Pigmentation is one of the most diverse characters in vertebrates, with a broad variety of colour phenotypes and colour patterns found in different species, populations and individuals. 1 The diversity and fascinating skin colours and pigment patterns are observed in fish (Figure 1), which coloration widely distributed in the hypodermis of the body as well as the epidermis of scales and fins. 2,3 According to the FAO, global fish production in 2018 was projected to be around 179 million tonnes, with a total first-sale value estimated at USD 401 billion, of which 82 million tonnes from aquaculture and the rest from capture production, and approximately 88 per cent of total fish production was used for direct human consumption, with the remaining 12 per cent going to non-food purposes such as fish oil, fishmeal and ornamental fish. 4 Body colour is an interesting economic feature in aquatic animals, for example fish and shellfish, and the global exports of ornamental fish increased from USD 177.7 million in 2000 to a peak of USD 347.5 million in 2014. 5 The global market for ornamental fish is currently competitive, with a relatively high demand for aquarium fish around the world, and more than

Farming of red tilapia is increasing rapidly. However, its commercial farming development is challenged by lack of clear information on genetic basis for skin colour and pigmentation differences due to environmental changes. This study investigated the effects of photoperiod (light:dark, L:D) on the growth and skin colour variation of Malaysian red tilapia. A total of 180 fish weighing 150.48 ± 0.44 g were reared under natural photoperiod (13L:11D, control), prolonged lightness (24L:0D) and prolonged darkness (0L:24D) in three replicates for 78 days. The weight gain of fish cultured under both prolonged light and darkness were significantly higher than fish under natural photoperiod. The tyrosinase level in ventral skin was significantly higher for fish cultured under prolonged darkness condition than in the other two photoperiod regimes. Contrary, the cysteine level in the dorsal skin was significantly higher in the fish cultured under natural photoperiod than in prolonged light and darkness. The relative mRNA expressions of SRY‐related HMG‐Box 10 (sox 10), tyrosine (tyr), tyrosine‐related protein 1 (tyrp‐1) and solute carrier family 7 member 11 (slc7a11) genes were significantly higher in ventral skin of fish under prolonged darkness than the other two photoperiods. This study demonstrates that photoperiod has an impact on melanogenesis and growth of red tilapia. Understanding the effects of photoperiod on genetic basis of red tilapia will help in selective breeding programme of the important economic traits for the development of commercial red tilapia farming.

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