Fish Coloration and Chromatophores: Pigment, Structure and Colour Change

How melanophores, xanthophores, erythrophores and iridophores make fish colours, how pigment and structural colour differ, why fish change colour, and what diet studies on carotenoids show.

Chromatophores: the colour cells of fish

Fish colours come mainly from chromatophores, colour-producing cells that arise from the neural crest during embryonic development. In zebrafish, melanophores, xanthophores and iridophores are all present by three days after fertilisation. Mature chromatophores are classed by their colour under white light.

Melanophores contain eumelanin, a black or dark-brown pigment packed in vesicles called melanosomes and made from tyrosine with the enzyme tyrosinase; when tyrosinase is defective, no melanin forms, which causes some types of albinism.
Xanthophores hold mainly yellow pteridine pigments, and erythrophores mainly red and orange carotenoids; one cell can contain both, so the boundary between the two types is not always sharp.
Iridophores reflect light from stacked plates of crystalline guanine separated by layers of cytoplasm, producing iridescent and metallic colours.
Leucophores, found in some fish, also use guanine crystals but give a white shine rather than iridescence.
Cyanophores, cells with a blue pigment of unknown structure, are known from some mandarin dragonets (Synchiropus splendidus).

Pigment colour versus structural colour

Colour-producing materials fall into two classes. Pigments (biochromes) such as melanin, carotenoids and pteridines absorb part of the visible spectrum and let the rest reach the viewer. Structural colours arise when light is reflected, scattered or diffracted by structures roughly a quarter of a wavelength in size; because the effect depends on geometry, such colours often shift with viewing angle. The guanine plates of fish iridophores act as microscopic mirrors whose orientation and spacing set the colour, and when combined with coloured pigment filters they can produce bright blue or green.

Black: melanin dispersed through the melanophore; grey and brown: melanin concentrated inside the cell.
White: light reflected by guanine crystals in iridophores and leucophores.
Red, orange and yellow: carotenoids that come from the diet, plus pteridines the fish can make itself.
Green, blue and violet: generally structural colours from the skin and scales.

The neon tetra (Paracheirodon innesi) shows how structural colour behaves. Its blue stripe is produced by light-reflecting guanine crystals and appears blue-green when light-adapted and indigo when dark-adapted; at night, when the fish rests, the blue and red turn grey or black.

How fish change colour

Physiological colour change is fast and works by moving pigment inside existing cells. When melanin spreads through flat melanophores lying over other chromatophores, the skin darkens; when it gathers at the cell centre, the colours of the underlying cells show through. This movement can be under hormonal control, nervous control or both. Melanocortins disperse pigment, while melatonin and melanin-concentrating hormone aggregate it; noradrenaline also moves pigment, and in many bony fishes chromatophores respond directly to light, UV radiation, temperature, pH and chemicals. The pineal body of the brain takes part in controlling colour change.

Morphological colour change is slower and involves changes in the number and form of chromatophores and in pigment synthesis. In the Midas cichlid (Amphilophus citrinellus) all fish start grey, but a minority of 8–10% turn gold or orange as the overlying melanophores die and more carotenoids accumulate in the skin.

Background, mood, stress and health

Background adaptation: most fish can lighten or darken somewhat to match their surroundings. The response depends on vision, and melanin movement in melanophores is the main mechanism. In juvenile leopard coral grouper (Plectropomus leopardus) reared for 56 days on different tank colours, fish on white backgrounds stayed red with a smaller melanin zone, while black backgrounds gave the darkest skin.
Signals of mood and status: colour change can be triggered by mood, temperature, stress or visual changes in the environment. Dynamic displays include erecting coloured fins and flaring brightly edged gills, as fighting fish (Betta) do in aggression.
Stress: Atlantic mackerel (Scomber scombrus) crowded in sea cages shifted from mostly green to mostly blue within the time of a typical crowding event; the shift grew with crowding severity and duration and was linked to higher plasma lactate (stress physiology is covered in a separate guide).
Health: in cichlids, carotenoid colour correlates with low parasite load and high social status; in red male Pundamilia nyererei it tracks parasite load, antibody responses and oxidative stress. Loss of colour is also listed among the signs of neon tetra disease.

Carotenoids and diet

Vertebrates cannot make carotenoids, so the red, orange and yellow carotenoid hues of fish depend on carotenoids that originate in photosynthetic organisms and enter through the diet. A review of cichlid studies (Sefc, Brown and Clotfelter 2014) reports that carotenoid colour is influenced by diet and body condition, but the results are species-specific and not uniform.

In feeding trials with Nile tilapia, dietary tunaxanthin was deposited in the skin unchanged, whereas dietary beta-carotene was neither accumulated nor converted there.
In Amphilophus citrinellus and Heros severus, dietary beta-carotene had smaller effects on body colour than astaxanthin.
Several studies found that supplementation raised skin carotenoid levels in cichlids, but not in all species, and the effect depended on the carotenoid used.
In the Midas cichlid, carotenoid supplementation did not change skin colour and did not enhance innate immunity in either colour morph.
In the firemouth cichlid (Thorichthys meeki), fish on a high-carotenoid diet more often won paired contests, but the advantage vanished under green light that hid the red colour.

Fish can also make yellow to red pteridine pigments, which in theory allow colour that is largely independent of diet, although pteridines appear to matter little in the few cichlids examined. Feeds marketed for colour are discussed in the separate guide on colour-enhancing foods.