Shipworms were already weird animals, but they just got weirder. The ocean-dwelling clams, which actually look much more like slimy worms, are some of the only creatures in the world that can eat wood. Now scientists have figured out that shipworms are even more unique than we thought: Instead of breaking down their food using bacteria in their guts (the way basically almost all animals do), shipworms break down wood using bacteria in their gills.
This revelation was published Monday in the Proceedings of the National Academy of Sciences, with research lead by Daniel Distel, director of the Ocean Genome Legacy Center of New England Biolabs at Northeastern University. Distel, along with researchers from the U.S. Energy Department's Joint Genome Institute, is interested in harnessing the shipworm's unique ability to digest wood to improve the production of renewable, environmentally friendly fuels.
When the shipworms gnaw on wood, they have to digest cellulose — the toughest plant material there is. The cellulose in agricultural crop waste such as corn and wheat stalks represents a massive reservoir of untapped energy.
"If you understand how to break cellulose down into component sugars, it's then a very easy step to go to ethanol — which is a potential biofuel," Distel said.
But in investigating how the mollusks digest cellulose, Distel and his colleagues were shocked to discover that the creatures have an as-yet-unheard-of relationship with the bacteria in their bodies.
Instead of living in the gut and secreting digestive enzymes there, the bacteria that live in shipworms actually take up residence in the gills. Somehow the enzymes they produce make the journey to the gut and aid in digestion, but Distel has no idea how.
"This is the only case described so far," he said, though he believes there must be similar systems out there in nature. "We've never seen an animal use bacteria outside the digestive system for digestion."
It's equally surprising that the bacteria can produce enzymes that survive the journey from one part of the host to the other.
"Our theories on how this might work range from the trivial to the really fascinating," Distel said. On the one hand, it could be very simple: Maybe the system is passive, with tiny holes in the cells in the gill allowing the bacterial enzymes to leak out and find their way to the gut.
The more exotic theory, Distel said, is that the bacteria manage the transport themselves.
"Some pathogens are able to inject proteins into the cells of their hosts, and they have special pathways to secrete those proteins," he said. "Generally those proteins are designed to do harm, to make the host more suitable to the bacteria's needs. But it could be possible that these bacteria have figured out a way to export enzymes into host cells for good, to support their symbiotic relationship."
Distel and his colleagues want to figure out how and why these bacteria do their magic on the shipworms' guts but are also committed to using their digestive skills for commercial applications.
"These enzymes turn out to be fairly complex proteins, made up of several active modules that are connected to each other," Distel said. These bits and pieces can be configured into different combinations to improve their function. "Other organisms do this, but these bacteria seem to take it to an extreme."
He hopes that this strategy of mixing and matching will help scientists find an ideal enzyme for breaking down cellulose, leading to faster and more efficient biofuel production. And, in the meantime, he's hoping to figure out more of the shipworm's secrets.
"They just have a certain charm," Distel said.