Code red. Siphonophores may use glowing red spots on their tentacles to attract prey.

Deadly Light in the Ocean Depths

For the Erenna siphonophore--a deep-sea relative of the jellyfish--red means go. A new study indicates that the creature is the only known marine invertebrate to generate red fluorescent light. But it's not a fashion statement. Researchers believe the glow baits hungry fish into becoming meals themselves.

Some sea creatures sport glowing blue patches on their bodies that help distract predators. Blue is the preferred color of such bioluminescence because blue wavelengths travel farther than those of other colors and are thus easier to detect in water. So Steven Haddock, a biologist at the Monterey Bay Aquarium Research Institute in Moss Landing, California, and colleagues were perplexed to find glowing red spots on the tentacles of siphonophores pulled from waters as deep as 2300 meters off the coast of Monterey, California. Red light travels such short distances in water that many scientists had questioned whether deep-sea animals could even see it. "We were really struggling to try to explain why these things ... are red," says Haddock.

Before they tackled the why, Haddock's team focused on the how. Younger siphonophore tentacles sport blue, not red, spots. Upon closer examination, the researchers discovered that, over time, fluorescent compounds cover these blue spots, absorbing the glow and re-emitting the energy as red light.

Although bioluminescence is primarily used for defense, siphonophores may have turned this tool around to capture prey. The creatures, which can reach tens of meters in length, "deploy large webs of tentacles that act like a spider web," says coauthor Casey Dunn, a biologist at Yale University in New Haven, Connecticut. The dangling fluorescent red lures may resemble copepods (tiny relatives of shrimps) eaten by some deep-sea fish, the team reports 8 July in Science.

"It's an exciting study in that it actually poses more questions than it answers," says Jennifer Nauen, a biologist at Harvard University in Cambridge, Massachusetts. Adds Edith Widder, a biologist at Harbor Branch Oceanographic Institution in Fort Pierce, Florida, "When you find exceptions like red emissions in an environment that is almost entirely blue, there's a great story that goes with it."

Related sites
Video of a siphonophore in action
More about Siphonophores
Bioluminescence: How does it work?
Haddock's home page