Red-flashing fish have chlorophyll eyes.

A fish that uses a form of plants' green magic to see provides the first documented case of a chlorophyll playing a physiological role in an animal.

The eyes of a fish called a loose jaw, or dragon fish, contain a derivative of chlorophyll, reports a research team from England and Finland. The compound endows Malacosteus niger with the rare power of seeing far-red light, the team says in the June 4 Nature.

Since most deep-sea creatures see only blues, the compound gives loose jaws a secret weapon. They pulse a farred light that illuminates prey, but victims don't know they've been caught in the glare. "It's a bit like an army sniper scope," says coauthor Julian C. Partridge of the University of Bristol in England.

"I'm sure they use it for communicating about mating things," he says, but cautions that researchers in submersible craft have difficulty studying life 500 to 1,500 meters down, where loose jaws prowl. "Any behavior you see is going to be abnormal because you're crashing around with whizzy motors and flashing lights," Partridge says.

Researchers have known that loose jaws are among the rare fish that glow--and see--red as well as blue, but only now have they identified the red-vision chemicals. The team studied the response to light of extracts from loose jaws' eyes and found that one type of pigment absorbs reds and then somehow excites the eyes' other light-detecting pigments. Studying various spectra of the red-catching pigments, the team identified them as modified chlorophylls.

Bioluminescence specialist Edith A. Widder from Harbor Branch oceanographic Institution in Fort Pierce, Fla., points out that the loose jaw's eye chemistry differs from that of a closely related species that flashes red. She says bioluminescence has probably evolved nearly 30 times.