Issue Date: February 15, 2006
It didn't take long for lab technician Raphael Ritson-Williams to realize that the pancake-shaped flatworm he had collected during a biodiversity study in the coral reefs around the Pacific Island of Guam was, at least to some of its ecosystem mates, a deadly menace.
"I had the flatworm and a cowry in the same jar," Ritson-Williams says, referring to a pretty-shelled marine gastropod. Next thing he knew, the cowry was gone. Its shell remained, but the penny-sized inhabitant was visible within the bulging digestive system of the see-through, cookie-sized flatworm, a previously undescribed species.
"The cowry has an inner chamber that it coils around," making it hard to pull it out of its shell, notes Ritson-Williams, who works with Valerie J. Paul at the Smithsonian Marine Station at Fort Pierce in Florida. The scientists figured the killer flatworm used a chemical agent to extract the cowry. Trolling the literature for clues, the scientists found a 1987 paper by Japanese researchers who reported finding tetrodotoxin, a powerful neurotoxin first isolated from pufferfish, in another flatworm species.
It was a vexing lead. Tetrodotoxin occurs in many marine and terrestrial creatures, but no one presumed the agent had an offensive function. "The prevailing, simplified view is that it is a defensive chemical," says William Fenical, professor of oceanography at Scripps Institution of Oceanography in San Diego. "Here we have a system with flatworms in which there is evidence that, in this case, tetrodotoxin is used as an offensive weapon."
Using standard analytical techniques, Ritson-Williams and Paul and Mari Yotsu-Yamashita of Tohoku University in Sendai, Japan, discovered tetrodotoxin and a few analogs in the flatworm's tissues, particularly in the animal's feeding organ. They also found that the toxin concentrations decrease after feeding but then increase over time—another indication that the toxins are for killing, not for fending off predators (Proc. Natl. Acad. Sci. USA, published online, dx.doi.org/10.1073/pnas.0506093103).
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