratio) levels. The relevance of this approach is unquestionable and has allowed the advance of ecotoxicology in the last decades as a tool for environmental risk assessment, not only identifying the contamination‐related threat but also protecting the wildlife (Calow and Forbes 2003; Rudén et al. 2016). However, another important question about the effects of contaminants is how repellent are they for organisms? This repellence can be measured in the traditional forced exposure, in that any behavioral changes may provide us with an idea of how repulsive the contaminant may be. But by using forced exposure, a key question remains unanswered: Could the presence of contaminants influence the spatial distribution of aquatic organisms? The reduction in abundance or even the absence of some species in a contaminated area is commonly attributed to the toxicity of contaminants; nevertheless, if a contaminant is repellent, it could trigger an avoidance response in the organisms that then move to a more attractive area. A sine qua non condition to measure any possible real spatial displacement is that organisms are able to choose different environments. This approach requires a methodological change because the exposure to contaminants should not be forced. This change in the exposure method brings a new conceptual aspect to the effects of contamination: if organisms avoid the exposure, the consequences affect not only individuals themselves but their spatial distribution instead (Araújo et al. 2016). A description of nonforced exposure approaches has existed for many years (see review in Araújo et al. 2016), but here we focus on the 2 multicompartmented exposure systems (Figure 1): linear system (Lopes et al. 2004) and heterogeneous multihabitat assay system (Araújo et al. 2018). In both multicompartmented systems, different concentrations of contaminants can be provided simultaneously and organisms are given interconnected compartments that contain different levels of contamination. The goal of this multicompartmentalization is to simulate the environmental heterogeneity of aquatic ecosystems. This makes it possible to examine the ability of the organisms to detect and select their habitable area depending on its attractiveness/repellence (in a cost–benefit approach). The complementary steps proposed here are intended not to raise doubts concerning the relevance of the traditional forced exposure approach but to suggest other key ecological concepts possible to integrate in ecotoxicology when a multicompartmented system is used. If used complementarily, it serves to integrate the organisms’ choice with the potential toxicity of the contaminants, including some concepts (Figure 1) such as “avoidance” and “preference” (both responses assess the repulsive and attractive potential of contaminants in a gradient or patchy contamination scenario), “habitat selection” (this concept could be applied to assess the