A lesson from fish: don’t let stress ruin procreation

Posted on March 26, 2022

Annual fishes live in temporary ponds that dry out completely in summer.  As a result they have the shortest life spans of all vertebrates: after a few months they perish and the next generation depends on the survival of their drought-proof eggs, which hatch when the pond is eventually flooded.  As the shallow ponds dry out, environmental factors such as temperature, pH, oxygen, turbidity and salinity can reach extreme, survival-threatening levels, and these changes increase the concentration of the stress hormone cortisol in a fish’s body.  In most vertebrates high cortisol levels trigger the reallocation of resources away from delayable activities like reproduction in order to deal with immediate environmental challenges.  However, because annual fish are severely time-limited, they have to reproduce and cope with environmental stress simultaneously.  To find out how they respond to this predicament, a group of researchers surveyed populations of the rivulid fish Austrolebias reicherti in temporary ponds in Uruguay at different stages after hatching, and measured cortisol levels in the blood of sampled individuals.  As the season progressed, conditions in the ponds deteriorated and blood cortisol concentrations increased significantly: late season levels were twice those of earlier levels.  Reproductive effort increased the weight of female ovaries and decreased the weight of livers (and therefore energy stores) in males.  The researchers also kept laboratory-bred fish in dilute concentrations of hydrocortisone to examine the behavioural and physiological effects of cortisol, and found that after a ten-day treatment cortisol levels in male fish had doubled.  The same fish also spent more time on courtship displays and became more brightly coloured.  Because cortisol promoted reproduction at the expense of maintenance, it seems that annual fish are one of the few types of vertebrates where the stress response doesn’t inhibit breeding. 

Reference:  Passos, C. et al. 2021.  Stress promotes reproduction in the annual fish Austrolebias reicherti.  Animal Behaviour 174, 105-114.   https://doi.org/10.1016/j.anbehav.2021.02.003