Changes in Rainbowfish
Some rainbowfish of the genus Melanotaenia show a range of body colour changes that makes them very variable in colour. For example, an adult, male M. lacustris shows rapid changes in the iridescence on the body of a live fish, as the the chromatophores and iridiophores change appearance with the mood of the fish. The colour changes are located on the body, generally, or along a blaze located along the dorsal edge of the fish, from "forehead" to the dorsal fins.
The Australian Museum fish site reports: "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." (Source 1) They are also responsible for the iridescence in rainbowfish.
The picture at left, of a young, male Melanotaenia lacustris, shows the iridescence given by its iridiophores on the sides of the fish and fins, most noticeably as silver on the underside and as an opaline play of yellow, green, turquoise, blue and violet colours on the upper body and fins. The dark blue stripe in the caudal end of the fish and the orange-red rows of dots between the scales are due to the chromatophores located there. This species changes colour very noticeably, depending upon mood, time of day and whether the fish is displaying for some reason.
Two types of colour change are notable. The first being in the general ability to change body colour, stripes and patches, and the second being the ability to change the colour of a specific colour blaze that runs along the top of the body from the forehead to the dorsal fins. The colour blaze is shown clearly in the photograph, at right. The black chromatophores are also more prominent in that individual.
It is this second colour change that I find fascinating, as the colour changes very rapidly in flashes. One commentator described the flashes in Melanotaenia lacustris in the following way: a "dominant or spawning male is a sight to behold and a true wonder of nature. His forehead turns a really bright orange in colour and then flashes various shades of blue, green, purple and violet. These colour changes may last up to about 1 second for each shade but continually change whilst the male is spawning which in turn can last for 30 minutes or more. These colours are almost as bright and attractive as a series of neon lights." [Source 2]
In M. boesemani there is a similar, bright, iridescent, green stripe or blaze running from the mouth, along the top of the head and back to the second dorsal fin. The picture, at left, clearly shows a pale, blue colour along the head of M. boesemani but does not capture the iridescence well. When viewed from above, this stripe is bright and very prominent. However, it is not always present. It comes and goes, brightens and flashes, with the mood of the fish. It varies in colour, too, from iridescent green to nearly white.
I have seen a similar flashing blaze in M. duboulayi but of a bright, golden-yellow colour. It is as though the fish turns on this iridescence in response to another fish (or me observing above him) as a threat or warning signal. M. maccullochi (Harvey Creek) shows a vivid red-orange flashing blaze of true neon appearance and M. splendens displays in orange-yellow.
In the cases observed in M. boesemani, M. lacustris and M. splendens, the colour flashes were seen on occasions when a dominant male was defending territory, showing aggression or displaying to a female. It appears to be a remarkable adaptation for display that is more highly developed (or visible) in some species of rainbow.
It would be interesting to test whether other species of rainbowfish exhibit a similar flashing colour blaze and under what circumstances or stimulus.
Link to Notes on Keeping Australasian Blue-eyes- fishes also related to the the Rainbows.
GOOD LUCK WITH YOUR RAINBOWS