Colouration
Rainbowfishes are recognised for their bright, brilliant and beautiful colouration. However, their colouration often fades when maintained under aquarium conditions. Therefore, one of the greatest challenges for the aquarium hobbyist is to accurately replicate the natural colouration of rainbowfishes that they maintain in captivity. Various products have been introduced to alleviate this problem, but none has performed so effectively and consistently as carotenoid pigment. A variety of carotenoids pigments are used in the diet of fishes for colour enhancement. The most promising carotenoids proved to be successful in enhancing colour is astaxanthin that shows marked improvement in colour on most species of brightly coloured ornamental fishes including Tetras, Cichlids, Gouramis, Goldfish, Koi, Rainbowfishes and many other species.
The colouration of fish is mostly shaped by a variety of selective pressures imposed by their predators and prey, their competitors, and their potential mating partners. Some of these interactions favour reduced conspicuousness, or crypsis, while others promote the opposite. Plasticity in colour patterns, or in associated display behaviours, appears to be a common solution to balancing opposing pressures. Their colour patterns are often multicomponent signals that are used to communicate within species, in both interand intrasexual interactions, and between species. Most fish species have fairly inconspicuous coloration that matches the background in their natural habitat. Many fish are darker dorsally than ventrally, a pattern termed “countershading”, and this pattern may serve to reduce conspicuousness against bright downwelling light when viewed from below, and against darker upwelling light when viewed from above (e.g., a dark dorsum blends with deeper water, while a light ventrum blends with the light surface).
The visual capacities of fish can vary dramatically; some species are tetrachromatic and possess the ability to see ultraviolet or far-red light, while others are partially or completely colour blind compared to humans. Field studies involving spectrophotometric measurements of both the colour pattern and background, and physiological measurements of the viewing species visual capabilities are necessary to determine if a colour pattern is indeed inconspicuous (or cryptic), because what appears obvious to the human eye may not actually be conspicuous in the natural habitat.
The conspicuousness of a colour pattern can be affected by environmental factors, such as ambient light conditions. In an aquatic environment, variation in the spectral transmission of the water stemming from differences in colour and opacity may affect the perception of colour patterns by other individuals viewing from a distance. Under the same water conditions, however, the visual perception of the same colour patterns by receivers in close proximity, like females attending to courting males, could be affected to a lesser degree.
Fish can display broad swaths of uniform colour or complex patterns. These patterns serve a variety of functions including camouflage and warning colouration and influence aspects of behaviour such as mate recognition, mate choice, and shoaling preference. Predation avoidance and mate attraction in particular have been studied extensively as possible ecological explanations for specific colourations.
In a classic series of experiments, Endler (1980) demonstrated that orange spots on male guppies made them more attractive to potential mates, but also made them easier targets for predators. Therefore, high predation pressure resulted in duller-coloured guppies, while lower predation pressure allowed sexual selection to drive the evolution of guppies with larger, brighter orange spots.
Physiological mechanisms (colour pattern change) and behavioural decisions (shoaling) interact to mediate antipredator tactics in rainbowfishes. Longitudinal stripes are often associated with highly developed shoaling behaviour in rainbowfishes. However, rainbowfishes cannot know their own colour, so how do they acquire this shoaling preference for fish of the same colour? Previous studies in other fish suggest that prior exposure to potential shoal mates — familiarity — makes an important contribution to shoaling preferences.
Another environmental factor that can affect whether an organism is easily detected is the background; if there is a good match between an individuals colour pattern and the background colour pattern, there will be a reduced likelihood of detection. The local environment, therefore, could affect the colour pattern variation in a population residing there. When environmental conditions are similar across a species range, colour patterns may be similar. However, when environmental conditions are dissimilar across a species range, colour patterns may be very different across populations. This may explain why the same species of rainbowfishes have different colour patterns in different environments. Natural rainbowfish populations are highly polymorphic to the extent that no two stream populations are alike. However, this colour polymorphism is expressed mainly only in adult males. Most females lack the bright pigmentation of males.
Females can choose male phenotypic traits, including courtship displays, nuptial coloration, pheromones, body size, and territorial quality. All these traits are sources of information about male quality that attract females and that can enhance female reproductive success. Although female choice can be based on several male traits, another mechanism involved in sexual selection is mating through resource competition. Thus, intrasexual selection, usually more strong among males, will select for fighting ability. Because male-male competition is an honest signal, females can easily detect male abilities, and choose dominant males.
Other types of environmental variation could also result in variable selection on male size and colour. In addition to being highly variable, size and colour have measurable effects on male fitness. For example, colour affects male mating success and predation risk. In general, males having more colouration are preferred by females, while males with brighter or more conspicuous colouration are at greater risk of predation. Male size has also been implicated as a factor in female choice, in male mating success, and in susceptibility to predation. Larger male Melanotaenia australis generally have a reproductive advantage (Evans et al. 2010; Young et al. 2010). In contrast, Pseudomugil signifer females prefer males that exhibit more courtship movements and also provide more care to the brood, instead of dominant males (Wong 2004).
Coloration is also known to play a role in both mate choice and intrasexual competition. During courtship most (but not all) rainbowfish males frequently “flash” an ephemeral coloured “blaze” that runs from the upper lip to the first dorsal fin, which they flash on and off like a neon sign during a “head-down” display. The colours can range from white to yellow, orange, rustic red and light blue, depending on species. It is possible that these rapid changes in colour come about by changes in the structural layer of reflective cells (the iridophore) that can alter which wavelengths are reflected back through the layer of pigment, which lies on top.
Bright colours represent a balance between selection for crypsis by predators and selection for conspicuousness by sexual selection. Males cannot have colour patterns which are too conspicuous, or they will be consumed by predators; but they cannot be too inconspicuous, or females will choose other males. However, it is difficult to define what is and what is not conspicuous to predators and mates. A colour pattern on a bare aquarium background can be quite conspicuous, yet be quite cryptic against the natural background in their natural habitats. One fish which may seem more conspicuous than another in an aquarium may actually be less conspicuous in the field.
In order to be cryptic (inconspicuous), a colour pattern must represent or approximate a random sample of the background normally seen by predators. Any deviation from the background in the distribution of colour or brightness will make the colour pattern conspicuous; and the degree of conspicuousness is proportional to the deviation from the background distributions. In areas of high predation, the background match should be better than in areas with weak predation. On the other hand, sexual selection favours colour patterns which deviate from the background. If both sexual selection and predation occur together, then a compromise must be made.
The body colours of rainbowfishes are predominantly dependent on the presence of multiple pigment cells located in the dermis of the skin, above or below the scales, called chromatophores. Dermal chromatophores are generally composed of three cell layers: the xanthophore (contains carotenoid and pteridine pigments), the iridophore (reflects colour structurally), and the melanophore (contains melanin).
Colour changes result from the 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 often responsible for the bluish or silvery appearance of many rainbowfishes.
There are two types of chromatophores, those that contain biochromes and those in which colours result from purely physical properties, the schemochromes. Biochromes are true pigments, such as carotenoids (red, yellow, and orange), pterins (white, red, orange, and yellow), purines (white or silver), and melanins (reds, yellows, browns, and blacks). Chromatophores that contain the red and orange carotenoids are termed erythrophores, and those that contain the yellow carotenoids are termed xanthophores. Although, both colours can be produced by a range of pteridine and carotenoid pigments and both xanthophores and erythrophores can contain both pigment types. Schemochromes are colours created by an optical effect (e.g., via interference, refraction, or diffraction). The detection of structural colours can be ephemeral. For example, detection of the chromatic effect of iridescent structural colours depends on the angle at which they are viewed; changes in angle can produce the “flashing” appearance seen in moving fishes.
Pigments are compounds that absorb particular wavelengths of light and contribute to the colour of rainbowfishes. Two types of pigments commonly found in rainbowfishes are carotenoids (usually yellows, orange, and red) and melanin (browns, blacks, and greys); they are deposited in the skin. Short wavelength (blue and violet) and silvery colouration are almost always structurally based, the result of selective light scatter owing to variable refraction within the skin. This type of structural colouration, which occurs in the iridophores, is undoubtedly an important component of the fishes colour pattern. The melanin pigments are contained in chromatophores termed melanophores. Melanin is a product of the oxidation of amino acids, such as tyrosine and tryptophan. A fourth type of biochrome, pterinophores, contains pterins that contribute to eye colouration as well as colouration of other fish tissues. Pterins can be synthesised in developing pterinophores by vertebrates and other animals.
Aspects of colour are also affected by food availability. Carotenoid availability has a direct effect on the brightness of colour in adult males. Most of the naturally occurring colour pigments are derived from phytoplankton, algae and photosynthetic bacteria consumed in their natural diet, and these are not normally available in captivity, as they are mainly dependent on artificially prepared diets. Colour intensity can also be related to the environment and will often decrease when the fish are stressed, or from the nature of captivity itself. Even the substrate or background colour of the aquarium can affect their colouration. Light intensity has also been shown to affect the colouration of rainbowfishes and the coloured background (or substrate) may affect light intensity. Rodgers et al. (2010) reported that Melanotaenia australis individuals can change their colour patterns to match their background environment.
Carotenoids are the most common natural pigments responsible for many of the bright colours found in fish and crustaceans in their natural environment, as well as a variety of biological functions. More than 800 different naturally occurring carotenoids are known. Carotenoids commonly occurring in freshwater include beta-carotene (orange), lutein (greenish-yellow), astaxanthin (red), tunaxanthin (yellow), doradexanthin (yellow), and zeaxanthin (yelloworange). Certain fishes have the capacity to convert one form of carotenoid into another carotenoid.
Carotenoids are a class of natural fat-soluble pigments found principally in plants, algae, photosynthetic bacteria and some non-photosynthetic bacteria, and they play a critical role in the photosynthetic process. They also occur in yeast and moulds where they carry out a protective function against damage by light and oxygen. Carotenoids also play other important functions as pro-vitamin-A, antioxidants and immunoregulators. It has also been observed that fishes with a high level of carotenoids are more resistant to bacterial and fungal diseases.
Rainbowfishes are not able to synthesise carotenoids, but they can obtain carotenoid pigments in their diet, including plant material, algae, and other animals. In many cases, the level of colouration is dependent on an individual’s foraging success and physiological efficiency. The level of colouration an individual expresses can therefore indicate their quality as a potential mate or competitor, or of their escape potential in the eyes of predators. In addition to this variation in expression based on diet, pigments can be expressed differentially depending on development (juveniles can possess colour patterns that are very different from adult conspecifics), seasonally, and even ephemerally. Therefore, rainbowfishes maintained in captivity should be fed a complete diet that includes a colour enhancing agent, such as astaxanthin, to supplement natural feeds that might be limited in captivity. Typical commercial diets used for aquarium fish are very low in total carotenoids.
Carotenoids are also vital nutrients for healthy growth, metabolism, and reproduction. Male rainbowfishes use colouration to produce sexual signals associated with breeding. It is well known that the most highly coloured males have the best chances of finding a mate. One reason why females are not usually highly coloured is that they need to pass on their carotenoids to their offspring. At the time of sexual maturation, they mobilise stored carotenoids to the ovaries and finally, on to the progeny. This active transfer of carotenoids from the female to the eggs has led to the hypothesis that carotenoids are vital for larval survival and growth.
Colour enhancement through the use of carotenoids in feed has been confirmed by a number of authors. Ako et al. (2000) reported intense colouration of Xiphophorus helleri, Pseudomugil furcatus and Archocentrus myrnae when fed diet containing carotenoid rich strain of Spirulina platensis and Haematococcus pluvialis. Pseudomugil furcatus became significantly more intensely coloured when fed a diet containing 1.5~2.0% of a carotenoid-rich strain of Spirulina platensis and 1.0% of a specially grown Haematococcus pluvialis for 3 weeks. Although colour enhancement was apparent after only one week when the fish consumed these doses of algae, lower doses (0.5% and 0.4%, respectively) were not significantly different. Both treatments were significantly more effective than control treatments with no added carotenoid. Alagappan et al., (2004) also obtained higher carotenoid level in the blue gourami, Trichogaster trichopterus with 4 gm/kg spirulina in feed. Goldfish fed diets containing 100 mg/kg astaxanthin had significantly higher red than all other treatments while there were no significant differences in other colour parameters.
Research has also shown that the addition of astaxanthin can enhance the colouration of rainbowfishes. Several species responded positively, and maintenance of the bright colouration was achieved with incorporation of 25 mg/kg astaxanthin into various diets on a continual basis. Some species also had faster growth rates. However, more research is required to establish the optimal level of astaxanthin to use in diets. The raising of rainbowfishes in outdoor ponds with or without supplementary feed will enhance their colour development, most likely due to the natural food items in the pond.
It has been recommended that feeds containing colour enhancers or pigments such as astaxanthin should be fed to bring out the full colour of the fish. Duckweed has high concentrations of carotenoid pigments, particularly betacarotene and xanthophyll that make duckweed an especially valuable supplement for rainbowfishes. Feeding Chlorella vulgaris and Spirulina also produces positive pigmentation results in certain fish, and the microalgae Chlorococcum seems to be a promising source of astaxanthin, canthaxanthin and adonixanthin. These algae have strong concentration in particular carotenoid pigments.
It is worth noting that aside from giving an appropriate colouration, microalgae sources could provide better growth performance as has been shown in a number of studies. Many aquatic species including crustaceans, amongst which are krill and shrimp store carotenoids in their shell. Carotenoids can be found in many fish species as well. These animals eat crustaceans and other organisms that ingest astaxanthin-containing algae and plankton as a major part of their diets.
Alternatively, commercially available alga products (e.g., NatuRose) can be incorporated into their diets. When these alga products are incorporated into the diet at 1~2%, an enhancement of coloration in the treated fish may be noticeable within two weeks. NatuRose algae meal is a natural source of astaxanthin derived from the microalgae, Haematococcus pluvialis. It is a spray-dried, dark red powder, and is currently used worldwide as a coloration and nutrition source for numerous species of animals. The flakes of Haematococcus pluvialis contain the largest concentrations of astaxanthin found in nature (some 40,000 parts per million).