Green gleam helps fish see violet

CHARLESTON, S.C. — The greeneye fish views its stygian home through fluorescent lenses that turn one color into another, researchers propose, making glowing green images of hard-to-see violet objects.

“Crazy” is what Yakir Gagnon of Duke University cheerfully called the fish-vision idea he and his colleagues presented January 4 at the annual meeting of Society for Integrative and Comparative Biology. Fluorescent materials known to science so far, he explained, respond to incoming light by glowing in a different color in all directions. Yet lenses on the bulging, upward-looking Chlorophthalmus fish appear to have materials that direct that fluorescent glow in the same direction and pattern as incoming light.

Like many deep-sea fishes, greeneyes have only one kind of light-detecting pigment in the retina, the surface at the back of the eye that catches images. That pigment is optimized to pick up a particular wavelength of green light. Alone, the pigment doesn’t even detect blue-violet light.

Yet greeneyes’ fluorescent, glowing lenses appear to translate blue-violet light into a more detectable green color. The Duke team has found that incoming blue-violet light (with a short wavelength of 410 nanometers) zaps lens substances into fluorescing a blue-green that’s just a twinkle away from the color the retinal pigment sees best. The lens glow peaks mostly at 485 nanometers, and the retinal pigment picks up light best at 488 nanometers.

Even light made fish-visible might not be so fish-useful if it’s just a blur. Yet the fish lens appears to cast its glow mostly in the same direction that incoming light is traveling, raising the possibility that it’s transmitting something like a fluorescent image, Gagnon said.

The first hint of this came from work by Alison Sweeney, now at the University of California, Santa Barbara, who removed a greeneye’s lens and aimed light at it through a pattern of slits. Capturing what the lens would have projected on the retina, Sweeney found the lens produced a blue-green glow in roughly the same pattern as the incoming light.

Even when ground up into soup, the lens material somehow still cast its glow in particular directions, Gagnon reported. Blue-violet light aimed at a laboratory container of lens material excited a glow directed out the opposite side, as if the shaft of light had just traveled through the soup.

Just how a fluorescent substance might preserve light direction is itself extremely blurry. “This is all too new,” Gagnon said.

What he did say, though, was that the sea does hold violet things a greeneye might like to see. Reports place the fish from about 50 meters to 1,000 meters deep, where plenty of creatures flash lights in the blue-violet range to attract prey or mates or scare away predators.

“Really neat,” said Geoffrey Hill of Auburn University in Alabama, who studies colors in birds. Biologists have argued that fluorescence in birds, such as the glow-power of budgerigars’ heads, benefits the animals. So far, Hill said, the argument for benefits from the fluorescent fish lens looks tighter.