Pigment in goldfish is found in
pigment-producing cells called chromatophores,
including xanthophores (containing yellow pigment)
and erythrophores (containing red pigment). When
fish are outdoors, they will receive ultraviolet
light (including UVA and UVB light) from the sun.
UVA and UVB are both high-energy light, and can be
harmful to organisms, however, pigments can
provide some protection from this, and ultraviolet
light stimulates the chromatophores to produce
more pigment. Artificial lights usually only
provide a limited spectrum of light compared to
sunlight and usually do not produce ultraviolet
light. This means that over time, goldfish
chromatophores exposed to artificial light will
produce less pigment and the goldfish will look
paler in color.
The UV light not only impacts the amount of
pigments produced, but also the density of the
pigments or how spread out in the cell they are,
which also affects the coloration of the fish.
This response is pretty rapid, so you might notice
that when you turn on your fish tank light in the
morning, the fish may look paler, but will look
more vibrant later in the day.
The coloration of the fish is also due to which
pigments are expressed in which combinations, so
the type of artificial light may impact the
balance of the pigment color types and therefore
the overall color of the fish over time. The food
the goldfish eats will also affect its color; most
goldfish diets contain natural pigments called
carotenoids and the amount and type the fish eats
will impact the color of the chromatophores.
The longer a fish is away from natural sunlight,
and the type of artificial light it receives, as
well as the type of food it eats, will all impact
the degree the color of the fish will change, and
how long this process will take. As the
chromatophores die, if they are not replaced by
other similar chromatophores, the color of the
fish will change. These effects can be observed in
some fish in less than 30 days.
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