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What causes the colouration of a fish and can it change? Read on ...
Some fishes cannot change colour. Examples include many of the uniformly dark-pigmented deepsea fishes and some unpigmented cave fishes.
Many species of fishes however can change colour. The changes can be slow or fast. Slow changes of colour (eg breeding colouration) are generally under the control of hormones and are usually semi-permanent.
Rapid colour changes (eg stress responses) are largely under control of the nervous system although hormones may also be involved. The colour of fishes can also vary with the seasons, between day and night and even with changes in habitat and food.
There are two kinds of cells that give colour to fishes, chromatophores and iridiophores (also called iridocytes). The Chromatophores are located in the dermis of the skin, above or below the scales. They impart true colour (rather than structural colour) and contain black, red, yellow, blue, white (and rarely green) pigment granules called chromatosomes. Only one colour is found in each chromatophore. Colour changes result from chromatosomes concentrating in the centre of the chromatophore or dispersing throughout the cell.
Iridiophores contain highly reflective guanine crystals. The crystals act as mirrors, which reflect the colours of the outside environment. Iridiophores are responsible for the silvery appearance of many pelagic fishes.
Slow colour change
Slow colouration changes often occur as a fish grows from a larva, through juvenile and adult phases. The top two images show the different colours of juvenile and adult Axilspot Hogfish.
Colour changes in the breeding season are also common. Males of many species exhibit breeding colouration at certain times of the year (see third image from top). Females generally do not show the same degree of colour change.
Some fishes show different colouration in different habitats. The Longfinned Eel has a transparent leptocephalus, a countershaded freshwater phase and a silvery oceanic phase.
Rapid colour change
When most people hear the term ‘colour change’ they think of rapid colour change. Many species of fishes can change colour rapidly. A fish caught on hook and line may change colour as a result of stress.
Divers often observe fishes rapidly changing colour as they swim over different bottom types (see the three images of Southern Pygmy Leatherjackets). As a fish swims over a light substrate the chromatosomes are transported into the centre of the cell (aggregation) resulting in the fish appearing paler. Swimming over a dark substrate results in the chromatosomes spreading out throughout the cell (dispersal) which leads to a darkening of the fish’s colour.
Some species of Flatfishes are remarkably good at quickly changing colour to match the background.
The video, below, provides information on why the flesh of fishes can be different colours.
- Helfman, G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.
- Wallin, M. 2002. Nature’s palette. How animals, including humans, produce colours. Bioscience explained. 1(2): 1-12. (www.bioscience-explained.org/EN1.2/pdf/paletteEN.pdf)