Promoting zebrafish embryo tool to identify the effects of chemicals in the context of Water Framework Directive monitoring and assessment

The Water Framework Directive (WFD) addresses the European Union Member States to achieve a good status of all water bodies. The WFD measures have allowed to reduce and eliminate the discharges, releases, and emissions of several priority substances with the aim of limiting the risks for the ecosystems and public health. Additionally, a Watch List (WL) for the monitoring of emerging contaminants was adopted to assess the environmental risk of new chemicals potentially toxic for water bodies. However, the amount of toxicants widespread in the environment is incredibly high and only a tiny fraction of substances is regularly monitored as established by the European legislation. Furthermore, the WFD does not involve a monitoring plan for chemical mixtures. Chemical analysis of water samples is essential for the monitoring programmes. However, it does not give full answers about the adverse effects of contaminants present in water bodies. In this context, the effect-based methods (i.e. bioassays and biomarkers) are essential tools to implement the monitoring strategies and reach the ambitious goals included in the WFD. Zebrafish early stages, i.e. embryos and early larvae, represent a very successful vertebrate model to assess the toxic effects on aquatic organisms and to subsequently perform a valid ecosystem monitoring. Indeed, this animal model raises many advantages and allows the definition of many toxicant modes of action (MoA). In this review, we report a large number of literature studies that performed experimental analysis using zebrafish embryos and early larvae to investigate the effects of the compounds included into the WFD and the related WL. We show how the zebrafish embryo model is able to detect and identify different toxicity mechanisms and specific effects with a great level of accuracy. Our goal is to promote the use of this effect-based method in the water monitoring strategies and to improve its use for regulatory purposes.