Do you think humans have mastered the fine art of marketing before any other species? Think again: coral reefs are among the most colorful places on Earth for a reason. Fishes and inverts have been master marketers for a very long time.
They do it so well, in fact, that we’ve decided to keep them in our aquariums and spend millions of dollars working toward their survival in the wild.
But on the reef, and in our aquariums, color displays often play a very different role than our impression of fishes and inverts as beautiful aquatic “jewels” would lead us to believe.
The Colors Don’t Make the Creature
Reef organisms display various bright colors for a wide variety of reasons. We have to keep in mind that we perceive color very differently from reef species. Light travels through water differently than it does through air.
For example, water filters out the longer wavelengths of red light quickly, while blue light penetrates the farthest into the water column. This is part of the reason some corals do better under actinic (blue-colored) lighting.
Reef animals aren’t necessarily trying to advertise their presence on the reef. In many cases, what we perceive to be bright colors in our reef aquaria actually serve to camouflage creatures in the ocean.
The bright yellows and blues of some angelfishes, for example, actually mimic color patterns and shapes that predators might see swimming near a reef, allowing the angelfishes to blend in with their surroundings.
Turn on the Red Light
Many fishes use color to signal information relevant to mating behavior. In some species, color changes in one sex can instigate color changes in another sex.
The role of color in reef fish mating behaviors is complex. Some species are able to flash colors briefly, putting on a miniature light show for prospective mates.
Some species breed very quickly. Males of certain fish species flash bursts of color to indicate a readiness to spawn, and females respond by quickly releasing eggs into the open water column. Once the eggs are fertilized, both the males and females are able to return to the safety of the reef and away from predators.
These specialized light shows are controlled in many species through cells called chromatophores. These specialized skin cells allow species to communicate using light. Certain cephalopods – octopuses, squid and cuttlefishes, for example – can use these special skin cells in very elaborate coloration displays to communicate information, show dominance and initiate mating.
Some squid species can actually display different messages on each half of their bodies, simultaneously warning other males to stay away while sending love notes to a potential mate.
Color conveys all kinds of information, including dominance, genetic fitness and readiness to spawn. Fishes and other reef animals have learned to use color displays to their advantage in these ways.
Make a Meal of It
Some species use color to advertise their usefulness to others. For example, one recent study found that cleaner fishes, such as cleaner wrasses, which would otherwise be prey to larger reef fishes, use blue and yellow patterns to advertise their willingness to perform cleaning duties.
Cleaner wrasses normally set up “cleaning stations” on the reef, and various fishes visit when the cleaner uses color to display its readiness to clean. The “customers” move in and signal that they are ready to be cleaned, and the cleaner wrasses go to work removing dead skin and parasites.
The arrangement is mutually beneficial. Without the ability to advertise their services, cleaner fishes might not ever get a good meal.
Evolution Drives Adaptation
In every case, the colorations we observe on the reef are the direct result of competition and adaption.
Fishes use coloration as camouflage to hide from predators and signal to friends. Predators, in turn, adapt to see better or otherwise counteract their prey’s adaptations.
A sort of evolutionary duel ensues as species change over the course of time. Thus, we end up with the dazzling multi-colored world we see in coral reefs around the world.
Some fishes use color to lull their prey to dinner. A study conducted in 2007 found that some predatory hamlet fish species track similarly colored nonpredatory species. It seems that the hamlet fishes’ prey is less likely to detect danger if their coloration matches that of the hamlet fishes.
Other fishes, such as the false cleaner fish or saber-toothed blenny (Aspidontus taeniatus), mimic color patterns of cleaner fishes to get the benefits of a free meal without having to do the work. In fact, the false cleaner fish is known to bite off small chunks of its “customer’s” skin.
Hidden Worlds on the Reef
The use of coloration to aid in survival even extends into the nonvisual (at least to humans) portion of the electromagnetic spectrum. Many reef species, including several corals and many fishes, use fluorescence to communicate or otherwise aid in their survival. (Check out this article on bioluminescence.)
Some fishes can apparently see ultraviolet (UV) light and use this ability to locate phytoplankton, which do not reflect UV light and stand out, appearing black against the UV backdrop.
Other fishes use UV coloration to communicate, keeping their displays hidden from some predatory species that are not capable of seeing UV.
Many coral species fluoresce. We observe this in our aquariums on occasion. But we don’t really know why they do this. Some suggest the ability acts as a sort of “sun screen” in the ocean. Other theories exist, but we don’t really know the answer to the mystery of coral fluorescence yet.
There is so much we don’t know, so much the denizens of the reef still keep hidden from us and each other.
More Than Meets the Eye
All this deception has its price, however. Several marine species and especially reef species seem to have very acute senses.
Predatory fishes have developed all kinds of sophisticated methods of detecting their prey. Sharks are a perfect example. It is frequently said that sharks can smell blood a quarter of a mile away. Sharks have good eyesight and can see in color.
They also see well in low light, something that is crucial beneath the waves. Sharks have a tapetum lucidum, a special reflective layer in the back of their eyes that reflects light back into their eyes, enhancing low-light vision. The tapetum lucidum is the same thing that causes a cat’s eyes to shine in the dark.
But the all-time king of vision in the ocean is the mantis shrimp. Many reef hobbyists are familiar with these community tank terrors. Often hitchhiking in on live rock, mantis shrimp are totally efficient, ruthless killers that are capable of taking out most of the life in a reef aquarium.
Their eyes are the most sophisticated in nature. They can see several parts of the electromagnetic spectrum that humans are blind to. Their eyes have 16 different types of photoreceptor cells.
Humans only have four such cells in their eyes (one type of rod photoreceptors, and three different types of cone photoreceptors). Mantis shrimp also seem to have very good spatial perception skills.
What a Show
Whether predator or prey, big or small, nearly all creatures on the reef use light displays to interact.
Reef aquarists take advantage of all these different interactions and light displays when they keep beautiful, colorful fishes and corals in their reef aquaria.
But remember, the beauty we are privileged to see in our aquaria isn’t the result of random chance but millions of years of evolution and a deep will to survive.
The next time you look into your reef aquarium try to see these intricate displays of color for what they are: nature’s magnificent display of survival at its fittest.