Intersex in fishes and amphibians: population implications, prevalence, mechanisms and molecular biomarkers

Intersex is defined as the abnormal presence of both testicular and ovarian cells in gonads of gonochoristic animals. Its occurrence is widespread and reports on its presence in the gonads of vertebrates continues to increase. In this review, we use standardized terminology to summarize the current knowledge of intersex in gonochoristic fishes and amphibians. We describe the different indices that have been used to assess the severity of intersex and synthesize reports discussing the prevalence of intersex in relation to different types of pollutants. In addition, we evaluate the geographic distribution and chronology of the reported cases of intersex in fishes and amphibians, their pathological descriptions and severity and discuss species sensitivities. We also summarize molecular biomarkers that have been tested for early detection of intersex in wild populations and highlight additional biomarkers that target molecular pathways involved in gonadal development that require further investigation for use in the diagnosis of intersex. Finally, we discuss the needs for future research in this field. Copyright © 2015 John Wiley & Sons, Ltd. Gonadal intersex is a widespread condition among gonochoristic fishes and amphibians. In this review, we use standardized terminology to describe the different forms of intersex reported in earlier studies and identify the indices used to assess their severities, and summarize reports regarding their frequency of occurrence. We also evaluate the global geographic distribution and chronology of the reported cases and highlight molecular biomarkers that have been used or should be investigated for diagnosing intersex in fishes and amphibians.