Unusual haul casts doubt on theory that fish can't see red.

    

Deep-sea fish are suckers for lures lit up in red, say California researchers, challenging a long-held belief of marine biologists. They claim that a deep-sea relative of jellyfish uses glowing tentacles to catch its supper.

Using a remotely operated submersible, up to 2,300 metres beneath the waves off the coast of California, Steven Haddock and colleagues collected three specimens of a new species of siphonophore, a group closely related to jellyfish and corals. Two of the three had the remains of fish in their guts. As fish are rare at those depths, this indicates that the species (which has not yet been named but is part of the genus Erenna) has a knack for angling.

In the 8 July issue of the journal Science1, Haddock, a bioluminescence expert from the Monterey Bay Aquarium Research Institute, Moss Landing, and his team suggest the siphonophore uses red light as bait to capture its prey. The newly described species has glowing red spots inside its stinging tentacles, which it flicks rhythmically.

"The motion and shape of the lures is quite distinct and nearly identical to that of a copepod," says Haddock, suggesting that the siphonophore mimics the movements of plankton to catch the attention of fish. "To us, the accumulated evidence is hard to explain any other way," he adds.

A red herring?

But this explanation is controversial, because red light doesn't travel very far through water, and most biologists think that deep-sea creatures are adapted to see blue light rather than red. "Of the 2,000 to 3,000 known species of deep-sea fish, we have looked at the visual pigments of around 250. Only three species are sensitive to red light," says Ron Douglas, an expert on fish visual systems at City University, London. "If this is a lure, it would have to be specific to these three species of fish."

Haddock couldn't identify the fish in the guts of his siphonophores, so he doesn't know if they are of species known to be able to see red. But he suspects that the siphonophores wouldn't have developed the lures unless they could attract a good proportion of fish. Perhaps red vision is more common than assumed, he says.

Haddock points out that deep-sea fish caught by researchers are now often sorted on deck under red light, in an attempt to preserve their visual apparatuses for study. Ironically, Haddock says, this would destroy any red-sensitive visual pigments.

"In challenging the 'conventional wisdom' we are not saying that blue-vision is not the dominant mode of operation," says Haddock, "just that there is evidence for red light being used."

Douglas suggests that the siphonophores might be using their lures for something other than fishing. "It's at least as likely that they are using them for signalling to one another," he says, although he has little idea what siphonophores might have to talk about.

• Nature