The wild, psychedelic colors of reef fish are often what first enraptures divers. Is there an evolutionary explanation for why small, tasty animals adorn themselves in vivid colors instead of camouflaging themselves from hungry predators?
Color-vision specialist Justin Marshall, PhD, noted about coral reef habitats that it “is almost inconceivable for only one evolutionary force to be behind the colors of such a diverse assemblage.”
Alfred Russel Wallace, a 19th-century evolutionary thinker, proposed that reef fish are colorful because they camouflage themselves against colorful reefs. Wallace, however, was not a diver and could not see that most coral reef fish constantly swim past highly variable backgrounds, which are often less colorful than the animals. Frogfish and seahorses are notable exceptions that are mostly sessile and often match their colors to the soft corals and sponges where they reside.
More recently, some ecologists have proposed that fish might adopt bright colors simply because they can. Most reef fish swim close to the safety of the structurally complex coral reef environment and can easily take shelter from approaching predators, so it might not matter how visible they are. Fish that swim well above the reef are usually less colorful, enabling them to blend into open water.
Another theory postulates that most reef fish don’t camouflage themselves because they can’t. In clear, brightly lit tropical waters with the highly varied background of a coral reef, there is virtually no camouflage pattern that would provide safety to a swimming fish.
Evidence for this theory is a comparison of fish on either side of the Isthmus of Panama that share a common ancestry but have diverged into different species. These fish evolved separately to adapt to different habitats. Fish in the vibrant, clear Caribbean waters are more brightly colored than their relatives in the more turbid and monochromatic Pacific environment.
Some features of a bold color pattern can impede predation. Predators often search for the eye of their prey to know which way it will swim when they strike and take a correct approach angle. Many brightly colored fish have dark markings that obscure their eyes. Some have false eyes, usually near the tail, to confuse a predator into striking in the wrong direction.
Bright colors may highlight defensive biology, such as the orange-spine unicornfish’s bright patch of orange around the sharp spines on its tail. Other fish may use bright colors like nudibranchs and poison dart frogs do to advertise their toxicity or unpalatability. Examples include small puffers and boxfish with their noxious slime, fangblennies that can deliver a venomous bite, and lionfish with their venomous dorsal spines.
These warning colors comport with famed behavioralist Konrad Lorenz’s explanation of fish colors as “advertising billboards.” A fish might want to proclaim many things about itself, just as humans use clothing, hairstyle, and material goods to advertise wealth, social status, or group affiliation. Species identity is a key attribute for fish. Coral reefs are among the most biodiverse habitats on Earth, and fish need to be able to quickly distinguish their own species for mating and to drive off competitors for mates and food.
Emperor angelfish are highly territorial and aggressively run off members of their own species that might compete for resources. Juveniles, like adults, are brightly colored but have a very different color pattern that enables them to share a reef with an adult without being attacked. Their color pattern advertises, “My diet is different from yours, so don’t waste energy coming after me.”
Many reef fish, notably wrasses and parrotfish, have differently colored sexes. Distinct color patterns can advise to a territorial male that one fish is a female he wants to keep inside his territory and another is a male that he must displace.
In addition to marking their species, sex, and age, color patterns can distinguish some fish as individuals. Researchers can learn to identify individual fish by variations in their color patterns, so it is likely that fish can do the same to recognize mates or aggressors.
Some fish can change their color pattern or intensity almost instantaneously, often in concert with a display of aggression. Some fish display or intensify colors during courtship. Black triggerfish, for example, display lines of electric blue and yellow instead of their usual solid black during courtship and other social interactions. They also use neon patterns to entice cleaner fish to choose them among the many fish vying for attention at a cleaning station.
Cleaner fish use bright colors to advertise their services. This behavior leads to another reason that reef fish might have bright colors: mimicry. Several fang blennies sport colors that mimic the cleaner wrasses with which they associate. Innocent victims expecting a parasite removal service are instead subjected to a blenny’s dine-and-dash attack. The fang-like incisors that do the dirty work can inject venom if a predator attacks.
Some other nonvenomous blennies mimic the fang blenny’s bright colors and benefit from reduced predation. Other fish may copy the bright colors of especially numerous species of reef fish to reduce their individual risk of predation. Juvenile orange-band surgeonfish, for example, are bright yellow and often school with yellow tangs.
A dynamic display is another way to discourage predation. This behavior could be suddenly revealing a bright color pattern that startles a predator just long enough for the fish to escape. Several types of scorpionfish have rainbow patterns hidden on their pectoral fins. The rest of the fish is a drab color that blends with the reef rubble, where the scorpionfish rests in ambush. If it has to flee a larger predator, it flashes the bright rainbows, which disappear as the fish comes to rest in a new location. Other fish have startle patterns on their dorsal fins that they can show off as needed.
Dynamic color patterns usually communicate something to nearby fish. It is not known, however, if the fish changing its color is intentionally sending a particular message or if it is an involuntary response to an emotional state.
Fish may evolve bright colors that don’t offer any survival advantage as long as there is no disadvantage. Many deep-sea fish and nocturnal fish that shelter under reef ledges by day — such as squirrelfishes, soldierfishes, and bigeyes — are deep red. Red is not visible at night or below about 60 feet (20 meters), so it could be functionally equivalent to black.
Fish do not necessarily see colors the way that we do. Many fish that live in deeper water cannot detect red light, but some fish in these environments are not only sensitive to red light but also can produce it (or other colors) by one of two mechanisms. Some fluoresce, absorbing the energy of blue to ultraviolet (UV) light that penetrates deep into the ocean and re-emitting it as red light. Others generate red light through a chemical reaction (bioluminescence). Either method gives these fish a private communication channel to signal members of their species without tipping off predators.
Some reef fish can detect at least four colors, including UV wavelengths, which we cannot see. The species that can control their UV reflectivity may use it for intraspecies communication undetectable to a predator. The two-bar damselfish, for example, has a highly UV-reflective patch on its dorsal fin, which it can raise and lower like a signal flag. Some fish can mix UV with other pigments to produce additional colors, such as UV yellow, and some detect polarized light, potentially using it for navigation.
Reef fish use their delightful colors in many ways, but how do the fish experience them? It’s doubtful that any fish looks at another and thinks about how skin pigmentation indicates metabolic fitness for a more suitable mate. It is more plausible that appraising another fish’s appearance prompts a noncognitive emotional response and perceptions of attractiveness or threat.
Carl Safina points out in his excellent book, Becoming Wild, that it’s natural that animals view their own kind as beautiful, but the real question is why humans also find them beautiful. Safina concludes that nature has endowed all creatures with a perception of beauty that enables them to feel comfortable in the world to which they are adapted.
He further posits that humans inherited not only the capacity to appreciate beauty but also our visual aesthetic values from distant ancestors we share with fish and birds. While I cannot confirm or disprove this provocative idea, I have seen fish flee from approaching tourists in unflattering swimwear.
© Alert Diver – Q1 2026