Thousands of meters below the sea, a tiny worm wriggles through the darkness, its dozens of paddle-shaped bristles moving in beautiful coordination. Suddenly, a hungry predator appears. The worm releases a glowing green sac, and the fish homes in on this bright new trophy. By the time the fish realizes the sac is no meal, the worm is long gone.
In tomorrow's issue of Science, researchers report the discovery of these bioluminescing "bombs" and the creatures that make them. The worms, relatives of earthworms and leeches, belong to the annelid phylum and have been given the group designation Swima. In all, Karen Osborn of the Scripps Institution of Oceanography in San Diego, California and colleagues have identified seven species. Their discovery "is a substantial increase in annelid biodiversity," says Torsten Struck, a marine biologist at the University of Osnabrück in Germany.
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Team member Steven Haddock of the Monterey Bay Aquarium Research Institute in Moss Landing, California, first saw one of these annelids flit across the camera viewfinder of a remotely operated submersible in 2001. Since then he, Osborn, and others have come across dozens more individuals. Most were spotted in canyons off the coasts of California and Oregon, but a few lived in the Philippines. They ranged in size from 18 to 93 millimeters and were between 1863 meters and 3793 meters below the surface, sometimes along the sea floor and other times in midocean.
About 2 millimeters across, the glowing bombs are actually modified gills that consist of four chambers, likely holding apart fluids that react when they come into contact with each other to create light. Each worm has eight appendages for holding the sacs. When released, the sacs glow green for about a minute, Osborn reports. "It's a different chemistry than has been found in other polychaete worms in the same phylum," she says. She presumes the worms drop the bombs as a way of distracting predators.
Although some worms drop appendages during reproduction and others use bioluminescence, "these two features were never found in combination [in annelids] until now," says Struck. The find drives home that even "simple" worms have sophisticated behavior, he says. And given that the researchers have found these creatures on both sides of the Pacific Ocean, Struck says, there are probably many more varieties out there, lighting up the sea as they go.