‘Unicorn of mollusks’: scientists discover mysterious clam, edible and as big as a baseball bat, in Philippines lagoon

PUBLISHED : Tuesday, 18 April, 2017, 8:57am
UPDATED : Tuesday, 18 April, 2017, 10:22pm

For hundreds of years, biologists knew of the giant shipworm - an enormous species of clam more than a metre long - only from shell fragments and a handful of dead specimens.

Those specimens, despite being preserved in museum jars, had gone to mush. Still, the shipworm’s scattered remains made an outsized impression on biologists. Its huge tubular shells were so striking that Swedish taxonomist Carl Linnaeus included the animal in his book that introduced the scientific naming system “Systema Naturae.”

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,” said Margo Haygood, a marine microbiologist at the University of Utah.

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 ended the shipworm’s near-mythical status. A TV station aired a short documentary segment in 2010 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.

The clam is quite edible. Local divers were shown gathering the baseball-bat-sized molluscs, which were then cut up and stir-fried.

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.

“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 more than a metre long. “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.

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.

“This paper 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 provide 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.”