Coloration is controlled by the endocrine and nervous system, but dietary sources of pigment also play a role in determining color in fishes. The endocrine and nervous system both influence coloration in fish. The pituitary gland secretes hormones that direct the production and storage of pigments throughout the life of a fish, and particularly as maturity is reached. Pigment production and storage often increases at the onset of maturity. Many species use color to provide camouflage and attract a mate. Fish of the family Cichlidae are particularly known for brilliant coloration of mature males. The autonomic nervous system directs rapid color changes in response to stimuli such as a predator or an aggressive tankmate. Anyone who has observed fish knows this color change can occur at a spectacular rate.

Specialized pigment containing cells called chromatophores are located beneath the scales. These cells are branched, permitting pigment granules to be near or away from the surface and aggregated or dispersed. These cells are the reason for the variable and sometimes rapid changes in fish color. Additionally, colorless purine crystals are contained in specialized chromatophores called iridophores. These crystals are too large to move in the iridophores but are stacked to provide a reflecting surface and the base or structural coloration of fishes. The iridophores are responsible for the silver sheen, particularly of small pelagic fish. These cells are capable reflectors of light and are responsible for the counter shading effect where fish appear darker when viewed from above and lighter when viewed from below. This mechanism helps detour predation.

Pigments are characterized by their colors. Carotenoid pigments are red and orange. Xanthophylls are yellow. Melanin pigments are black and brown. Phycocyanin is the blue pigment derived from blue-green algae. Cells containing yellow pigments overlying those containing blue pigments can produce green hues. Fish are capable of producing some pigments, but others must be supplied in the diet. Black and brown pigments are produced in cells called melanocytes. Fish are incapable of producing carotenoid and xanthophyll pigments. Therefore, these must be supplied in the diet.

Natural sources of pigments are available in the diets of most fish. Color enhancing diets may contain additional natural pigments to enhance colors of ornamental fishes. The carotenoid pigment found in most marine and a few freshwater invertebrates is astaxanthin. This pigment gives the characteristic color to the flesh of salmon and is available in the diet of aquarium fish in shrimp and krill meals and salmon (fish) meal used as sources of protein in some feeds. Pure astaxanthin or canthaxanthin (synthetic astaxanthin) may also be added to fish feed to enhance red and orange coloration. These carotenoid pigments are often added to feeds for farm raised salmon and trout to give fillets a desirable red color. Xanthophylls (yellow pigments) are found in corn gluten meal and dried egg that may be added to the diet to enhance yellows. The ground petals of marigold flowers have also been used as a source of xanthophylls. The blue-green algae spirulina is a rich source of phycocyanin and may be added to a diet to enhance blue coloration. The expense of supplementary pigments often limits the amount used in tropical fish feeds. These natural sources of pigments are in contrast to several methods routinely used to enhance colors of ornamental fish.

A discussion of enhancing colors of ornamental fish would be incomplete without mention of dyeing and painting fish, and feeds containing hormones. The practice of painting essentially colorless fish (e.g. glassfish) has become widespread. The neon colored paint is non-toxic, but the handling and painting, coupled with shipping stress often invites disease problems. These fish often contract ich (Ichthyophthirius multifilis) and fungal infections. The paint is shed in time and the fish returns to being colorless which may be more disturbing to someone paying a premium for "painted" fish. Dyeing colorless fish has recently become popular. The fish are immersed in water containing dye and the immersion and handling may lead to the aforementioned disease problems. Hormones may be used to enhance fish coloration by causing a false early maturity. Testosterone supplied in the diet likely allows a premature storage and expression of pigments in the chromatophores. Fish that often exhibit drab juvenile coloration may then show full adult coloration. Fish treated with hormones often become all male, sterile, and require a continuous dietary supply of hormones to maintain coloration. The sex of juvenile fish is often ambiguous and hormone diets, most often containing testosterone, create all male fish. Uncontrolled doses of testosterone sterilize fish. Endogenous production of hormones ceases, so coloration is not maintained when fish are taken off the hormone treated feed. Fish feeds containing hormones do have legitimate commercial uses in Tilapia (Oreochromis spp.) diets (Teichert-Coddington et al. 2000). Tilapia growers are hampered by the fact that this cichlid often matures prior to reaching market size. The fish farmer often ends up with mixed size classes and stunting of fish in growout ponds if the tilapia are allowed to mature and reproduce. Feed energy also goes into producing gametes instead of fish flesh. Feeds containing hormones have been used to provide all male groups of tilapia for growout. These diets contain testosterone since males grow faster. The feed is administered to juvenile fish prior to growout and is currently undergoing FDA approval for food fish. Given the current status, this feed is probably not widely available to ornamental fish growers and hobbyists and would be of little use enhancing color of fish already sold as adults which encompass most ornamental fish with the notable exception of cichlids. There is no specific way to tell if a fish has been fed a diet containing hormones except to be vigilant of the fish you purchase. If it looks to good to be true, it probably is!

Water quality may also play a support role in determining the color of ornamental fish. Degraded water quality increases stress on captive fish and may dull fish colors. A high quality biological filter and routine -at least bi-weekly- water changes will provide an environment enabling fish displaying their brightest colors.

Hobbyists may wish to experiment with their own color enhancing diet. There are several recipes for gelatin-based feeds available in other publications, notably Moe (1982) and Konings (1993). I would recommend the protein portion of these diets (e.g. shrimp, fish, squid) be replaced with salmon fillets. Salmon are a good source of carotenoid pigments that enhance reds. Additionally, all essential amino acids will be supplied using salmon as a protein source and the higher lipid content in salmon will promote better utilization of the protein. The addition of high quality pure spirulina powder will enhance blue pigments. This can be purchased from aquaculture suppliers. Any gelatin based diet should be stored frozen to maintain freshness and used within several weeks. Feeding a varied diet rich in sources of pigments along with good water quality will ensure captive fish develop vivid colors.

REFERENCES:

Moe, M.A. 1982. The marine aquarium handbook – beginner to breeder. Green Turtle Publications, Plantation, FL.

Moyle, P.B., and J.J. Cech. 1988. Fishes an introduction to ichthyology. 2nd Edition. Prentice Hall, Englewood Cliffs, NJ.

Fujii, R. 1969. Chromatophores and pigments. pp. 301-353 in W.S. Hoar and D.J. Randall (eds.). Fish Physiology. Volume III. Reproduction and Growth. Bioluminescence, Pigments, and Poisons. Academic Press, New York, NY.

Konings, A.(ed.). 1993. Enjoying Cichlids. Cichlid Press.

Teichert-Coddington, D., B. Manning, and J. Eya. 2000. Concentration of 17alpha-Methyltestosterone in hormone treated feed: Effects of analytical technique, fabrication, and storage temperature. Journal of the World Aquaculture Society 31: 42-50.