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How do carotenoids affect fish coloration?

Carotenoids are a group of natural pigments comprising about 1,100 identified compounds with a wide variety of structures. They can be commonly found in both plant and animal worlds. They are fat-soluble, so in the body they are found embedded in cell membranes, dissolved in fat droplets or bound to proteins in the system used for photosynthesis. Only plants and some species of fungi and bacteria are capable of producing carotenoids. Fish must take them in with their food. The most well-known effect of carotenoids is color-enhancement of fish. However, these compounds are also involved in many physiological processes, improving health and well-being of fish.

Color-enhancement in fish fed with carotenoids

In fish’s skin you can find three types of pygment cells: melanophores, chromatophores and guanophores. The melanophores contain the black pigment: melanin. Chromatophores, on the other hand, contain red and yellow pigments from the carotenoid group. Pigments present in melanophores and chromatophores reflect or absorb light, depending on its length. We can only see the light that has been reflected. Guanophores, the third type of pigment cells do not contain pigments but colorless guanine, crystal substance which, depending on its structure, reflects or disperses light like a prism. Thanks to guanine white, grey and silver colors are visible on fish’s skin. As a result of interaction between these three types of pigment cells a number of other colors and shades is formed, including green and blue. So in reality blue and green coloration in fish is in most cases just an optical illusion, the light-and-shadow effect and the effect of the presence of black, yellow and red pigments. By using color-enhancing foods rich in corotenoids you can influence the coloration of fish.

Antioxidant properties of carotenoids

Carotenoids are antioxidants, meaning that, along with vitamins C and E, they protect fatty acids and cell membranes from free radicals. At the same time, the effect of astaxanthin was found to be more effective compared to other carotenoids and vitamins C and E (astaxanthin is 500 times more potent antioxidant than vitamin E).

Carotenoids as a source of vitamin A

Some carotenoids, including beta-carotene, astaxanthin, zeaxanthin and canthaxanthin, are converted in the fish body into vitamin A, which is responsible for, among other things, embryonic cell differentiation, reproduction, vision or healthy skin.

Effect of carotenoids on fish immunity

Carotenoids increase fish’s resistance to viral, bacterial and fungal infections (beta-carotene increases the secretion of lysozyme, a natural enzyme produced by leukocytes that has bactericidal properties). In addition, carotenoids increase the intensity of phagocytosis, which is one of the body’s defense mechanisms against pathogens.

Effect of carotenoids on fish reproduction

Carotenoids have been found to have beneficial effects on all stages of reproduction (maturation, germ cell production, fertilization, embryo development, larval and fry growth).

Carotenoid assimilation by fish

Differences in the chemical structure of carotenoids as well as mutual interactions between them and with other compounds in the food affect the differences in their assimilation by fish and color-enhancement at the same time. It also depends on specific species and individual living conditions. Of the carotenoids commonly used in fish nutrition, astaxanthin is the best absorbed, followed by canthaxanthin and beta-carotene. The most popular caroteoid in ready-made foods for aquarium fish is astaxanthin. High concentration of astaxanthin can be found in D-50 Plus, Astacolor, Vitality & Color Granules, for example.

There are also differences in the bioavailability of a given carotenoid due to the presence of its many chemical variants – isomers. Their assimilability is also determined by the location of the carotenoids in the cell. As I mentioned earlier, they can be embedded in cell membranes, attached to proteins of the photosynthetic system or dissolved in fat droplets (fruits). This explains why beta-carotene from the fruit is better absorbed than that embedded in the leaves. Also, the processing of raw materials containing carotenoids significantly affects their bioavailability. You can compare what the availability of beta-carotene looks like depending on the processing of carrots:

raw carrot < 1%

raw carrots heavily shredded – about 25%

cooked carrots lightly shredded – about 5%

cooked carrots heavily shredded – about 57%

That explains why fish fed with color-enhancing foods change their coloration at various rates and intensities. As a result, it can be observed that the coloration of fish within a flock can vary.

dr inż. (Ph.D.Eng) Aleksandra Kwaśniak-Płacheta

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