Killer flatworm: new species hunts with puffer fish toxin.

A newly discovered saltwater flatworm, pale yellow and about the size of a silver dollar, can take down mollusks in their shells, thanks to a powerful neurotoxin also found in puffer fish.

Yet this formidable hunting tool flops as a defense against fish eating the flatworm, says Raphael Ritson-Williams of the Smithsonian Marine Station in Fort Pierce, Fla.

The puffer fish poison, tetrodotoxin, has turned up in a variety of other creatures that spend at least part of their lives in water. These include other flatworms, frogs, North America's rough-skinned newt, and the blue-ringed octopus.

Tetrodotoxin and other toxins from marine organisms have caught the attention of biomedical researchers for basic research and drug development. However, Ritson-Williams and a few other biologists are studying what animals actually are doing with the poisons. His new experiments provide an unusual look at how a flatworm uses its formidable chemistry.

A small-scale mystery led Ritson-Williams to recognize the new flatworm's predatory use of toxin. Five years ago, while collecting flatworms in Guam, he picked up a cowrie to give to a friend who was doing a research project on that speckled-shell mollusk. He put the cowrie in a container with the new flatworm. When Ritson-Williams got back to his base camp, the flatworm was really fat," he says. And all that remained of the cowrie was an empty shell.

Since flatworms don't have teeth or any obvious weapons, Ritson-Williams' Smithsonian colleague Valerie Paul suspected poison. Ritson-Williams sent samples of the worms to chemist Mari Yotsu-Yamashita of Tohoku University in Sendai, Japan. The flatworm has tetrodotoxin along with some closely related chemicals, the researchers report in an upcoming Proceedings of the National Academy of Sciences.

To see how the flatworm uses its toxic arsenal, Ritson-Williams offered it various mollusks. The flatworm killed at least 30 species, including ones with trapdoors that block out most danger. By engulfing its prey or covering the victim's trapdoor, the flatworm presumably seals its quarry in a pocket of toxin laced water (for Ritson-Williams' video, go to www.sciencenews.org/articles/20060218/ worm.mov).

Ritson-Williams next tested for defense capabilities by offering flatworms to fish swimming free in the wild. Many of the fish readily swallowed the worms. He says that he doesn't know whether the fish suffered any long-term ill effects but points out that the poison failed as a defense against being eaten.

The flatworm's tetrodotoxin probably comes from bacteria that live in its body, says Marian Litvaitis of the University of New Hampshire in Durham. Marine flatworms are good at borrowing weaponry, she notes. The ones that she studies acquire their toxins from sponges and sea squirts.