Pigmentation and colour changing mechanism in fishes.

Skin pigmentation of fish is one of the key factors influencing the market value of the ornamental fish industry. The process of fish pigmentation is regulated by genetic, morphological, physiological, and environmental factors. Several biochemical pathways and a cascade of enzymatic reactions are responsible for the production and dispersion of pigments within chromatophores. Melanin, pteridine, carotenoids, and purine-based pigments are the principal pigment classes responsible for fish colouration. Several genes such as mc1r, sox10, and pax3 have been identified as crucial regulators of pigment biosynthesis, migration, and dispersion within the fish chromatophores. The colour expression in fishes is modulated by various environmental factors, including temperature, light intensity, and photoperiod, which are mediated through hormonal pathways involving alpha melanocyte-stimulating hormone (α-MSH), melanin concentrating hormone (MCH), melatonin, and their receptors. Additionally, nutritional contents acquired from food, such as carotenoids, also play a key role in the supply of precursors for pigment synthesis in fishes. The present review, therefore, focuses on pigment biosynthesis, different mechanisms of colour change, and factors affecting colouration in fishes.