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REVIEW ARTICLES
CURRENT SCIENCE, VOL. 119, NO. 7, 10 OCTOBER 2020 1093
*For correspondence. (e-mail: aquaparo@gmail.com)
The quintessence of colour enhancement in
ornamental fishes: an empirical pathway
towards rainbow revolution
Paramita Banerjee Sawant1,*, Srijit Chakravarty1, Subrata Dasgupta1,
Narinder Kumar Chadha1 and Bhawesh T. Sawant2
1ICAR-Central Institute of Fisheries Education Versova, Mumbai 400 061, India
2Taraporevala Marine Biological Research Station (Dr Balasaheb Sawant Konkan Krishi Vidyapeeth), Mumbai 400 051, India
One of the greatest challenges in the ornamental fish
industry is to replicate accurate natural colour of
fishes in captivity. Numerous attempts to preserve
colour in captivity have been ineffective in reducing
its fading, making it an important determinant in the
selection of ornamental fish species for trade in terms
of saturation, brightness and hue. Colour development
of ornamental fishes has been widely studied, yielding
curious insights about evolutionary genetics and hav-
ing a discerning role, either as deceptive or attractive
(aposematic) signals in mating as well as in camou-
flaging (Delphic) patterns during predator–prey inter-
actions. This article discusses colour enhancement
strategies with reference to nutritional interventions
through carotenoid-rich feed ingredients, genetic mani-
pulation or injection of colour in subcutaneous layers
of the skin. An insight into the mechanism of pigmen-
tation shows that motility and pigment dispersion of
chromatophores are the two drivers by which fishes
control integumentary colour variation. Research on
colour development and its enhancement has wit-
nessed novel techniques to support the ornamental
fish industry. Therefore, this article also sheds light to
answer questions on various issues pertaining to envi-
ronmental and physiological effects on colouration. It
attempts to provide insight on potential research
areas, with caution on ethical and legal issues to
ensure sustainability, so as to restrict risks of
unwanted inheritance of colour patterns. It also high-
lights the problems of identity crisis among conspeci-
fics thereby bringing a ‘rainbow revolution’ to the
ornamental industry.
Keywords: Colour enhancement, chromatophore, orna-
mental fish, rainbow revolution.
COLOURATION is the general appearance of an animal re-
sulting from the reflection or emission of light from its
skin surface. Vibrant colours in different shapes and
forms in ornamental fishes make their rearing a favourite
hobby, which has a rich history that flourished with major
advancements in civil aviation after the Second World
War, when ornamental fishes started to be exported glo-
bally to the developed countries1. Although the global
ornamental fish trade is punitive compared to the edible
fish industry, it is growing at an enviable rate of 14%
annually with an annual corpus of US$ 200–300 million2.
Global aquarium trade is mostly dependent on wild
catches. In addition to freshwater ornamental fishes, 1471
species of marine finfish, 140 species of corals and
more than 500 species of marine invertebrates are traded
globally adorning 1.5–2 billion marine aquaria, more
than 600,000 in USA alone, making it a second most
popular hobby in the world after photography3. Illegal
trade of native ornamental organisms raises concern on
their sustainability (being detrimental to the local flora
and fauna), and this is further aggravated by climate
change4,5.
Fishes exhibit a variety of beautiful colours and colour
patterns like the rainbow, from violet to red, tints and
shades of green, bright yellows, subdued yellows,
oranges, vibrant reds and all colours between blue
and red. This diversity in colour patterns appropriately
gives rise to the word ‘ornamental’, also forming a
basis for their descriptive names such as blue damsel,
yellow cichlid, orange chromide, etc. Although fishes
inherit skin colour, they are unable to produce red,
orange, yellow, green and some blue pigments, which
must be obtained from the food they consume. A number
of colour enhancement strategies are presently in vogue,
but only a few have been documented. Besides conven-
tional strategies such as nutritional supplementation of
carotenoids and genetic manipulations colour enhance-
ment strategies can be as gruesome as injecting toxic
dyes into nearly colourless fishes such as Indian glassfish
(Chanda ranga), to add glitter to the trade by unethical
means6.
In addition to visual pleasure for the onlookers, colour
in ornamental fishes provides a conspicuous route to un-
derstand the underlying principles of signalling in them
in response to variations in environmental factors such as
water-quality alterations, genetic manipulations, adapta-
tions, reproductive stimuli, etc.6,7.
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The process of colour development
Chromatophores: origin, arrangement, motility and
pigmentation mechanisms
Variation of colouration and pigmentation patterns in fish
is attributed to six different types of chromatophores, viz.
melanophores, xanthophores, erythrophores, iridophores,
leucophores and cyanophores, in contrast to only one type
(melanocytes; secreting eumelanin for black and pheo-
melanin for yellow/brown colouration) in mammals8,9.
Chromatophores are mostly derived from the neural crest,
except leucophores and cyanophores, and their arrange-
ment patterns vary structurally according to skin location,
age and physiological state of the fish10. Both melanopho-
res and xanthophores are dendritic cells having innerva-
tions from preganglionic nerve fibres originating from the
15th vertebra of the spinal cord11. Ultrastructure of
melanophores (as revealed by electron-microscopic stud-
ies), shows them enclosed within a single cell membrane
encompassing melanosomes and other cell organelles
(Figure 1). Most of the observations favour the view that
melanosomes are selectively moved through the cellular
processes, leaving the cell contour rather fixed12.
The motile activities of the chromatophores are con-
trolled mostly by electrical signals or through adrenergic
monoamines and the stimulation of
α
-adrenoceptors by
catecholamines. The process is accompanied by an in-
crease in intracellular Ca2+ levels, which in turn trigger
the aggregation of melanosomes13. Paracrine factors such
as endothelins (ETs) are also reported to be involved in
these processes14. Recent studies on stripe formation in
zebrafish reveal existence of a complex interplay between
the pigment cells in which iridophores promote and sus-
tain melanophores and attract xanthophores, whereas xan-
thophores repel melanophores. Stripe formation, though
initiated by iridophores appearing at the horizontal myo-
septum, gradually becomes a self-organizing autonomous
process15.
Environmental factors affecting colour development/
colour change in fishes
Colour change in fish can be broadly categorized into two
basic categories: (i) physiological colour change, a com-
paratively faster process which is evident within seconds
(attributed to rapid motile response of the chromato-
phores), and (ii) morphological colour change, involving
the change in morphology and density of chromato-
phores16. This can be further elucidated by the fact that
fishes adapted to a darker background have a greater
number of melanophores concentrated at the head and
dorsal trunk region stimulated by the action of melano-
cyte stimulating hormone (MSH), secreted from the pars
intermidia of the pituitary. In contrast, fishes adapted to a
lighter background have higher number of dopa-positive
melaoblasts corresponding to the enhanced secretion of
melanocyte concentrating hormone (MCH), released from
the pars nervosa, favouring effective camouflage mechan-
isms to evade predation. Such dispersion or contraction
of melanophores is controlled through a cAMP-mediated
protein kinase A pathway and Ca2+ ions located within
the melanophore17,18. In an experiment with rainbowfish
(Melanotaenia australis), Kelley et al.18 showed increase
in area and brightness of colour (especially red coloura-
tion) when reared in an environment rich in dissolved
organic matter. These fishes also moved in lightly packed
shoals with a probable explanation that red colouration,
being least diffractive, could attribute to conspicuousness
of colour patterns leading to better communication bet-
ween individuals in visually compromised environments.
This ability of changing colour may have a suppressive
impact on selection also, as the chromatic plasticity
evades natural selection methods of elimination through
predation or natural mortality19.
Strategies for colour enhancement
Several viable strategies for colour enhancement of
ornamental fishes (to earn a competitive edge in the
growing market) include dietary supplementation of col-
our enhancers, genetic manipulations and injecting dyes
in the subcutaneous layers of fishes (commonly known as
juicing or painting). Nutritional strategies remain the
most widely used method of colour enhancement, where
fish diets are supplemented with colour enhancers rich in
carotenoids (mostly microalgae, plant/animal sources and
synthetic derivatives). Figure 2 provides a summary of
the inter-relationship between various factors instrumen-
tal in bringing out efficient pigmentation in ornamental
fish.
Nutritional strategies for colour enhancement
Carotenoids: structure and function: This may be consid-
ered as the most potent route for colour enhancement,
owing to the simplicity of the method and eco-friendly
nature of such practices. Carotenoids are non-nitrogenous
fat-soluble pigments synthesized from geranyl diphos-
phate (GPP) by all photosynthetic organisms20. Photosyn-
thetic plants can synthesize lycopene and
β
-carotene and
in their biosynthetic pathway, lycopene is converted to
β
-
carotene, which in turn is further metabolized to astaxan-
thin, which is a non-plant carotenoid21. Dietary carote-
noids play an important role in the regulation of skin and
muscle colour in fish. Over 750 known natural carote-
noids have been described since the structure was first
elucidated by Kuhn and Karrer in 1928–1930 (with a
basic structure of tetraterpenoids having a carbon back-
bone of 40 carbon atoms), and are broadly divided into
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CURRENT SCIENCE, VOL. 119, NO. 7, 10 OCTOBER 2020 1095
Figure 1. Teleost chromatophore showing (a) aggregated and (b) dispersed state.
Figure 2. Relationship among various factors bringing out effective pigmentation in fish (modified from Diler and Dilek95).
carotenes (comprising carbon and hydrogen) and xantho-
phylls, which are oxygenated derivatives of carotenes22,23.
They can exist either in free or esterified forms, or in
association with protein as keratin as in salmonid eggs,
birds and ornamental fishes20.23.24. Fishes, unlike other
animals, do not possess the ability to biosynthesize caro-
tenoids de novo, but can modify alimentary carotenoids
and store them in the integument and other tissues20.
Farmed fish feeding on formulated diet has no or little
access to carotenoids and therefore, the necessary carote-
noids must be added to their diet. For example, astaxan-
thin supplementation in the diet of cultured snapper,
Pagrus pagrus and goldfish, Carassius auratus led to
improvement in redness of their skin, which however,
diminished over time in the absence of astaxanthin sup-
plementation25.
The effectiveness of carotenoid sources in terms of
deposition and pigmentation is species-specific26,27. This
is usually attributed to the pigment conversion ability of
the fishes. For example, goldfish converts the dietary
yellow pigment zeaxanthin to a more conspicuous red
astaxanthin28, while trout, Oncorhynchus mykiss converts
astaxanthin to zeaxanthin29. The third variety is seen in
red sea bream, Pagrus major, which does not convert
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xanthophylls to either canthaxanthin or astaxanthin30. The
most effective carotenoids used for the colour enhance-
ment of ornamental fishes are astaxanthin and canthaxan-
thin, alone or in combination31. In addition to colour
enhancement, these also function as antioxidants and
immunostimulants, triggering interest in research32,33.
This is in agreement with the carotenoid trade-off hy-
pothesis, according to which animals that display carote-
noid-based signals should experience a trade-off when
allocating carotenoids between physiological and pig-
mentation demands34. Carotenoid demands for develop-
ing immunity and antioxidant protection can reinforce the
intensity of carotenoid-based signals leading to enhanced
mating success. Females have a greater demand for caro-
tenoids than their male counterparts during the active re-
productive phase for allocating carotenoids needed for
the formation of yolk protein in their eggs35–39.
Application of carotenoids for colour enhancement: Plas-
ticity of skin colouration of ornamental fishes and their
susceptibility to dietary manipulations has sparked re-
search interest in this field since the 1970s, starting with
the phenomenal work of Tom Lovell and his team using
marigold petals as carotenoid supplement in the diet of
tiger barb40. This paved the way to the search for more
potent carotenoid-rich natural ingredients such as
spirulina, marine algae, crustacean exoskeleton, etc. with
dietary levels varying from 4% to 20% in the feed41–44,
microalgae (Haematococcus, Arthrospira, Dunaliella,
Chlorella, Chlorococcum, Leptolyngbyatenuis, Nostocel-
lipsosporum), cyanobacteria containing high amounts of
pigments (~3–5% of biomass)45 as well as several macro-
algae such as Porphyra, Gracillaria and Palmaria46,47.
During the course of time, research interest inclined
towards finding alternatives or rather unconventional in-
gredient sources such as yeast (Rhodotorula sanneii),
chesnut flowers48, paprika and red pepper (Capsicum an-
num)49, carrot (Daucus carota)50, marigold petal (Tagetes
erecta)40, China rose petal (Hibiscus rosasinensis)51, rose
petals (Rosa chinensis)52, Ixora coccinea, Crossandra in-
fundibuliformiss53, apple peel meal54 and oleoresin obtained
from marigold flowers55. Marigold petals are also a rich
natural source of lutein, beta carotene and xanthophyll in
esterified forms of palmitic and myristic acids56. Crusta-
cean wastes like shrimp head and crab offal also yield
considerable amounts of carotenoids and astaxanthin, and
are thereby finding increasing use in fish feed for colour
enhancement as also the pharmaceutical industry57. While
immediate effects of carotenoid (as an antioxidant)
appear positive, a potentially negative effect of its high
doses could lead to high plasma creatine kinase, indica-
tive of increased breakdown of skeletal muscle58. There-
fore, carotenoproteins (carotenoids with protein
complexes) having enhanced digestibility and stability
(than carotenoids) may be used in low doses to express
beautiful colours, different from the pigment itself59.
Carotenoproteins from shrimp shell waste have been re-
searched as the most potent in fish diets owing to their
superior 2,2-diphenyl 1-picryl hydrazyl radical scaveng-
ing activity, ferric reducing antioxidant power assay and
2,2-azinobis (3-ethylbenzothiazoline-6-sulphonic acid)
diammonium salt radical scavenging activity60. Again,
free forms of carotenoids have been argued to be better
utilized by fishes61. This paved the way for use of syn-
thetic astaxanthin in fish feed for colour enhancement,
especially in ornamental fishes (Table 1). Considering its
indiscriminate use and easy availability, safe limits of
astaxanthin are considered to be 100 mg/kg by the US
FDA for salmonids, rainbow trout and crustaceans62. In
addition, astaxanthin doubles up as a potent antioxidant
to boost immune responses in crustaceans, specially
shrimps63–65.
Genetic manipulation as a strategy of colour
enhancement
Constant jeopardy of colour loss, need for technical
expertise in carotenoid supplementation and escalating
improbabilities arising due to dependent and independent
variables, led to the development of genetic means of
colour enhancement, which were backed by advantages
of being stable and heritable. Inheritable traits were
emphasized for developing colour patterns in ornamental
fishes solely based on the findings on inheritable genes
linked to varied colour patterns in guppies (Poecilia re-
ticulata). Since Winge66 attempted to compose possibly
the first genetic linkage map among vertebrates, fishes
from family Poeciliidae have been a model for the study
of evolution, ecology, behaviour, tumour genetics and
genomics, but most importantly, for sex-linked inheri-
tance of a variety of masculine ornamental traits impor-
tant for sexual selection and adaptation in natural
populations67–69. Extensive genetic research elucidated
that guppy has an XY sex-determination system, wherein
the Y-chromosome harbours male sex-determining locus
in tight genetic linkage to masculine ornamental genes69.
Interestingly, recombination rates in the swordtail
(Xiphophorus spp.) do not show any sex-specific procli-
vity, unlike zebrafish70.
With advancements in transgenic technology, the fish
became a good model for transgene induction in the mid-
1980s owing to its easy husbandry practices and potential
for fast growth. Although the first transgenic fish was
produced in 1985 (ref. 71), initial research attempts suf-
fered redundancy in real-time data due to non-availability
of promoters from homologues species and laborious
work involved in the analysis of classical reporter genes
such as CAT (catecholamine transferase),
β
-galactosidase
and luciferase. Successful demonstration of fluorescence
reporter proteins, such as green fluorescent protein
(GFP), enhanced green fluorescent protein (EGFP), and
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Table 1. Astaxanthin supplementation in ornamental fishes
Fish species Astaxanthin supplementation (mg/kg) Reference
Penaeus monodon (Tiger shrimp) 50–100 63
Carasius auratus (Gold fish) 45 89
Hyphessobrycon callistus (Serpae tetra) 40 90
Colisa lalia (Dwarf gourami) 100 91
Cichlasoma citrenellum (Midas cichlid) 160 92
Premnas biaculeatus (Spinecheek anemonefish) 214 93
Amphiprion ocellaris (Common clownfish) 80–160 94
subsequently, blue (BFP), cyan (CFP) and red, opened
new horizons in live cell imaging for confirming trans-
genic technology in live embryos and cells without sacri-
ficing the fish for fluorescence microscopy71. Transgenic
status can now also be confirmed with immunohisto-
chemistry using confocal microscopy and fluorescent in
situ hybridization to narrow down cellular arrangements
for induced colouration. Colourful as well as fluorescent
varieties of zebrafish and medaka have been generated
using this technology, adding luxurious hues over physio-
logical colourations. One of the pioneering attempts was
that using muscle-specific gene, mylz2, and a skin-
specific gene, keratin 8 promoters for GFP and develop-
ment of a transgenic zebrafish showing green fluores-
cence bright enough to be observed under daylight.
Subsequent attempts also yielded fruitful results with red
and yellow fluorescent proteins71–74. This became the
most successful commercial application of transgenic
technology, marketed under trade name of ‘Glo Fish’
after sufficient media stir regarding ANDi (the first
transgenic primate), and Alba (the transgenic GFP rab-
bit)75. With advancements in transgenics, use of CFPs
like AmCyan1 (mutant version of the CFP, amFP486
having excitation spectra 458 nm and emission spectra
489 nm) has been attempted for displaying a yellow–
green colour under normal daylight or white light, a bril-
liant green fluorescence under ultraviolet light and a
cyan-like fluorescence under blue light (from a light-
emitting diode) in marine medakas (Oryzias dancana),
showing promising results and scope for further stu-
dies76,77. Various studies have expressed concern over the
release of such transgenic fishes in public water bodies,
foreseeing ecological orchestration that may result from
such introductions78–80.
Subcutaneous injection of pigments and dyeing
as strategies for colour enhancement
The booming ornamental fish trade gave way to many
unethical practices of colour enhancement, foremost
being painting, more commonly known as ‘juicing’81,82.
This practice started in the late 1970s with glassfish (Pa-
rambassis ranga), whose glassy or transparent appear-
ance presented a perfect canvas for subcutaneous
injection of dyes to earn a better price83. Studies have
shown that such subcutaneous injection of industrial
synthetic dyes is traumatic to an extent that it may be
fatal83,84. Furthermore, the process of colour injection
itself is gruesome, wherein numerous wounds are in-
flicted on tiny fish via needles on the dorsal and ventral
musculature, making them immunocompromised82. Other
methods of colour enhancement are dipping the fish in
caustic dyes (cause sloughing-off of the natural pigmen-
tation pattern), superimposing the intended colours on the
fish body. These caustic dyes damage the first barrier of
defence, i.e. skin, destroying lysozymes and other immu-
noreactive molecules thus making the fish susceptible to
pathogen attack85. There have been enough protests to
deter people from buying such painted fish, suggesting
the need of a legislative ban to be imposed on such type
of heinous techniques. However, conclusive action is
awaited, keeping avenues open for moral and legal police
to do their part.
Conclusion
Research on colour development and enhancement has
traversed a long journey and future research may provide
answers regarding the effects of abiotic parameters in
governing colouration. Bio-reporter genes may be in-
strumental to indicate a shift in the colour in response to
the desirable titre of the chemical or concerned pollutant.
Caution should be exercised during use of these tech-
niques following ethical and legal guidelines, ensuring
their sustainability. Strategies for inducing selective
fertility in transgenic fishes (on/off strategy) and other
reproductive containment techniques are essential to
prevent unwanted integration of the transgene into non-
target organisms86.
Insights from research on the role of micronutrients
(calcium, iron and zinc; enhancing the plumage colour
pattern in zebra finches) provided another avenue for a
study87 warranting attention of fish biologists on their
potency in augmenting general health status and disease
resistance88. However, their roles in colour enhancement
still remain unexplored. Future research must ensure total
abstinence from unethical painting/juicing techniques,
and should be directed towards creating a mass awareness
against the ill-effects of such heinous practices on fish
health. Menaces like ornamental fish colour pollution
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should be discouraged at its advent to safeguard the inter-
ests of stakeholders and native biodiversity. There should
be enough considerations to protect the feral exit of such
colour-enhanced fishes into the wild, which might pro-
duce risks of unwanted inheritance of colour patterns and
problems of identity crisis among conspecifics. Although
the nature of inheritance of colouration and pigmentation
remains poorly studied in fishes, future techniques, if
harnessed with caution, are bound to bring a global ‘rain-
bow revolution’ to the ornamental fish industry.
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Received 8 June 2019; revised accepted 23 July 2020
doi: 10.18520/cs/v119/i7/1093-1100
