Gender bent but not in the mind of fish

There is strong evidence that environmental exposure to endocrine disrupting chemicals (EDCs) is resulting in significant alterations to the reproductive system of many wildlife populations. Most of these studies measure chemically induced changes to the endocrine system or reproductive morphology and mostly provide simple markers of exposure. The heightened concern over the effects of EDCs is however primarily driven by the hypothesis that this disruption may have serious deleterious consequences on reproductive success. In extensive laboratory studies on breeding populations of zebrafish, I have shown that multigenerational exposure to environmentally relevant levels of endocrine disruptors cause very significant reductions in reproductive success. Lifetime exposure to 5 ng/l of ethynylestradiol, for example, caused complete reproductive failure. These reproductive failures were not caused by exposure proximate to the timing of spawning but by the disruption of development during earlier embryonic and larval sexual differentiation. Histology revealed that male gonads had not differentiated into functional testes. Significantly, these sterile males still initiated spawning in females and resulted in unfertilized eggs. This differential in the sensitivity of behaviour compared to gonadal disruption raises important issues in understanding the implications of endocrine disruption in wild populations. Moreover, the particular mode of reproduction behaviour that is used by a species fundamentally affects the population level impact of endocrine disruptors. This paper will discuss these results and how a greater understanding of the dynamics of group spawning may help in assessing the potential impact of endocrine disruption