Ever wonder why the ocean is blue? Sunlight can only reach so far below the surface, so the deeper you go, the more limited the color spectrum becomes. For the marine organisms that inhabit those depths, living beyond the reach of most of the sun's rays makes color pointless: Most are a non-descript blue. The fairy wrasse (Cirrhilabrus solorensis) is a vivid exception. Since it inhabits depths of between 32 to 213 feet (10 to 65 meters)---well below the reach of red sunlight---the small Indonesian fish uses biofluorescence to make its own deep crimson light. But until now, scientists have been unsure why, since it's generally thought that fish at those depths can't see this part of the color spectrum.
Researchers at Germany's University of Tubingen decided to test this assumption. First, they went to the protected reef surrounding Indonesia, the fish's main stronghold, to observe the animal's behavior in the wild. Then, in lab experiments using specimens purchased from an ornamental fish trader (they couldn't remove fish from the marine park), fisheries biologist Tobias Gerlach and his fellow researchers put 27 males in isolated tanks. To mimic natural deepwater conditions, ambient red light was blocked. Then the researchers placed a mirror on one side of each tank and used different color filters to alter the fish's own reflection. One masked red fluorescence; another allowed the fish to see itself in full color. The scientists also tried filtering the image so that only the red fluorescence was visible, without the outline of the fish's body. A video camera recorded each fish’s response to the two-minute tests.
A Multi-Purpose Glow
The researchers found that when the fish faced a mirror image of themselves as they appear in the ocean's depths, they reacted with strong "threat displays," chasing and biting their own crimson mirror image. That behavior matched the aggressiveness of males the scientists had observed in the fish’s native waters. When the researchers concealed the red, the fish were calmer. "The males really react aggressively to the color," Gerlach told Discover. "It’s some kind of signal, I think." Gerlach speculates the red luminescence has two purposes: To fend off would-be competitors and to demonstrate brawniness to females. "It tells other fish about their strength and quality as a mate," Gerlach says, adding that the fish are so aggressive he had to limit the duration of the experiments so they wouldn't become too stressed. The male fairy wrasse's conspicuous red glow doesn't leave it more vulnerable to predation, however, since most other reef fish at the same depth can't see red very well, if at all. The study
, the first to demonstrate the function of biofluorescence in a reef fish, was published today in the Proceedings of the Royal Society B. The next step will be to test whether female fairy wrasses are, indeed, attracted to fluorescence, Gerlach says.