Stress and sex: does cortisol mediate sex change in fish?

Cortisol is the main glucocorticoid (GC) in fish and the hormone most directly associated with stress. Recent research suggests that this hormone may act as a key factor linking social environmental stimuli and the onset of sex change by initiating a shift in steroidogenesis from estrogens to androgens. For many teleost fish, sex change occurs as a usual part of the life cycle. Changing sex is known to enhance the lifetime reproductive success of these fish and the modifications involved (behavioral, gonadal and morphological) are well studied. However, the exact mechanism behind the transduction of the environmental signals into the molecular cascade that underlies this singular process remains largely unknown. We here synthesize current knowledge regarding the role of cortisol in teleost sex change with a focus on two well-described transformations: temperature-induced masculinization and socially regulated sex change. Three non-mutually exclusive pathways are considered when describing the potential role of cortisol in mediating teleost sex change: cross-talk between GC and androgen pathways, inhibition of aromatase expression and upregulation of amh (the gene encoding anti-Müllerian hormone). We anticipate that understanding the role of cortisol in the initial stages of sex change will further improve our understanding of sex determination and differentiation across vertebrates, and may lead to new tools to control fish sex ratios in aquaculture.