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Seeing red: scientists overlook fluorescent fish
Marlowe Hood, Yahoo News 16 Sep 08;
It was staring them in the face, but somehow generations of marine biologists have failed to notice that a lot of fish in the sea glow a fluorescent red, according to a study published Monday.
This unheralded talent for neon-like crimson displays is more than a curiosity, and is sure to create waves -- and a bit of embarrassment -- among ichthyologists, as fish experts are called.
It has long been axiomatic that red light is simply not part of the mental universe of marine fish because the sunlight's longest visible wavelengths do not penetrate below a depth of ten metres (30 feet).
A fire-engine red diving suit at 20 metres, for example, will appear dark grey or black to anyone -- or any fish -- that happen to be in the vicinity.
Dive far enough beneath the surface, and there is simply no red to be seen.
This foreshortening of the color spectrum under the waves was also assumed to correspond to a narrowed field of vision in fish, said the study's lead researcher in an interview.
"The general consensus, which dominated fish literature for 20 or 30 years, was that fish don't see red very well or at all," explained Nico Michiels, a researcher at the University of Tubingen in Germany.
From an evolutionary standpoint, in other words, why develop a skill that you will never be able to use?
But conventional wisdom, it seems, was flat out wrong.
"We have been blinded, literally, by the blue-green light that is available on reefs in the daytime," said Michiels.
At least 32 species of reef fish -- including pygmy gobies and some wrasses -- can shine like a red Christmas bulb, not by reflecting sunlight but by emitting their own, the study found.
Dissection revealed that the fluorescence originates in guanine crystals, a chemical compound that is added to nail polish and car paint to give added luster.
And because the light is coming from the fish themselves, it remains visible at depth and is easily seen -- but only at close distances.
Michiels and his colleagues saw the light, as it were, by accident.
Looking through a filter while scuba diving that blocked out the brighter green and blue light waves, leaving only red ones, they suddenly saw a whole universe of sea creatures glowing various hues of cherry, crimson, ruby and rust.
"Besides fish, there are lots of fluorescent organisms on the reef, including algae, coral and other small organisms," Michiels said.
That some fish glow is certain, and was confirmed using spectrometry in laboratory experiments. It is also very likely that many can see red perfectly well, and do so for a reason.
One possibility is that they emit the color -- visible only within a small circumference -- as a means of intimate communication, perhaps for mating or to signal danger.
There is also evidence that turning red could serve as a kind of camouflage.
"It may seem strange, using fluorescence to make yourself invisible," Michiels said. "But fish that sit on a reef have a lot of fluorescence around them, so they blend in."
The next step is to figure out how day-glo fish might use their color-generating power to exchange information. Adapting techniques proven in other experiments with fish, Michiels is creating an environment in which fish can court each other via video.
By controlling which part of the color spectrum the fish can see, the researchers will seek to verify that some fish do, indeed, see red.
