How fish use colour

Since Darwin’s time we have known that, throughout the animal kingdom, the flashiest males have the best chances of finding a mate.  But it was only recently that researchers discovered why this is so.  Males that can afford to use carotenoids as ornaments are healthier and stronger.  One reason why females are not usually highly colored is that they need to pass on their carotenoids to their offspring.  Carotenoids have been shown to improve young fish survival and growth.

Researchers are now pretty sure that carotenoids play a major role in evolution.  Carotenoids stimulate the immune system as well as detoxification processes.  Animals that are more susceptible to disease and parasites have to use their carotenoids to defend themselves.  Genetically resistant males use fewer carotenoids for fighting disease and so can make a better show.  Healthier females have more carotenoids for their young.  Embryos with adequate stores of carotenoids have a better chance of survival. 

It is clear that carotenoid pigmentation of salmon is an important natural indicator of biological superiority.  The bright pink color of salmon flesh and the bright red of salmon eggs are therefore reliable signs of good health in farmed salmon.

Where fish get their colour

There are four main groups of pigments that can be used to provide colour in these cells: melanins, carotenoids, pteridines, and purines.  Melanins are responsible for the dark colouration seen in fishes.  Carotenoids, which are lipid-soluble, dominate in giving the yellow to red colours.  Pteridines are water-soluble compounds and result in bright colouration like the carotenoids.  Pteridines play a small role in colouration when compared to carotenoids.  In the purine compounds, guanine predominates, and large amounts of guanine can be found in the silvery belly skin of most species of fish. 

These basic compounds can be combined with other components, like proteins, to produce the blue, violet, and green colour ranges seen in fishes.  In the flesh (muscle), the carotenoids are the dominant pigment.  In lobsters and shrimps, astaxanthin is attached to a protein to produce the carotenoprotein, crustacyanin.  This carotenoprotein imparts a blue colour in living Crustacea; in the presence of heat the carotenoprotein molecule is cleaved, which subsequently results in the characteristic red colour of cooked lobsters and shrimps.  Carotenoids are the major pigmenting compounds and cannot typically be synthesised by fish.  In contrast, most other pigmenting compounds can be made by the fish.

Which carotenoids are in fish

Astaxanthin is the pigment that provides salmon flesh with its characteristic rich pink-red colour and the major carotenoid in the integument and internal organs of red sea bream.  Astaxanthin accounts for more than 90% of the total carotenoid content found in the flesh of wild salmonids (salmon and trout).  In the wild, fish absorb astaxanthin from the crustaceans they eat.  The absorbed carotenoid is then transported in the blood to the muscles and skin where it is deposited. 

In other fishes and to a limited extent in salmonids, additional carotenoid compounds are the source of other bright colours.  Research results indicate that tunaxanthin is a rather common pigment in marine fish.  This carotenoid is especially abundant in yellow-coloured fishes, like yellowtail (Seriola quinqueradiata).  Astaxanthin, by contrast, seems to be dominant in red marine fish.

Lutein is also widely found in many marine species.  Carotenoids commonly occurring in freshwater fish include beta-carotene, lutein, taraxanthin, astaxanthin, tunaxanthin, alpha-, beta-doradexanthins, and zeaxanthin.

• Lutein (greenish-yellow)
• Tunaxanthin (yellow)
• Beta-Carotene (orange)
• Doradexanthins (yellow)
• Zeaxanthin (yellow-orange)
• Canthaxanthin (orange-red)
• Astaxanthin (red)
• Eichinenone (red)

Carotenoids in Aquaculture

Carotenoids are used in aquaculture feeds to provide the colour associated with consumer products, such as the bright vibrant colours of ornamental fish.  The same carotenoid, astaxanthin, found in wild salmon is used in aquafeeds to impart this natural, pink-red colour to farmed salmon fillets.  Colour matters, particularly in regards to consumer preference for aquaculture products.

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Information provided by Roche S.A.