How do Koi get their colors?

Koi are one of the largest fishes to display attractive color patterns and metallic iridescence that most aquarium fishes possess. No matter what their pedigree is, or how cheap or pricey they are, the colors are due to the presence of cells containing color pigments or Chromatophores.

The most superficial layer of fish skin (epidermis), contains mucus glands, sensory cells and Chromatophores. Scales lie in the deeper layer of skin (dermis). Dermis also contains Chromatophores. If the scales are transparent and the epidermis is thin, dermal pigments can show through the skin, adding 3D effect.

A Chromatophore can contain different types of color pigments. Color reflected by a Chromatophore is dependent on the type of pigment present. There are 3 types of chemicals that give color to Koi.

Cells containing Melanin pigments are responsible for black and brown color (even in humans). Animals can make their own melanin pigments.

Cells containing Pteridines and Carotenoids reflect red, yellow and orange colors. Carotenoids must be supplemented in diet.

Cells containing Purines refract light to produce iridescence or metallic sheen. Animals can make their own Purines.

The intensity of color depends on the distribution of color pigments in the cell. For example, if melanin crystals are grouped together, skin appears lighter but if crystals are spread out, then the skin appears darker.

When a Kumonryu is stressed or netted, its black color turns Grey. This is because the nervous system produces adrenaline that aggregates the melanin crystals making the skin appear lighter. When adrenaline is metabolized, the crystals will disperse and the black color will return.

Purines however, work in an opposite manner. When they are grouped together, refraction is higher and intensity is greater. Purines are precursors of DNA!! They refract different wavelengths of light depending on their alignment and shape. They give the white, silver and blue sheen.

An animal can contain all pigments -Purines, Carotenoids and Melanins or some combination. The combination gives more complex colors. Moreover, the arrangement of chromatophores defines color patterns. Color distribution is controlled by hormones and nervous system of Koi. Light, water quality (pH, salinity, alkalinity, turbidity and temperature), health conditions influence hormones and neurons to change color. Colors can fade or intensify when
i) color pigments decrease
ii) number of cells containing color pigments decrease
iii) the color pigments move from deeper to superficial layers or vice versa.

Some color changes are morphological. When a Shiro Utsuri is placed in a pond with black background, it can turn Grey by reducing the number of melanin containing cells. But the change is gradual and happens over days or weeks because the change is induced by hormones.

Most talked about and most researched color pigments are Carotenoids. They produce vivid reds and yellows in fruits, vegetables, flowers and animals. There are more than 600 varieties of pigments in Carotenoid family. They are fat soluble and are precursors of Vitamin A. Animals have no capability of producing them. They are produced by plants, fungi and bacteria. But animals that consume the plants, fungi containing these pigments either deposit them in their tissues or transform them into other forms and integrate them into their tissues. Carotenoids can be deposited in skin, muscles, fins, eggs, milt, eye and reproductive organs.

Example: Pink flamingo diet contains beta-carotene. If you deprive them of beta carotenes, they turn white! The pigment they consume is actually a yellow carotenoid pigment. But they convert it into red colored Canthaxanthin and Astaxanthin which get deposited in their feathers and tissues. Similarly, Koi have the ability to convert Carotenoids into other forms, like Astaxanthin.

Carotenoids absorb blue-green and blue light, therefore reflect yellow light. Chlorophyll absorbs red light. When cells containing Carotenoids also contain Chlorophyll as in plants, together they absorb blue, blue-green and red lights making the plants appear green. Since Carotene is more stable than Chlorophyll, as temperature drops in autumn, it remains in the leaves even after chlorophyll disintegrates. If the leaves do not have any other color pigments, they appear yellow (Birch) in autumn.

Chemistry: Carotenoids contain molecules linked with double bonds. The color absorbed by a Carotene is dependent on the number of double bonds present. For example, carrot roots contain a type of carotene that has 9 double bonds which gives it yellow-orange color. If it had 10 double bonds, the color would be deeper orange. Tomatoes and watermelons contain 11 double bonds that give them red color.

Astaxanthin, a carotenoid pigment, is responsible for red color in Koi. It consists of 9 double bonds but has Keto and hydroxyl groups that give skin a deep red color. It is also a powerful anti-oxidant and has oxygen carrying capacity. Some scientists believe that it helps Koi to cope with low-oxygen situation. It is produced by marine algae and some strains of bacteria like Haematococcus pluvialis. It is passed up in food chain and is deposited in the shells of shrimp, crabs and lobsters and salmon. Benthic organisms like Brine Shrimp and Cyclops found in pond water also contain Astaxanthin. Koi fed with food containing Astaxanthin develop deep red color. You can add shrimp, krill, cyclopeeze, freeze-dried brine shrimp to koi food and get the same color enhancing effect as many expensive color enhancing food pellets.

Paprika, ground marigold petals and carrots can enhance orange color. Yellow Carotenoids like Zeaxanthin, lutein are present in turmeric, corn, yellow snap beans, yellow bell peppers and egg yolks. When fed, these bring out the yellows in Koi. Phycocyanin present in Spirulina algae brings out blue color in Koi.

Koi have the ability to consume simple beta-carotenes and convert them into other color pigments. Can we turn a white Koi into a purple Koi by feeding eggplant?
I don't know. But you can certainly conduct this experiment at home and let us know the results. Please do not inject dyes to make Koi colorful. Dyes overload kidneys and liver and may be toxic to Koi. Many dyes are carcinogenic in nature.

Bibliography:
Bagnara J. and Mac Hadley. 1973. Chromatophores and pigments. Fish Physiology v. III. W.S. Hoar & Randall (ed.s). Academic Press, New York, London.

Parker, G.H. 1948. Animal Colour Changes and Their Neurohumours. A Survey of Investigations 1910-1943. Cambridge University Press.

Noakes, David L. G. 1992. Fish colors. The colors we see in fishes involve complex physical and biological processes. TFH 2/92.

Selong, Jason. 2001. Enhancing color of ornamental fish. FAMA 11/01.