Back on land, the fish were housed in aquariums and tested for fluorescence the same day using a spectrometer, which measures the spectrum of light generated by an object. The measurements revealed that fish caught at depths of 66 feet fluoresced red more readily than fish of their same species caught at 16 feet below the water's surface.
"In some species, the difference is quite impressive," Michiels said. "Some of these species are six times more fluorescent in deeper water than in shallow water."
Red wavelengths of light are rapidly absorbed by water and aren't present in deep waters, Michiels said. Thus, traditional pigmentswould be useless for creating red coloration. Red pigments simply look gray without that portion of the spectrum to bounce off them, much like the clothes in your closet all look indistinguishably gray when you try to pick out a sweater without turning on the lights.
The fish "can use the ambient light, which can be blue or green — it doesn't matter —and transform it into red or yellow," Michiels said. The fish can be red in a blue environment, if they fluoresce, he added.
The researchers are now working to measure the fishes' fluorescence in their natural environment. They've also noticed that fish often have spots of fluorescence around their eyes. These spots could be used as a miniature personal light source, kind of like the headlights on a car, Michiels said. The next step, he added, is to find out if the fish really use their fluorescent spots in that way.
The researchers report their findings today (July 15) in the journal Proceedings of the Royal Society B.
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