Colouration in Fish

The colouration and patterns of the fish that we keep in our aquaria are in many cases, the things that attract people into the hobby of fishkeeping. For enthusiasts who show fish, these two factors are also very important in determining the quality and therefore the value of any particular fish.

Yet our understanding of fish colouration is still a relatively inexact science, with the known scientific facts clouded by theories and old wives tales. In the following paragraphs I would like to have a look at the colouration of fish and help to explain some of the changes you may observe in your own fish.

What makes colour?

The colouration of a fish is produced by three colour pigments which are largely contained within cells called Chromatophores. The 3 pigments are Erythrin (Red), Melanin (Black), and Xanthin (Yellow) each of which occurs in different chromatophores. Complementing the colour pigments are irridocytes, which are best described as tiny reflective spheres within the skin.

All of the colours we see in freshwater fish are a mixture of these components. For example orange is a combination of red and yellow chromatophores, brown is a mixture of black and yellow and red is just the red chromatophores. If there are no chromatophores the fish will appear white due to the presence of the irridocytes or the background colour of the skin and muscle will show through.

The position of the irridocytes within the skin of the fish will influence it’s reflective properties. Irridocytes on the surface of the scales will have a silvery appearance such as that found on tinfoil barbs or hatchet fish. If the irridocytes are in the lower layers of the skin the fish have a matt white colour.

In certain cases the irridocytes can combine with the chromatophores to produce reflective colours. Irridocytes combined with chromatophores containing Erythrin , for example, will result in a shiny gold appearance.

Blue is an unusual colour in fish in that it is a result of black pigment deep in the skin, with irridocytes in the middle layers of the skin. The irridocytes interfere with the light to give a blue colour.

Density of colour

The chromatophores may be positioned on the surface of the skin (above the scales), immediately under the scales or deeper in the skin. If the chromatophores are very dense the colour will also appear dense, with the chromatophores on the surface of the skin blocking those below.

The chromatophores on the surface of the skin tend to be less stable due to them being more easily removed (by rubbing against underwater objects) or spreading as the fish ages and grows. Those deep in the skin are more stable and less likely to break up.

Where does the colour come from?

In general, fish cannot make their own colour pigment therefore they have to consume it in their diet. In the wild these pigments would originate from eating algae, shrimps, snails etc. In the confines of an aquarium or pond there is not enough algae or other natural supplies of pigment, so it has to be included in the food that you provide. As with all foods it is important that the colour enhancing food given is of high quality to ensure that the pigments are in a form that the fish can absorb into its body.

If foods containing colour enhancers are not given, the chromatophores may not be filled with pigment and the fish may look pale or poorly coloured.

When the chromatophores are filled with pigment, any excess will circulate through the body before being passed out in the faeces.

The colour enhancing ingredients in fish food can be either natural or artificial, but all are a source of the pigments mentioned previously. Natural ingredients which are rich in colour pigments that can be utilised by our fish include krill, spinach, spirulina algae and carrot. These ingredients were specifically selected in the FishScience foods to optimize fish colouration.

Aging

Each fish is born with a fixed number of chromatophores, with this number remaining relatively constant throughout its life. As the fish ages and grows, these chromatophores have to cover a larger area of skin and therefore with some fish, there is a tendency for the colour to become paler (due to the chromatophores becoming less dense) or to fragment.

Changing colour

A chromatophore is a very branched cell with finger like projections, within which the colour pigment can be moved. Two extremes are

A) For the pigment to spread throughout the chromatophore, in which case the cell is the colour of the pigment.

B) For the pigment to be concentrated into one area within the cell, which results in the background colour showing through (usually pale or dark).

The distribution of this pigment is affected by a number of things.

  • The nervous system and hormonal system. In some fish species there is evidence that they can control the distribution of pigment in the chromatophores, allowing them to change their colour for camouflage or display, for example.
  • Water quality. Different conditions can have a marked impact on the distribution of colour pigment. Raised levels of pollutants such as ammonia, nitrite and chlorine tend to cause the pigment to concentrate, resulting in the fish becoming paler or darker. The pH and hardness of the water can also have an impact.
  • Background colour. Fish tend to adjust their colour intentionally or unintentionally, in order to be less conspicuous against the background. The flatfish species that live around the British coast are a great example of this, with many adjusting their colour to mimic the substrate. Aquarists who show their fish recognize this and will, where possible, avoid having ‘colourful’ fish in a pale container. Showing the fish against a black background and with dark coloured gravel ensures they look at their optimum.
  • Treatments. When added to an aquarium, many medications will result in a noticeable change in the colour of a fish. Adding salt is a good example and, for freshwater fish that can tolerate it, will result in the colouration fading.
  • Algae. Koi and goldfish which have lived in an algae rich green pond for some time are usually intensely coloured due in part to the lower light conditions and partly to the impact the algae have on the water.
  • Temperature. Cold water fish are usually at their best colour-wise in the autumn and winter when the cold temperature causes the pigment in a chromatophore to spread throughout the cell. In the middle of summer when temperatures are (hopefully) hot, the reverse occurs and the fish appear less intensely coloured.

The subject of fish colouration is a fascinating one. The genetics of any fish plays perhaps the major role in what colour the fish will appear, but there are other variable factors which can have an influence – and which we can adjust. Hopefully this article will have given you some insight into why your fish are the colour that they are – and why that colour can change in some instances.