Octopus suckers glow in the deep, dark sea

Science News, March 13, 1999 by S. Milius

A red octopus that drifts through deep waters off the eastern United States shines in a novel way: Its suckers flash on and off.

Finding luminescence in any of the known octopus species surprised codiscoverer Edith A. Widder of Harbor Branch Oceanographic Institution in Fort Pierce, Fla. The seas where octopods live sparkle with constellations of bioluminescent fish, squid, crustaceans, and other creatures. Yet the only octopods previously found to luminesce belong to two or three species in which females develop bright rings around their mouths just before mating.

The light organs that Widder and her colleagues describe in Stauroteuthis syrtensis lie in a single row of raised buttons stretching down each arm. The bumps look like suckers that evolved the ability to glow, the researchers suggest in the March 11 NATURE.

Such a glimpse into the evolutionary history of a bioluminescent structure holds special interest because of its rarity, notes Widder. Tracing histories of light organs has proved difficult because "there's no fossil record for bioluminescence," she laments.

In 1997, Widder brought a live, foot-long specimen from the Gulf of Maine back to a lab. When the researchers turned off the lights, the suckers glowed blue-green, emitting the most light at 470 nanometers, a wavelength that travels well underwater. "It's kind of a twinkling effect," Widder says.

Most octopods catch fish or other decent-size prey, but this octopus, oddly enough, had been eating mostly tiny crustaceans called copepods. "It's like a raccoon living on a diet of mosquitoes," Widder explains.

She saw no structures to filter copepods from the water, so she speculates that glowing suckers might lure them the way porch lights attract moths. Shallowwater copepods flock to light, but researchers know less about the deep-water counterparts. Glands around the octopus' mouth secrete mucus, which might snag bedazzled copepods for dinner.

Widder points out that many deep-sea creatures hunt with glowing lures. Light may be attractive in part because of the nutrient bonanza of luminescent fecal matter that drifts to the depths from creatures above. The abundance of light-generating microbes in the diet of upper-ocean animals gives this fecal rain its gentle glow.

For about 100 years, scientists have recognized S. syrtensis without realizing that it glows, notes cephalopod biologist Michael Vecchione, who heads a National Marine Fisheries Service lab in Washington, D.C. Despite the earlier reports of glow-in-the-dark octopus lipstick, the new find was startling.

S. syrtensis "is in a completely different group," Vecchione says. It swims with fins and the webbing between its arms, while the other glowing octopods motor through the sea by squirting water jets.

Richard E. Young of the University of Hawaii at Manoa in Honolulu calls Widder's discovery "a spectacular find." As one of the authors of a report on mouth luminescence, he's long mused that more octopods ought to glow. The greatest hope for finding overlooked bioluminescence, he predicts, lies with the hard-to-observe, deep-water finned species. There, he says, "we haven't looked hard enough."