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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". These are the same types of cells present in some invertebrates: crustaceans, insects, cephalopods, and are found in other cold-blooded vertebrates (reptiles, amphibians).
The first to review the literature on the physiology and behavior of animal color (Animal Color Changes) was Parker (1948). This work was further compiled and added to by Bagnara and Hadley (1973) in the third volume of Fish Physiology: Chromatophores and Pigments. Importantly this last work covers how fishes use their chromatophores in a variety of responses to internal and external stimuli.
Physiology of Color, Change:
Delving through even recent literature one will find some disagreement re defined terms, but the following terms are generally agreed upon. A single color cell is a chromatophores, a group of chromatophores a chromatosome. Chromatophores are defined on the basis of the pigments they contain:
Principal Types of Chromatophores and Properties
Types of Chromatophores:
Color and its change in fishes is a matter of colors already present being manifest, clumped or masked.
Of the most common types of chromatophores are melanophores. These contain black or brown colored melanin crystals. The degree or intensity of darkness of these cells and hence the fish they're found on depends on the amount of dispersion of the melanin pigment within the cell. Intracellular migration and aggregation is the fundamental chromatophores activity. When the pigment is dispersed widely throughout the cytoplasm, the skin macroscopically appears darker (black or brown). When the pigment granules aggregate or contract the cell loses its darkness.
Some examples of this loss of dark color are the dark bands on freshwater angels (Pterophyllum) and sunfishes (Lepomis) that can quickly blanch from dark to light or come back again given fright or excited states.
Other colored cell types include xanthophores and erythrophores. These chromatophores are generally slower to change. An example of these cells can be understood readily by looking at a swordtail (Xiphophorus).
Studies of skin pigment cells in recent years have helped our understanding of pigment biology. Two important concepts have become established re the function of chromatophores re the dermal unit and the epidermal melanin unit. The epidermal unit is only concerned with morphological color change (in warm and cold-blooded animals). The dermal chromatophores unit contains three layers:
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.
Chromatophore Responses: Physiological (Nervous, Hormonal) and Morphological
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. We'll discuss these states a bit later.
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. This change can take from scant seconds in the Killifish Fundulus, to about twenty days in the European Eel (Anguilla). The significance of the different time intervals in color change is that it is related to the mechanism of change. When the time for the change exceeds two hours, hormonal coordination is indicated. Alternatively, a change in the span of ten minutes of less is likely largely due to neuron-control.
Physiological responses are defined as either primary or secondary and as stated are moderated by nervous and endocrine system control. Primary color changes generally occur through routes other than the eyes, i.e. by direct action of light on the chromatophores themselves (known as extraocular reflex). Primary responses are functional before the eyes are operative in embryonic and larval stages. Secondary responses depend on the nature of the environmental background and not specifically on the intensity of ambient light. Here the degree of pigment dispersion is determined by the ratio of the amount of light directly incidental to the eye to the quantity and quality reflecting on the eye from the background.
Though color and its change are mediated physiologically by the nervous and endocrine systems, they are determined by direct environmental influences like pH, salinity, and temperature dispersing pigments without the fish's direct control. Hence a third type (morphological) of color and pattern control exists in addition to hormonal and nervous.
Morphological Color Changes:
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. These changes involve the synthesis and destruction of large amounts of pigment through a few methods:
More often than not, morphological color changes are actionable with the other types of color change. For example, a fish placed on a white background will contract the pigments in its cells first physiologically (under nervous impetus), and then slowly lose its morphological color.
Research has shown that the pituitary gland, through its production of melanophore stimulating hormone (MSH) participates in morphological color changes. Further, that if an animal is sensitive to both physiological and morphological changes by the injection of MSH then the morphological process will be slower and more permanent, perhaps irreversible.
Nervous/Neural Mechanisms of Chromatophore Control:
There is good evidence that melanophore control by advanced bony fishes is principally actuated by the autonomic nervous system. Sharks and other cartilaginous fishes colors are apparently not controlled by nervous mechanisms, and consequently do not change rapidly.
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.
Hormonal Mechanisms of Chromatophore Control:
Hormonal color changes are slower, harder to observe and more subtle typically. Several "chemical messengers" (hormones) affect color. In fishes, all these are carried by the blood system (endocrine means "to cry within"). Most arise fro the pituitary, although they can be produced in the hypothalamus, pineal, thyroid and other endocrine glands. A comment for those unfamiliar with the structure and physiology of the "lower vertebrates" called fishes: they share about the same organs, chemicals and pathways as "higher ones" like ourselves.
For sure the "less advanced fishes", the Lampreys and Hagfishes and Cartilaginous fishes (sharks, rays, skates, chimaeras) do not have neurological color control. Instead MSH produced by their pituitaries is responsible of pigments. "Higher fishes" (teleosts) have a slightly different structure to their MSH, resembling that of mammals.
Melanophore Stimulating Hormone: The most abundant and studied, though not the only hormone affecting color change in fishes is MSH (or Intermedin in older literature). A similar hormone MCH, Melanophore Concentrating Hormone has the opposing effect of lightening individuals.
Estrogen: We'll mention as many fishes exhibit "sexual dichromatism" or difference in color associated with sex due to this hormone and other sex hormones. Wrasses (family Labridae) are good examples here. Other sexual hormones of fishes include Methyl Testosterone, Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH). These all influence color in fishes though not as pronounced or widespread as estrogen.
Functional and Behavioral Significance of Color, Changes
Behavior of Color:
Apart from its physiology, the study of the function of color in fishes is a rapidly expanding scientific field of study. As presently understood and classified, color falls into three general categories of function:
Of course, there are other ways to classify color. For instance a classification scheme might measure the amounts and variability of changes therein. One group here would be those fishes with a fairly stable color pattern, like the jawless fishes and sharks. Though there are sharks (e.g. the genus Carcharhinus) that show color change diurnally. Fish with numerous iridophores often do not exhibit elaborate color changes (they're "shiny" all the time). The Tinfoil Barb (Barbonymus schwanenfeldii) is a good example here.
Another scheme of arranging fishes by color would be by how they do so with their behavior. Most fishes do change color by one or more of the methods discussed earlier.
This is the most widespread and important type of coloration. This really makes sense when you consider the importance of not being seen to avoid being eaten. Camouflage coloration is accomplished in a few ways.
a) The most common type of coloration effect occurs in most, but particularly pelagic fishes, counter-shading. This is the matching of reflectivity from below and above with ambient reflection. Melanophores cover the topside of the fish, making it darker, and iridophores start about halfway down their sides of the body, increasing in density until almost solid underneath the fish. Next time you see a show with tunas, jacks swimming in the open sea, or visit a public aquarium displaying herrings or anchovies, look for this.
b) Deepwater fishes have bizarre and unusual coloration. Most are black or red, with the red usually being very bright. As the red-end spectra of sunlight are the first to be selectively absorbed in the upper water of the sea this red coloration is a mystery. It could be that the color is in some way related to storage of oxygen.
c) Cavefish of a few species live in worlds w/o light and have lost their pigment through evolution. Most have lost their eyes as well and use other senses to navigate. The blind Cave Tetra (Astyanax jordani) is a fish sold in the aquarium trade that is almost devoid of pigment and totally blind.
d) Matching camouflage is extremely widespread, especially in the tropics. There are several excellent examples of fishes that have active color patterns that match their immediate environment. Some fishes even have a repertoire of several adaptive color patterns. George Barlow (1981) recognized up to eleven such color patterns in Badis badis. Most of these patterns were found to match different surroundings.
Another classic example for pet-fish keepers is the pencilfish Nannostomus beckfordi that has a twenty-four hour rhythm of day and night coloration. A horizontal band runs the length of the body during the day while at night three vertical bands are visible to match the water plants it hides in during these hours. It was found (Abbott 1973) that this fish did not depend on light intensity or vision at all and that blind fish still went through the diurnal matching periods.
The vertical bands of freshwater angelfishes (Pterophyllum) also match vertical bands of plants, and lighten and darken with the presence of such physical objects.
e) Transparency. One of the more clever forms of concealment is to be transparent. Indian Glassfish (Parambassis ranga) and Glass Catfish (Kryptopterus bicirrhis) are two good examples of fishes that use this scheme that are common in the ornamental fish trade. This is obviously a good strategy for avoiding predators that rely on sight.
f) Another widespread color scheme is termed disruptive coloration. These markings serve to distract a predator, the fish thus creating a diversion. False eye spots as in many of the marine butterflyfishes (Chaetodontidae) or the Oscar (Astronotus ocellatus) are examples of disruptive coloration, potential predators either scared away or attacking this area allowing the would-be prey to escape. There are many tropical marine fishes with types of disruptive coloration.
Advertising is another function of color and color changes in fishes. In such large and sometimes dark, always relatively dense environments as the waters of the world advertising ones presence, species identity and possibly sex are critical.
a) An excellent example of bold advertising off the U.S. California coast is the Garibaldi (Hypsypops rubicunda). The Garibaldi uses its bright orange coloring as an advertisement to others of its own kind to defend its territory. These territorial boundaries are set by sight'¦ becoming smaller in more turbid water, and larger in more clear water.
b) Recognition is a very important function of coloration. Both pattern and color are often used in different types of recognition.
c) Another example of advertising involves color and cleaning symbiosis. Cleaner fishes (e.g. Labroides wrasses) are brightly colored so they will attract larger fishes that want to be cleaned. This "signal releaser" coloration also advertises that the cleaner is not to be ingested as prey. Some other species of fishes utilize this protection (as mimics) like Aspidontus, the saber-tooth blenny.
This is the third major type of coloration and change used by fishes as means for survival. Some species of fishes utilize the looks (and behavior) of others as a means to "get by" or get a meal. An example is a mimic of the common Labroides wrasse Labroides dimidiatus, Aspidontus, the Sabertooth blenny, that uses matching coloration for protection from predators and to gain proximity to hosts as food.
Other Uses and Responses of Coloration:
There are several "secondary" apparent uses, results of coloration and its change:
1) Thermoregulation. Some fishes are notably darker in the early morning hours, lightening up with increased temperature, even with loss or constant illumination, and darkening on one side if the water is warmed there.
2) Environmental Responses. Most fishes have the capacity to adapt to some extent to changes in the shade, color or pattern of their environment.
a) Response to background has been widely observed. The best example here are some of the flatfishes (Pleuronectiforms) that can adjust their body markings, even to checkerboard, to match the substrate. A curious observation is that "practice" shortens the span of time for these matching.
b) Response to darkness. This is a varying of coloration based on direct illumination on the animal.
3) "Psychic Response". Reactions by fishes from being handled or darkening when angry (think of a red devil cichlid (Amphilophus) or triggerfish (Balistidae) by what it observes outside.)
Control of coloration has been found to be a matter of the presence and concentration of pigment and reflective elements in types of cells called chromatophores in fishes. These colors and their change are under environmental, hormonal and neural control. There are differing classification schemes for lumping/splitting color functions and means, but there is no doubt that such markings and abilities to change are of great survival value and multiple functions.
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