What is the most beautifully colored fish in the world? This is a question that can be passionately debated for hours. Surely the Harlequin tuskfish, mandarin fish, coral trout (which despite its common name is really a member of the family of seabasses), fairy basslet, Catalina goby, lined fairy wrasse, flasher wrasse, Scott’s fairy wrasse, regal angelfish, ornate ghost pipefish and stoplight parrotfish would all have their supporters. So would dozens of other species. Perhaps the more relevant questions are, “Why are some fishes so colorful while others are rather drab,” and, “How do fishes use color?”
Clearly, coloration in fishes is something that gets our attention. But the questions posed here ponder the issue of how various species of fishes can possibly utilize their coloration, especially considering the fact that at depth our eyes perceive the underwater world as one that is illuminated by predominantly blue light. We all learned in our basic scuba class that the red part of the spectrum is quickly filtered out in extremely shallow water, with oranges and yellows following not too far behind. At a depth of 100 feet/30 m, most fishes appear to be blue fish swimming in a blue world unless we “paint in the colors” by illuminating them with a dive light, strobe or video lights, or use a mask with a color-correcting filter. You can’t help but wonder why so many fishes are so colorful if their world is so blue.
What Colors Do Fishes See?
For many years it was believed that the eyes of most fishes were very similar to human eyes, meaning that fishes see the same part of the spectrum we do. However, some specialists questioned the theory based on the fact that the riotous colors of so many fishes would be heavily muted at comparatively shallow depths and in low light levels. Why, they questioned, would any fish that lived below 50 feet/15 m, or even 30 feet/9 m, display brilliant reds, oranges and yellows? And why would fishes like tuskfish, mandarin fish and many other species so active during dusk and dawn be so colorful if these colors served no purpose? Why would frogfishes, sea horses, flounders and so many other species alter their coloration to match that of their immediate surroundings if other marine creatures could not see colors?
These specialists simply did not accept the premise that fishes do not use their coloration in meaningful ways. After all, some studies had shown that in many cases in the competition for mates, the most brightly colored fishes were the most successful. Color seemed to be so significant in mate selection that the results couldn’t possibly be coincidence. It was clear that other members of their species as well as other animals could see the colors.
Continuing studies soon demonstrated that the eyes of many fishes are very sensitive to a large part of the color spectrum, including ultraviolet light, and that ultraviolet light penetrates far deeper into the water column than previously thought. In fact, ultraviolet light penetrates well below 100 feet, and because of its comparatively short wave- length, ultraviolet light tends to scatter less than light waves from other parts of the spectrum. While human eyes are not sensitive to ultraviolet light, the presence of ultraviolet light combined with the sensitivity of some fishes to some other parts of the spectrum opened the door to understanding much more about how fishes use color.
Experiments have shown that some fishes can locate and capture prey when ultraviolet light is the only light present, while other studies have demonstrated that lemon sharks and some snappers can discriminate between widely varying colors outside the ultraviolet range. Lemon sharks have shown a preference for yellows, while snappers are able to distinguish between the same species of baitfish that have been dyed different colors. In the snapper experiment, some baitfish were dyed red and treated with foul-tasting chemicals before being presented to the snappers. The dyed bait repelled the snappers, while they feasted on untreated bait of the same species. For a long time afterward, the snappers continued to avoid baitfish that had been dyed red, even though the foul-tasting chemicals were not present.
Color-Coded
Current thinking suggests that while color does indeed play a prominent role in the lives of many fish species, different species are thought to use colors in a variety of ways. Color has proven to be an important element in species recognition and mate selection in many fishes. Some groupers and other fishes alter their color as a signal that they are ready and willing to spawn. On a reef inhabited by dozens, or perhaps hundreds, of species, it would be a big waste of energy to court any fish that swims past. It is vitally important for members of a species to be able to recognize their own kind, and to be able to distinguish adults from juveniles and males from females, so that potential mates can be more easily recognized.
In many species, such as Anthias, sheephead and parrotfishes, there are clear color differences between adults and juveniles, as well as between males and females. However, this is not always the case with all species. In garibaldi, blue sharks and many other species, color alone is not enough to distinguish males and females, though the spectacular blue spots found on the orange body of juvenile garibaldi do enable even casual observers to recognize juveniles and adults. Other factors, from size and shape to deep singing voice to nest-building skills, can play important roles in mate selection.
Color as Camouflage
Not all fishes use color as a means of standing out. Many species use color in an effort to camouflage themselves. Flatfishes such as peacock flounders and turbot are masters of camouflage. When resting on the sea floor, they alter the coloration and pattern of their skin to match that of the surrounding bottom. As is the case with sea horses, trumpetfishes, frogfishes, clingfishes, scorpionfishes and many other species, flatfishes alter their color by expanding or contracting sacs of pigments and different kinds of cells in their skin. In controlled tests, flatfishes have demonstrated a remarkable ability to match complex patterns such as checkerboards and polka dots in a wide array of colors. Of course, shape also plays an important role in camouflage.
Disruptive coloration is the use of color to break up the shape of an animal so that its form or outline is more difficult to see. Some scientists suspect that fishes ranging from striped emperor angelfish to polka dot-covered whale sharks use disruptive coloration to make it difficult for potential predators and prey to perceive them as an individual animal. Upon first consideration, this theory can seem to be a bit of a stretch, but if you spend a dive looking at many coral reef fishes as they swim in, over, through and around a reef, it seems a lot more plausible.
Vertical bands and horizontally oriented stripes are the tools that are most commonly used in disruptive coloration. Many specialists suggest that stripes are most effective for open-ocean fishes, while bands serve as a better disguise for resting fishes. Some fishes change their color pattern between stripes and bands depending upon whether they are swimming or resting.
Some sort of facial “mask” or stripe that de-emphasizes or breaks up the face and hides the eyes is usually another important element in disruptive coloration. A dark stripe through the eyes of many fishes is thought to make it more difficult for potential predators to distinguish their head from their tail, and thus more difficult to anticipate the direction of an intended escape. Add a false eye spot near the tail, as is the case with the four-eye butterflyfish, and you have a design that has proven to be very successful over time.
Still other fishes, such as a variety of nearly translucent gobies and many larval fishes, are believed to use the absence, or near absence, of color to help camouflage them. These fishes are difficult to separate from their background, because in many instances you can see their background through their bodies.
Many groupers, barracudas and a variety of others are known to use color as part of their display to advertise to potential cleaner shrimp and fishes that they would like to be serviced. When being cleaned, these fishes often blanch or darken their skin to increase the contrast in color between themselves and pesky ectoparasites that they want the cleaners to remove. When they have had enough, the hosts (the animal being cleaned) will sometimes alter their color to communicate that they desire no further cleaning at that time.
But as is so often the case, there is a fly in Mother Nature’s soup. Mimic cleaners such as saber-toothed blennies appear very similar to some cleaner wrasse. After carefully picking their moment, the mimic cleaners surprise their hosts by biting a chunk of flesh from their skin instead of removing an unwanted parasite.
Warning Coloration
Color is also used by some fishes as a warning. In some instances, fishes that are not particularly gifted swimmers, such as lionfishes, are thought to use their bright colors as a means of communicating to other animals that it is best to stay away from them and their painful spines. In essence, the theory of warning coloration states that this defensive coloration works best when advertised. It isn’t as beneficial to be protected by sharp or poisonous spines if potential predators are not made aware of it until after they attack.
Caribbean barjacks usually display a silver coloration when they gather in schools. However, when individual barjacks associate with feeding stingrays, they often alter their skin color to black and become highly protective of the moving “territory” around the feeding ray. It is believed that the barjacks alter their color as a form of intraspecific communication (communicating to members of the same species) to tell other barjacks to stay away from “me and my ray.” Feeding stingrays often uncover prey items that are eagerly pursued by the barjacks, and sharing does not seem to be part of their nature.
Startle coloration is still another effective use of color. Fishes like scorpionfishes and sea robins (flying gurnards) are rather drably colored. However, when threatened they unfurl large, brightly colored pectoral fins in an attempt to startle potential predators long enough to allow them to make a quick escape.
In some instances, body pigments help to protect the fragile internal organs of larval fishes and shallow-water fishes from damage from sunlight. Skin that is positioned over the brain tissue of many larval fishes is often darkly hued, a characteristic that offers protection from sunlight.
Countershading
While many reef fishes are celebrated for their striking colors, many open-ocean fishes are rather blandly colored by comparison. However, this does not mean that coloration is unimportant in the lives of open-ocean fishes. In fact, strong evidence supports quite the opposite. Open-ocean fishes such as blue sharks, mako sharks, oceanic whitetips and many tunas utilize a color pattern known as countershading to help them blend in with the surrounding water, an adaptation that helps them avoid potential predators and capture prey. In general terms, these sharks and other countershaded fishes are darkly colored on the upper portion of their bodies and lightly colored underneath. This type of coloration is referred to as countershading, a form of camouflage in which the color tone of an animal’s skin closely matches the tone of its surroundings.
Camouflage is often thought of as a defensive mechanism for prey that is used to avoid detection, but it is also an important weapon for many predators. Camouflage helps predators go undetected by their prey. Even a slight delay in detection can make the difference between a successful attack by a predator and wasted effort.
Countershading enables many reef sharks, including great white sharks and tiger sharks, to blend into an environment that is a combination of lightly hued surface waters and the darkly shaded bottom of the sea floor. For example, if a loggerhead turtle, a favorite prey item for a tiger shark, is swimming above a tiger shark, the shark has a better chance of going undetected because the top of its body, the portion the turtle is looking down on, blends in with the dark water background or dark reef below. On the other hand, if the shark is swimming above the turtle, the shark’s light underbelly blends in with the light-colored surface water.
It might be difficult to believe that a 15-foot-/5-m-long, 2,500-pound/1,130-kg great white shark needs to rely on camouflage as part of its arsenal. However, what is even more amazing is how effective the countershading really is. From a relaxed vantage point in a shark cage, I have repeatedly seen great white sharks absolutely “disappear” from view when only 30 to 40 feet/9 to 12 m away by blending into their surroundings, even though water visibility is in excess of 100 feet.
Some comparatively small fishes also utilize countershading. For example, surface-dwelling species such as needlefishes and halfbeaks use their silvery-blue coloration to blend in with the underside of waves.
Bioluminescence, light produced by living organisms, takes the place of color in some cave-dwelling species and in the dark environment of the deep sea. Perhaps more surprising is the fact that a variety of shallow-water fishes such as flashlight fishes, lanternfishes, some cardinalfishes, pineapple fishes, ponyfishes and bullseyes possess a light-emitting organ, usually located on their head. In some cases this organ is associated with the fish’s digestive tract, but it is usually a colony of luminescent bacteria that produces the light. Bioluminescence is thought to serve a variety of purposes, from breaking up the silhouette of potential prey to mate recognition to attracting potential prey.
A Curious Rainbow
As convenient as it might be to make the definitive statement that “all fishes use coloration in this way or that way for these specific purposes,” it is easy to see that not all species use colors in the same way or for the same reasons. However, as scientists continue to pursue the question of how various fishes use color, it becomes more and more obvious that body color is an important element in the underwater world.
For More Information
Watching Fishes by Roberta and James Wilson (1992, Pisces Books) includes a chapter on color; the text is geared to the casual fish-watcher.
A good scientific reference is The Ecology of Fishes on Coral Reefs by William McFarland (1991, Academic Press). Chapter 5, authored by Peter Sale, is titled “The Visual World of Coral Reef Fishes.”