Constantin Brancusi, Romanian sculptor (1876-1957)
What makes catching a mahi-mahi memorable? It's fighting spirit, of course, and the flashing displays of color. Does anything compare to these brilliant hues? From these striking colors, the mahi-mahi derives another of its common names: dorado (meaning gold, or golden, in Spanish). Beside this fish, even some of the bright fish of the tropics pale. But what makes the dorado, or redfish, or flounder, or any fish, colorful?
In their skin, fish contain specialized cells called chromatophores. There are two types of chromatophores: those that absorb light and those that reflect light. Chromatophores that absorb light contain pigments and include melanophores (black and brown), erythrophores (red), and xanthophores (yellow). Chromatophores that reflect light include leucophores (white) and iridophores (silvery).
These cells are responsible for fish coloration. The combination and overlapping of these cells creates the "overall palette of fish colors," a complete spectrum across the species.1 So if genetics are responsible for the variety of colors, why are several fish of the same species and gender different colors? That really dark red redfish must have had a little too much sun, right? No, many fish can actually change their colors (flamboyantly evident in the mahi-mahi). When a pigment-containing chromatophore expands, the pigment is pushed to the top skin surface, and the color is visible. When it contracts, the pigment is pulled to the center of the cell, away from the surface, and the color is not visible. So a dark red redfish has expanded most or all of the pigment in its skin, and a pale one has contracted all of it. Similarly, the yellow visible on the mahi-mahi is a result of the yellow spectrum being reflected by light-reflecting chromatophores; different spectrums can be reflected to produce different colors.
Why do fish change their color? Obviously not to match a new pair of shoes... No, there are a couple main reasons: camouflage (resembling the environment or other creatures) and advertisement (look at me! look at me!).
Camouflage is the most popular reason, and the purpose of blending in comes down to one relationship: predator vs. prey. If you're a tasty-looking flounder looking to avoid being a shark's mid-morning snack, it just won't do to wear your best black coat while lying around on a beige sand bottom. By turning a light, sandy color, adopting a grainy pattern to match the bottom, and stirring up some sand around its fins, the flounder can hide from the view of any impolite passerby. Similarly, if that same flounder wants to snag a passing shrimp to be its next meal, it uses a hidden element of surprise. And if the next day, the flounder camps out in a sketchier side of town, it can turn virtually any shade brown (including nearly black and nearly white) and mimic most patterns, even checkered, to hide out.2
A common type of camouflage employed in deeper waters is counter-shading. Since open-water fish, such as tuna, may have predators and prey both below and above them, they have to blend in from either view. This is accomplished by expanding their melanophores on top and capitalizing on iridophores underneath. From above, they match the darker depths, and from below, they match the silvery surface. The two types of chromatophores gradually merge on the fish's side.3
Advertisement is the second main reason for color change. This is when the fish wants to be seen. Poisonous fish often rely on being brightly colored as a warning, rather than trying to hide from predators. Colorful patterns can also specify territory disputes between neighbors, sexual readiness, or even job titles (such as cleaner fish). Male redfish, during spawning, turn dark red or bright bluish-gray above the lateral line and pale white underneath as a signal to the ladies: reproductive services now available.4
Of course, there are some other circumstances that cause color change, such as stress, pH balance, and salinity. A frightened fish produces adrenaline, which can cause pigment-containing chromatophores to contract, leaving a dull appearance.3 Temperature may have some effect, causing pigment to expand in response to cold and contract in response to heat.5 Deterioration of cells after death can also cause changes in coloration.6
That's the why. Now the how.
You know that color is caused by the contracting, expanding, and reflecting functions of chromatophores. But how are those cell actions triggered? Is it something the fish is aware and in control of? Could a flounder choose to be dark brown on a light tan bottom? No, there are two types of color change causes: physiological and morphological. Let's look at physiological.
There are two types of physiological color change responses: primary (controlled by the nervous system) and secondary (controlled by the endocrine, or hormonal, system). Both types occur quickly and are not permanent. Primary responses are a result of direct interaction between the chromatophores and light (no eyes involved). Light hits the fish's skin; chromatophores react. Secondary responses are a bit more complicated, involving the eyes and the environment around the fish. The number of expanded chromatophores is calculated by a ratio: the amount of direct light perceived by the eye to the amount and quality of light "reflecting on the eye from the background." 3 Plus, chemicals and hormones in the body, such as adrenaline or estrogen, will cause a chromatophores reaction. So the fish is not consciously causing these reactions.
Now the morphological causes of chromatophore change. Morphological color changes happen much slower, take much longer to complete, and are usually permanent. These changes are often inherent to growth, such as a flounder developing a white belly and brown back as it grows from larva to adult.7 Young trout have vertical or horizontal bar colorations on their sides as camouflage in their nursery stream habitat but will develop silvery sides and a blue or green back when they breach adult waters.8
The pigment itself comes from the fish's diet. Fish can't spontaneously create pigment in their skin; it's second-hand color passed down from what they've consumed in their environment: recycling in one of it's most beautiful forms.9 Some birds also make use of the pigment in their diet; roseate spoonbills have a pinkish color derived from the shrimp they feed on.
Whew, there's a lot going on behind the scenes in the mahi-mahi's show. Makes you appreciate the intricate details Mother Nature engineered in the amazing animals of our oceans (or bays). Now you can enjoy that beautifully dark red redfish without worrying about whether he has some sun block at home.
1 Jim Ames and Steve Schroder, "Chum Salmon Colors," Washington Department of Fish and Wildlife, 24 Jan. 2011 <http://wdfw.wa.gov/fishing/salmon/chum/chum_colors.html>.
2 "Flounder," How Stuff Works, 24 Jan. 2011 <http://animals.howstuffworks.com/fish/flounder-info.htm>.
3 Bob Fenner, "The Physiology and Behavior of Color in Fishes," Wet Web Media, 24 Jan. 2011 <http://www.wetwebmedia.com/aqscisubwebindex/coloration.htm>.
4 Dr. Charles Wenner, "Red Drum: Natural History and Fishing Techniques in South Carolina" (South Carolina Department of Natural Resources, 1992) 6, 16.
5 Dietrich C. Smith, "The Effects of Temperature Changes on Chromatophores" 200.
6 Dr. Lee A. Fuiman, Marine Science Institute, Email interview, 31 Jan. 2011.
7 "Biological Info: Southern Flounder," Louisiana Fisheries, 24 Jan. 2011 <http://www.lsu.edu/seagrantfish/biological/misc/southernflounder.htm>.
8 "Color, Light, Sound, and Electricity," ReoCities, 24 Jan. 2011 <http://reocities.com/RainForest/5530/fishes/ch19/ch19no02.html>.
9 Terry Cusick, "Where'd the Color Come From," North Florida Koi Club, 24 Jan. 2011 <http://www.nfkc.info/nfkconliine/Where%27d%20the%20color%20come%20from.htm>.