What—just because they're called gut microbes, you've been keeping them in your colon? How unoriginal. This is Bankia setacea, also called the Northwest or feathery shipworm. Humans usually pay attention to shipworms only when they perform their namesake activity: burrowing face-first into our boats or docks and eating their way through. Shipworms are bivalves, like clams or scallops, but their shells are shrunken into a set of rasping tools at each worm's front end. Animals that subsist on wood usually need help. Studies of termite guts, as well as wood-eating fish and beetles, have found specialized bacteria that break down the tough plant materials the animals themselves can't. Even humans and other omnivores and carnivores get a digestive hand from microbes. The abundant bugs in our intestines play a role in breaking down our food and getting the nutrients out. Yet the digestive tract of Bankia setacea is weirdly empty. Daniel Distel, a scientist at Northeastern University who studies symbiotic relationships in marine life, says researchers have known since the 1970s that shipworms keep plenty of bacteria in their gills (even though their guts are vacant). "It has been suspected for some time that they contribute to wood digestion by the host," he says. But an animal that stores its digestive bacteria someplace other than its digestive tract would be the first of its kind. So scientists need "an abundance of evidence" to prove it, Distel says. "Recently, advances in genomics and proteomics have given us the tools to answer many questions that were previously tough to address." Distel and his colleagues performed a whole host of experiments to look for the evidence they needed. They took bacteria from the gills of Bankia setacea and grew them in the lab. They showed that these bacteria, and their genes, are a good representation of the whole bacterial community in the shipworm's gills. They found that the gill bacteria can, and do, make enzymes for breaking down the cell walls of wood. Then came the crucial piece of evidence. Just as scientists had guessed, the bacteria's wood-digesting enzymes were also present in the shipworm's gut
—even though there were no bacteria there. "Although we suspected this result, there was no precedent for such a system," Distel says. The shipworm keeps bacteria in its gills that can digest wood, and steals their enzymes to use in its gut. It's still not clear how the worm moves the bacterial enzymes between its organs, but it is obvious that B. setacea is doing something pretty unusual. Distel thinks it's not the only one. "To our knowledge, all shipworm species house bacteria in their gills," he says. They may all be using similar strategies, harvesting the bacterial enzymes and somehow shipping them to where they're needed. (Distel notes that another "wood-eating critter" called a gribble also keeps a clean, microbe-free gut. But gribbles solve the problem by making their own enzymes.) There's still one outstanding question: why? "Why do shipworms house their digestive bacteria in their gills rather than in the gut, where any sensible animal would put them?" Distel says. "I can’t answer that." He speculates that keeping bacteria out of their guts might somehow let shipworms control the digestive process better. Maybe they don't have to share their broken-down food with the microbes, as they would if the microbes lived right in their guts. No matter their reasons, shipworms are showing us that when it comes to wrangling microbes, it's not necessary to stick to tradition. Image: Daniel Distel.
O'Connor, R., Fung, J., Sharp, K., Benner, J., McClung, C., Cushing, S., Lamkin, E., Fomenkov, A., Henrissat, B., Londer, Y., Scholz, M., Posfai, J., Malfatti, S., Tringe, S., Woyke, T., Malmstrom, R., Coleman-Derr, D., Altamia, M., Dedrick, S., Kaluziak, S., Haygood, M., & Distel, D. (2014). Gill bacteria enable a novel digestive strategy in a wood-feeding mollusk Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1413110111