Camouflage on the Reef: How Fish Hide in Plain Sight

When predators are at large, it obviously pays to be inconspicuous. One way is to blend into the background in the hope of avoiding detection by a hunter. Camouflage, or crypsis, is widely used on the reef for this purpose. An animal that can successfully blend into its background will live a great deal longer than one that stands out But one disadvantage of being inconspicuous is that you cannot advertise yourself to potential mates or warn competitors to keep off your patch. Some fishes solve this conundrum by changing color as they grow.Virtually transparent larvae can adopt dull, camouflaging colors as juveniles before becoming brightly colored, fully grown adults. Anatomically, animals can try to avoid predators in a variety of ways: they can match the colors of their background, they can disguise their outline, or they can pretend to be something else.

Is camouflage a common strategy on the reef?

Plenty of different animals use basic camouflage. Fish that occupy the waters surrounding the reef, but live freely in the water column, are often silvery, which is the most effective livery for open-water survival. Larval reef fishes are mainly transparent, again an adaptation that helps them to stay as invisible as possible to visual hunters. Countershading, where the dorsal surface of the fish is fairly dark in comparison to the pale -- even white -- coloring of the underside, is also common among fishes. It helps the fish to blend in, both against a dark background when seen from above and against the sky when viewed from beneath. But these adaptations are fairly basic compared to the camouflage seen in some reef inhabitants.

Do nocturnal fish bother with camouflage?

At night complex camouflage is less important because of the restricted light levels. One effective strategy in these conditions is to have an almost uniformly reddish body, as seen in nocturnal plankton-feeders, such as squirrelfish. This may seem strange, but because red light is absorbed rapidly by water, anything red is perceived as being a dark and neutral color. As anyone who has ever cut themselves while diving will know, the blood from the cut appears to be a dark green!

What about the bright colors of diurnal fishes?

It has long been thought that the dazzling colors of reef fish are a real disadvantage when it comes to avoiding predators. The vivid blues, yellows, and reds advertise effectively to rivals and to mates, but the by-product is that they act as a "come and get me" sign to visual hunters. Or so it was thought.

New research suggests that bright colors may work both as signals and as camouflage. To understand why, we need to take a predator's eye view, because the visual abilities of fish and of humans are different. When researchers used imaging techniques to adopt a predator's perspective, it became clear that although the bright color patterns of many small reef fishes are extremely conspicuous close up, at a distance of just a few feet these patterns function amazingly well at camouflaging the animals. This is because patterns such as stripes and spots blur together against the complex visual background of the reef. Moreover, blue and
yellow -- commonly seen in reef fish -- provide very good camouflage against both the reef and the open water when seen from a few feet away and from a predator's perspective.

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