And yet no one could get their hands on a living example of the giant shipworm, or Kuphus polythalamia. Unlike with other shipworms, named because they ate their way into the sides of wooden boats, no one knew where the giant shipworm lived.

“It’s sort of the unicorn of mollusks,” Margo Haygood, a marine microbiologist at the University of Utah, told The Washington Post.

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The habitat of the world's longest clam is a mystery no longer. As Haygood and her colleagues reported Monday in the Proceedings of the National Academy of Sciences, the search for the giant shipworm has come to an end.

Television news in the Philippines dealt the mortal blow to the shipworm's near-mythical status. A TV station aired a short documentary segment about strange shellfish living in a lagoon. The show filmed the mollusks growing in the muck, as though someone had planted rows of elephant tusks. As luck would have it, a colleague of Haygood's in the Philippines caught wind of the segment. Researchers investigated the lagoon, where they plucked a live shipworm from the mud, slipped it along with some seawater into a PVC pipe and shipped the animal to a laboratory.

“I’ve been studying shipworms since 1989 and in all that time I had never seen a living specimen of Kuphus polythalamia,” Daniel Distel, a co-author of the new study and the director of Northeastern University's Ocean Genome Legacy Center, wrote in an email. “It was pretty spectacular to lift that tube out of its container for the first time.”

Distel carefully chipped away at the giant shipworm's massive shell. Smaller shipworms are fleshy pink, beige or white, as are most clams. Not the giant shipworm. Its body is black.

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“To see this giant gunmetal black specimen was amazing,” Distel said. “On the one hand I was pretty excited to see what it looked like inside. On the other hand it was a little intimidating to dissect this incredibly rare specimen.”

A full-grown giant shipworm reaches up to three feet long, which means that when draped across the width of a twin bed, the clam would just barely fit. “It’s quite heavy. It’s like picking up a tree branch or something even heavier,” Haygood said. “The living animal is just magnificent.”

What's more, the giant shipworm barely has a digestive system. “It's not feeding in any normal way,” Haygood said.

The clam has a mouth and a small stomach, but its gills are supersize. Living within those gills are bacteria. That's not unusual for shipworms: The clams, as a rule, have symbiotic relationships with microbes. Usually, though, the microbes help shipworms digest wood.

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In the case of the giant shipworm, the scientists found grains of sulfur packed into the bacteria. The marine biologists suspect that, at some point in the shipworm's evolution, the animal traded its wood-digesting bacteria for bacteria that feed off sulfur compounds.

The study "provides a fascinating example of symbiont displacement, a phenomena we are only just beginning to observe more regularly in nature, thanks to advances in sequencing which have provided us with the tools to unravel the evolutionary history of microbes,” said Nicole Dubilier, director of the Max Planck Institute for Marine Microbiology, who was not involved in the study. “What we are now seeing is unexpected: symbioses are not as stable as we previously assumed.”

The symbiotic arrangement between microbe and giant shipworm was similar to one found in deep-sea hydrothermal vents. Thousands of feet below the surface, beyond the reaches of sunlight, tube worms also get their nutrients from bacteria that consume sulfides. Despite their similar names, though, tube worms and shipworms aren't close relatives. Tube worms are annelids — they're actual worms, like earthworms, not clams.

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But the symbiotic bacteria in both deep-sea worms and the lagoon-living clams are related to each other. “So this is a case of convergent evolution,” Distel said. That is, both the worms and clams independently arrived at the same conclusion: Housing bacteria inside their bodies was a fine way to stay nourished.

Haygood said the presence of the sulfide-consuming bacteria suggested that the lagoon, perhaps filled with rotting wood or other organic matter, produced hydrogen sulfide.

The discovery lends support to a hypothesis proposed by Distel in 2000 about the origins of animals that live in deep-sea vents. In Distel's theory, mussels that lived in wood and harbored the sulfide-eating bacteria might have sunk to the vents. Far below, they flourished on sulfide released from the vents.

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“Wood provided an ecological bridge, helping them to invade the vents,” he said. The discovery of the new shipworm indicated that shallow lagoons could have served as the location for the switch in bacteria types: First the wood served directly as food for clams. But once the clams began to take in the sulfur-loving bacteria, the wood provided a source of the hydrogen sulfide for the microbes.

“This is an extremely rare example where we were actually able to find fairly direct evidence about how this particular symbiosis evolved,” in which the clams traded one type of bacteria for the other, Distel said.