The melanocortin system regulates body pigmentation and social behaviour in a colour polymorphic cichlid fish

The melanocortin system is a neuroendocrine system that regulates a range of physiological and behavioural processes. We examined the extent to which the melanocortin system simultaneously regulates colour and behaviour in the cichlid fish Astatotilapia burtoni. We found that yellowmales are more aggressive than blue males, in line with previous studies. We then found that exogenous alpha-melanocyte-stimulating hormone (alpha-MSH) increases yellowness of the body and dispersal of xanthophore pigments in both morphs. However, alpha-MSH had amorph-specific effect on aggression, with only blue males showing an increase in the rate of aggression. Exogenous agouti signalling peptide (ASIP), a melanocortin antagonist, did not affect coloration but reduced the rate of aggression in both colour morphs. Blue males had higher cortisol levels than yellow males. Neural gene expression of melanocortin receptors (mcr) and ligands was not differentially regulated between colour morphs. In the skin, however, mc1r and pro-opiomelanocortin (pomc) b were upregulated in blue males, while asip 1 was upregulated in yellow males. The effects of alpha-MSH on behaviour and body coloration, combinedwithmorph-specific regulation of the stress response and the melanocortin system, suggest that the melanocortin systemcontributes to the polymorphism in behaviour and coloration in A. burtoni.