On the proper rearing density in domesticated zebrafish to avoid unwanted masculinization. Links with the stress response

Abstract

Zebrafish (Danio rerio) has become a well-established experimental model in many research fields but the loss of the primary sex determining region during the process of domestication renders laboratory strains of zebrafish susceptible to the effects of environmental factors on sex ratios. Further, an essential husbandry aspect such as what is the optimal rearing density to avoid stress-induced masculinization is not known. We carried out two experiments: one focusing on the density effects on survival, growth and sex ratio by rearing zebrafish at different initial densities (9, 19, 37 and 74 fish per liter) for three months (6–90 days post fertilization, dpf), and a second experiment focusing on the effects of cortisol during the sex differentiation period (15–45 dpf) on zebrafish reared at low density. Results showed an increase in the number of males in groups subjected to the two highest initial rearing densities. We also observed a reduction of survival and growth in a density-dependent manner. Furthermore, zebrafish treated with cortisol during the sex differentiation period showed a complete masculinization of the population. However, treatment with metyrapone negated the effects of exogenous cortisol. Our results indicate that the process of sex differentiation in domesticated zebrafish can be perturbed by elevated stocking density and that this effect is likely mediated by an increase of cortisol through the stress response. However, the underlying mechanism needs further study.