Coloration in fish
How do fish colours come about?
Fish coloration is formed by the reflection and absorption of various parts of the spectrum of light inside specialised cells. This is achieved by various layers of colour forming cells, such as crystals in the skin(iridophores), pigment cells(Chromatophores) and underlying flesh colours. Pigment cells are of various types.
Chromatophores can be classed according to colour under white light: xanthophores (yellow), erythrophores (red), iridophores (reflective), leucophores (white), melanophores (black/brown), and cyanophores (blue).
Combinations of these pigments and optical effects can produce many distinct colours.
Fish can also alter their brightness, colour and patterns. These colour changes are mostly autonomous and are effected by nerve impulses and hormonal releases during mating or fright. The nerve impulses and hormonal releases affect the chromatophores. Coloured chromatophores are branch liked structures that contract to remove the colour or expand fully to show full colour. Iridophores can reflect light thereby creating a metallic effect. Some iridophores refract light. This has the effect of creating a colour where that colour is not available as a pigment. Blue is often created in this way. Very few species of fish have blue pigmentation.
The pigments found in the chromatophores cannot be created by the fish but must be extracted from their diet. It has been demonstrated that when certain fish are denied a certain pigment from its diet that the fish will change colour.
Why do fish have colour?
Fish can change their colours to blend in with their environment called background adaptation, in terms of brightness and colour. They can also change their colours depending on mood and agression or submission.
Colouration in fish is vital for a fish’s survival. It plays a role in camouflage, fish recognition, mate selection ,mood display and warning of poisonous or danger. Most preyed upon fish have camouflage themselves by having a coloration, pattern or shade that closely matches it background environment. Fish can recognise each other as being from the same species and which gender and colour plays an important role in this. When in breeding mood usually the male will colour up with brilliant coloration. The brightest colours tend to attract the best females and is used as a warning against rivals. Fish can display dominance and submission by darkening or lightening their colours. In some species such as in many Malawi cichlids, submissive males will take on the colour of the females to avoid attack from dominant males. Some fish do the opposite of camouflage and in fact have developed very striking colours as a warning of being poisonous or dangerous.
In the tropical fish keeping hobby colour is one of the most important factors that determine whether a particular species of fish makes its way into the hobby. Hobbyists favour fish with brilliant colours and patterns. Hobbyists by selective breeding have created new “sports” with enhanced or even new colours. These fish are more valuable and score higher in fish keeping shows.
Experienced tropical fish keepers recognise that fish do change coloration according to their environment, water conditions and diet. Having a darker gravel, feeding foods with high levels of pigmentation and providing the correct ph, hardness and salt levels all help in getting the best out of fish colours. Some vitamins and amino acids that produce pigmentation have a short shelf life and will not survive in sufficient quantities in dried fish food. Only live food and vegetable matter can provide sufficient pigmentation to aquarium kept fish. Stressed fish will lose coloration as well as fish under bright lighting in bare aquariums or fish undergoing medical treatment.
Countershading is where there a fish’s body is dark when viewed from above and is light when viewed from below. This is for camouflage from predators above the water who will find it difficult to see a dark backed fish against the dark of the ocean, while the light bellied underneath of a fish makes it difficult for predators underneath to see the fish against the bright sky.
When in courtship mood fish usually enhance their colours to their maximum level of vividness. Usually male fish have better blue and red coloration and are generally more colourful than their female counterparts.
Young fish in most species are grey,green or black and have few distinctive markings. This is a form of camouflage because most young fish are preyed upon by adult fish. Young fish spend most of their youth near river or lake banks or near muddy bottoms between algae and plants and so have colours that resemble their bushy or earthy surroundings. Some sea fish have young that undergo a larval stage where they drift along with plankton. Most of these young are transparent to blend in with other plankton to avoid being eaten.