Colour of Fish
By Dougall Stewart BSc. (Hons).
Contents
The purpose of colour
How do we see the colour of a fish?
Basic Colours
Metallic colours
What if my fish is not displaying its normal range of colours?
References
When we admire the fish in our aquaria we often see the striking displays and colours as simple reflections. Rarely do we ask ‘why do our fish display these colours?’. It is my view that colour of a fish must have tremendous significance/benefit – to the fish. After all it carries a considerable cost/risk e.g. prey can be easily identified, the formation of purine bases from valuable food supply, and the development of site specific ‘colour organelle’ etc.
In simple terms, the colour a fish displays is dynamically multipurposeful i.e. it can be involved in:
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Communication
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Identification
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Camouflage
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Defence
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Mimicry
The colours we perceive are the result of certain wavelengths of light being reflected by objects, and towards the viewer’s eyes; whilst other wavelengths are absorbed. This simple model of ‘seeing light’ is further complicated in fish, where the scattering of light and its subsequent reflection back to the viewer is mediated by pigment producing cells and intracellular organelles.
Fish have the ability to change their ‘normal’ colour via pigment producing cells, called chromatophores, which can be found on/in their skin and scales. Chromatophores can be grouped into melanophores (melanin), erythrophores (red), xanthophores (yellow) and cyanophores (blue) and leucophores (light scattering whitish effect) (Oshima & Kasai, 2002).
Colour change in fish is said to occur when a receptor responds to ‘its environment’ and sends a nerve impulse to the brain; this elicits an appropriate response i.e. information is sent to chromatophores. Often this response is rapid and can involve the use of a number of hormones. An example of a hormone controlled colour change is the way that discus lighten their colours at night time, with the lights off. This is especially noticeable if you get up at 4am in the morning and turn the fish tank lights on – the discus normally look terrific and have a paler background. I suspect this response it triggered by the hormones melatonin – a hormone known for its lightening effect on fish during the night.
Metallic colours in fish are achieved via iridophores e.g. iridophores are responsible for the silver in herrings etc. Iridophores tend to consist of a layer of plates with crystals e.g. guanine sandwiched in-between or in front of the plates. Movement of the plates and the placement and or movement of the crystal causes an array of ‘muliti-directional reflective effects’ e.g. the colouration change along the lateral line of neon and cardinal tetras. In addition, a degree of control iridophore structures appears to be mediated by α-adrenoceptors on the iridophore membrane (Oshima & Kasai, 2002). To add to the range of possible effects, some fish have melanophores below iridophores in the skin. (Wallin, 2002).
If a fish is not displaying its normal range of colours e.g. a discus turns black/dark (and it is not in breeding ‘colours’, It is possible that the colour reflecting structures have ‘turned off’ this may be for a number of reasons.
Perhaps due to an intestinal disorder the fish is being robbed if essential nutrients to make the required hormones e.g. nor-epinephrine, guanine, melanophore stimulating hormone etc. A second hypothesis is that perhaps the fish has ample hormones but has ‘intentionally switched off’ its display processes in order to ‘hide and recuperate’ – or perhaps there is a combination of effects occurring. Whichever hypothesis may be correct, the one thing I do know if that a fish stops displaying correctly – this often means that the fish is unhappy; as a result I start checking all water parameters and starting making more in depth observation notes on my charges.
Oshima, N., Kasai, A., (2002), ‘Iridophores Involved in Generation of Skin Color in the Zebrafish, Brachydanio rerio’, Forma, 17, 91–101
Wallin, M., (2002), ‘Nature’s palette - How animals, including humans’, produce colours, Bioscience | explained, 1:2
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