Coloration is controlled by the endocrine and nervous system, but
dietary sources of pigment also play a role in determining color in
fishes. The endocrine and nervous system both influence coloration in
fish. The pituitary gland secretes hormones that direct the production and
storage of pigments throughout the life of a fish, and particularly as
maturity is reached. Pigment production and storage often increases at the
onset of maturity. Many species use color to provide camouflage and
attract a mate. Fish of the family Cichlidae are particularly known for
brilliant coloration of mature males. The autonomic nervous system directs
rapid color changes in response to stimuli such as a predator or an
aggressive tankmate. Anyone who has observed fish knows this color change
can occur at a spectacular rate.
Xanthophyll pigments are responsible
for the yellow color of the popular cichlid, Labidochromis caeruleus.
Specialized pigment containing cells called chromatophores are located
beneath the scales. These cells are branched, permitting pigment granules
to be near or away from the surface and aggregated or dispersed. These
cells are the reason for the variable and sometimes rapid changes in fish
color. Additionally, colorless purine crystals are contained in
specialized chromatophores called iridophores. These crystals are too
large to move in the iridophores but are stacked to provide a reflecting
surface and the base or structural coloration of fishes. The iridophores
are responsible for the silver sheen, particularly of small pelagic fish.
These cells are capable reflectors of light and are responsible for the
counter shading effect where fish appear darker when viewed from above and
lighter when viewed from below. This mechanism helps detour predation.
Pigments are characterized by their colors. Carotenoid pigments are red
and orange. Xanthophylls are yellow. Melanin pigments are black and brown.
Phycocyanin is the blue pigment derived from blue-green algae. Cells
containing yellow pigments overlying those containing blue pigments can
produce green hues. Fish are capable of producing some pigments, but
others must be supplied in the diet. Black and brown pigments are produced
in cells called melanocytes. Fish are incapable of producing carotenoid
and xanthophyll pigments. Therefore, these must be supplied in the diet.
Spirulina algae is a source of
pigments to enhance blues.
Natural sources of pigments are available in the diets of most fish.
Color enhancing diets may contain additional natural pigments to enhance
colors of ornamental fishes. The carotenoid pigment found in most marine
and a few freshwater invertebrates is astaxanthin. This pigment gives the
characteristic color to the flesh of salmon and is available in the diet
of aquarium fish in shrimp and krill meals and salmon (fish) meal used as
sources of protein in some feeds. Pure astaxanthin or canthaxanthin
(synthetic astaxanthin) may also be added to fish feed to enhance red and
orange coloration. These carotenoid pigments are often added to feeds for
farm raised salmon and trout to give fillets a desirable red color.
Xanthophylls (yellow pigments) are found in corn gluten meal and dried egg
that may be added to the diet to enhance yellows. The ground petals of
marigold flowers have also been used as a source of xanthophylls. The
blue-green algae spirulina is a rich source of phycocyanin and may be
added to a diet to enhance blue coloration. The expense of supplementary
pigments often limits the amount used in tropical fish feeds. These
natural sources of pigments are in contrast to several methods routinely
used to enhance colors of ornamental fish.
A diet rich in carotenoid pigments
will help this Labeotropheus trewavasae maintain the brilliant
red-orange hue.
A discussion of enhancing colors of ornamental fish would be incomplete
without mention of dyeing and painting fish, and feeds containing
hormones. The practice of painting essentially colorless fish (e.g.
glassfish) has become widespread. The neon colored paint is non-toxic, but
the handling and painting, coupled with shipping stress often invites
disease problems. These fish often contract ich (Ichthyophthirius
multifilis) and fungal infections. The paint is shed in time and the
fish returns to being colorless which may be more disturbing to someone
paying a premium for "painted" fish. Dyeing colorless fish has
recently become popular. The fish are immersed in water containing dye and
the immersion and handling may lead to the aforementioned disease
problems. Hormones may be used to enhance fish coloration by causing a
false early maturity. Testosterone supplied in the diet likely allows a
premature storage and expression of pigments in the chromatophores. Fish
that often exhibit drab juvenile coloration may then show full adult
coloration. Fish treated with hormones often become all male, sterile, and
require a continuous dietary supply of hormones to maintain coloration.
The sex of juvenile fish is often ambiguous and hormone diets, most often
containing testosterone, create all male fish. Uncontrolled doses of
testosterone sterilize fish. Endogenous production of hormones ceases, so
coloration is not maintained when fish are taken off the hormone treated
feed. Fish feeds containing hormones do have legitimate commercial uses in
Tilapia (Oreochromis spp.) diets (Teichert-Coddington et al. 2000).
Tilapia growers are hampered by the fact that this cichlid often matures
prior to reaching market size. The fish farmer often ends up with mixed
size classes and stunting of fish in growout ponds if the tilapia are
allowed to mature and reproduce. Feed energy also goes into producing
gametes instead of fish flesh. Feeds containing hormones have been used to
provide all male groups of tilapia for growout. These diets contain
testosterone since males grow faster. The feed is administered to juvenile
fish prior to growout and is currently undergoing FDA approval for food
fish. Given the current status, this feed is probably not widely available
to ornamental fish growers and hobbyists and would be of little use
enhancing color of fish already sold as adults which encompass most
ornamental fish with the notable exception of cichlids. There is no
specific way to tell if a fish has been fed a diet containing hormones
except to be vigilant of the fish you purchase. If it looks to good to be
true, it probably is!
Water quality may also play a support role in determining the color of
ornamental fish. Degraded water quality increases stress on captive fish
and may dull fish colors. A high quality biological filter and routine -at
least bi-weekly- water changes will provide an environment enabling fish
displaying their brightest colors.
Hobbyists may wish to experiment with their own color enhancing diet.
There are several recipes for gelatin-based feeds available in other
publications, notably Moe (1982) and Konings (1993). I would recommend the
protein portion of these diets (e.g. shrimp, fish, squid) be replaced with
salmon fillets. Salmon are a good source of carotenoid pigments that
enhance reds. Additionally, all essential amino acids will be supplied
using salmon as a protein source and the higher lipid content in salmon
will promote better utilization of the protein. The addition of high
quality pure spirulina powder will enhance blue pigments. This can be
purchased from aquaculture suppliers. Any gelatin based diet should be
stored frozen to maintain freshness and used within several weeks. Feeding
a varied diet rich in sources of pigments along with good water quality
will ensure captive fish develop vivid colors.
References
Moe, M.A. 1982. The marine aquarium handbook –beginner to breeder.
Green Turtle Publications, Plantation,
FL.
Moyle, P.B., and J.J. Cech. 1988. Fishes an introduction to
ichthyology. 2nd Edition.
Prentice Hall, Englewood Cliffs, NJ.
Fujii, R. 1969. Chromatophores and pigments. pp. 301-353 in W.S.
Hoar and D.J. Randall (eds.). Fish
Physiology. Volume III. Reproduction andGrowth. Bioluminescence, Pigments,
and Poisons. Academic Press,New York, NY.
Konings, A.(ed.). 1993. Enjoying Cichlids. Cichlid Press.
Teichert-Coddington, D., B. Manning, and J. Eya. 2000. Concentration of 17alpha-Methyltestosterone in hormone treated
feed: Effects of analytical technique,
fabrication, and storage temperature. Journal of the World Aquaculture
Society 31: 42-50.
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