Many things can affect fish coloration including water clarity and chemical composition, bottom color , mating , light penetration and intensity . Color is often affected by the current situation such as the fishes status as predator or prey, aggressive or passive, agitated or calm, angered or under attack etc. Their coloration scheme is often divided in categories such as warning coloration , cryptic coloration , disruptive coloration , or countershading . Much of coloration is controlled by cells called chromatophores and iridophores which react to the various stimuli.
A fascinating subject and probably more than anyone wanted to know about fish color including changes. As I read the description I could picture the changes occurring just as I have seen it many times.
http://www.wetwebmedia.com/AqSciSubWebIndex/coloration.htm http://www.clarku.edu/departments/biology/biol201/2002/CSantos/Colored%20Chromatophores.htm Long before aquariums we’ve known that fishes change color in response to their background, and that they change color during exercise and courtship. These changes in appearance are under the control of pigment containing cells called "chromatophores".
Some examples of this loss of dark color are ... sunfishes (Lepomis) that can quickly blanch from dark to light or come back again given fright or excited states.
Chromatophore changes can be divided into two categories, morphological and physiological. Morphological changes are usually evoked by maintaining an organism in a given setting, on a specific background for a number of days.
Physiological color changes involve alteration of pigment granules causing dispersion or aggregation consequent to various stimuli, e.g. light, temperature, chasing.
The control of aggregating and dispersing of pigment granules is caused by changes in the chromatophores ionic charge. A change of charge within the cell causes a change in color. There are two ways to change the ionic equilibrium within chromatophores, hormonal and neural. Both "paths" are often employed, one working more gradually, the other more immediately. For example, the time required to change from light to dark varies immensely.
There is good evidence that melanophore control by advanced bony fishes is principally actuated by the autonomic nervous system.
There are two principal chemicals that are produced and release by neurons (neurohormones) that affect color.
Epinephrine (Adrenalin): A nerve-activated hormone that’s released by an organism when it is excited or scared, causing pigments to contract and the animal to blanch, lose color.
Acetylcholine: A chemical that is active in muscle tissue, movement, almost always causing melanin to disperse, darkening the organism.
Morphological color changes are due to amounts of pigment present in the chromatophores of an organism. Morphological changes occur very slowly, generally over the course of a month or more, and are usually permanent.
Types of chromatophore are characterized by the color they carry. Erythrophores contain reddish pigments found in carotenoids and pteridines. Melanophores contain black and brown pigments called melanin. Xanthophores produce yellow pigments in the form of carotenoids. Fish are capable of producing some pigments, but others must be supplied in the diet. For example, they cannot produce carotenoids naturally. They accumulate carotenoids from their diet and transfer them into pigment cells to produce red, yellow, and orange colors. The intensity of the pigment is reliant on the quantity and types of carotenoids supplied in their food. The carotenoid pigment found in most marine invertebrates is astaxanthin. Another pigment that is derived from a food source is phycocyanin. This pigment is blue and is readily found in blue-green algae. Additionally, the ability of fish to store pigments they have acquired from their diet will greatly affect their appearance.
Various hues are made possible by the combinations of different layers of chromatophores. Cells carrying more than one pigment are called compound chromatophores. Most fish that appear to have green coloration on their scales actually have a layer with yellow pigment and another layer on top that scatters light and reflects a blue color. There are other types of chromatophores that do not retain pigments .
Note that there are two types of pigments true or based on color and reflective. Also note that not all chromatophores contain the actual pigment color that they appear. That is, that some work on different principles other than selective absorption/reflection. The iridophores contain quanine crystals that reflect different wavelengths of light, which give them an apparent color though no true pigment is present. Further, there are two types of iridophores or reflecting pigment cells, ones with decidedly larger and smaller quanine crystals. The larger crystals can change their orientation to reflect different colors of light. Cells with the smaller crystals can aggregate or disperse their pigments thereby controlling the intensity of color.
Because iridophores are typically light in color, the effect of dispersion and contraction is opposite that of melanophores. When quanine crystals are aggregated, the cell appears darker. The plate-like crystals give off iridescence as seen on the top and flanks of many fishes (e.g. Silver Dollars, Metynnis, Mylossoma, Anchovies, Engraulis).
Green, gold, red, blue and many other colors can be reflected selectively by iridophores. The iridescent blues of Neon Tetras (Paracheirodon innesi) are a result of quanine crystals for instance, and not blue pigmentation.
Fishes also utilize combinations of pigment cell types, with iridophores and melanophores mixed.
Much more in the links.
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