Northeastern professor Daniel Distel and his colleagues have discovered a dark, slithering 4-foot-long creature that dwells in the foul mud of a remote lagoon in the Philippines. They say studying the animal, a giant shipworm with pinkish siphons at one end and an eyeless head at the other, could add to our understanding of how bacteria cause infections and, in turn, how we might adapt to tolerate or even benefit from them.
Live specimens of the massive shipworm, which was captured by Distel's team with the help of researchers from the area, have eluded scientific description for hundreds of years. Distel, research professor at Northeastern's Marine Science Center, has been searching for it for two decades. He had examined fragments of its tusk-like shell, made of calcium carbonate, and gazed at dead specimens preserved in ethanol. But neither he nor any other living researcher had ever come across a live specimen of the ancient species: a bivalve mollusc named Kuphus polythalamia that was first described (though incorrectly classified) by Swedish taxonomist Carl Linnaeus in 1758.
In a new paper published in the Proceedings of the National Academy of Sciences, Distel and his colleagues present their research on the live shipworm. They describe how, remarkably, it does not eat, at least not much – it has a tiny digestive system. Instead, bacteria living inside its gills convert sulfur gas from rotting wood into nutrients to keep it alive.
"Most shipworms are very delicate, translucent, usually white, beige, or pink," says Distel, who directs the Ocean Genome Legacy at Northeastern, a unique biological bank where researchers acquire DNA from organisms around the world for genetic analysis. "They're mostly small, a few centimeters long. You have to be very careful not to damage them when you're taking them out of the wood, where they live. This thing was like a baseball bat. It was a beefy, muscular animal, jet black."
Indeed, the team had to edit much of the audio out of the original video of the creature's debut. "When I took that thing out of the tube, there was a collective gasp among the whole group," says Distel, "along with quite a number of expletives that had to be deleted."
Distel's genomic analysis of the host Kuphus, as well as the bacteria whipping up its food, revealed a symbiotic relationship between the two that elucidates an "evolutionary stepping stone," he says.
Shipworms, as a rule, eat wood – hence their being dubbed "termites of the sea." Bacteria in their gills, Distel discovered earlier, secrete enzymes that travel to their gut and break down the wood, which is made of cellulose, an organic material, and turn it into sugars.
But wood can also serve as a source of hydrogen sulfide – a sulfur gas that smells like rotten eggs. "We believe that somewhere along the line, a shipworm acquired a sulfur-oxidizing bacteria as a symbiont, and it was able to get energy not just from the wood but also from the inorganic gas hydrogen sulfide coming from the wood as it rotted," says Distel. "Eventually the new symbiosis completely replaced the old symbiosis."
Other marine animals also get their nutrients from sulfur-oxidizing symbionts, but the sulfur source differs. The giant tubeworm Riftia pachyptila, for example, gets its sulfur from the effluence of volcanic hot springs on the sea floor.
The symbiont bacteria convert the hydrogen sulfide into food similar to the way photosynthesis works in green plants. Green plants take energy from sunlight and use it to synthesize sugars from carbon dioxide. The bacteria take chemical energy from hydrogen sulfide, pull carbon dioxide out of the seawater, and synthesize sugars and other nutrients.
"These bacteria live inside the animals' cells, alongside the cytoplasm," says Distel. "If we or any vertebrate had bacteria living inside our cells, we'd be very sick. In the long run, studying these symbioses may tell us a lot about the process of disease. What is it about these bacteria that they can infect the host yet not harm it? How does the host learn to tolerate, and even benefit from, the bacteria?"