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:
-
Communication
-
Identification
-
Camouflage
-
Defence
-
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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