Known as the clam that sank a thousand ships, shipworms are odd-looking — and odd-behaving — animals.
The shipworm, a worm-like clam that can grow to at least two meters in length, feeds on wood by burrowing its clam-encased head into ship hulls and other sea wood and grinding the cellulose into particles with tiny clam teeth.
But unlike termites, which have bacteria in their gut to break down the wood they eat, the bacteria that shipworms need to produce digestive enzymes are at the other end of their bodies, in the gills.
In 1848, French scientist Gerard-Paul Deshayes described extremely tiny channels running the length of the shipworm from the gills to the mouth and stomach of the clam, but could not explain what they were for.
Scientists from the Northeastern Marine Science Center and the Coastal Sustainability Institute in Nahant, Massachusetts solved the mystery using the latest technological tools that sliced the shipworm into microscopic slices.
In an article published Nov. 9 in Proceedings of the Royal Society B — the Royal Society’s flagship biological research journal — research professor Dan Distel and his research associate Marvin Altamia explained how wood-dissolving enzymes make their way through the shipworm’s body found through canals that are only a fraction of the diameter of a human hair.
“The question has always been how these enzymes get from the gill to a place where they can actually help the animal digest wood,” says Distel, director of Northeastern’s Ocean Genome Legacy Center. “There is this physical transport from one place to another. We solved that in this work.”
His lab drew on Distel’s postdoctoral work with John Waterbury at the Woods Hole Oceanographic Institution, where Distel used fluorescent DNA markers — then one of the latest technologies in molecular biology — to show that the bacteria that Waterbury had in the lab grown was the same organism invade shipworm cells.
It was one of the very first applications of fluorescent in situ hybridization, or FISH, says Distel.
The fact that shipworms have bacteria that help them function is of interest to scientists.
“For vertebrates like us, if you have bacteria in your cells, it’s very sick. Invertebrates can have bacterial infections in their cells that are actually beneficial,” says Distel, so-called symbionts.
For this latest round of research, his lab took tiny one to two centimeter shipworms and cut them into thin slices using a guillotine-like device called a microtome.
Shipworms are like tubes, with their heads sticking out of the wood and their gills at the other end, where they absorb oxygen from seawater.
Distel and his researchers created lab-grown antibodies that synthesized shipworm-produced enzymes from genetic sequences. Then they applied the antibodies to the shipworm discs to see where they stuck.
A second fluorescent antibody was then applied to illuminate and visualize the enzymes under microscopic examination, providing a road map of the digestive enzymes’ travel route, which turned out to be exactly as Deshayes had described, Distel says.
“We could see spots in the gills where bacteria are, we can see spots in the gut and we can follow the pathway from the gills back to the stomach and mouth. We were able to connect the dots,” he says.
The channels in question are very tiny – about 30 to 50 microns in diameter. A human hair is about 90 micrometers in diameter.
“But apparently (the channels) are big enough to act as a little fire hose to carry the enzymes to the mouth and intestines,” says Distel.
Next, he wants to study how the enzymes produced by the bacteria in the shipworm host cells cross the cell membrane to reach the ducts, from which they can then be transported to the digestive tract.
In many ways, “the shipworm is kind of a tiny, natural model for a large cellulosic ethanol plant,” says Distel.
Understanding how it works can help produce renewable liquid fuel from agricultural and paper waste, he says.
Scientists are also interested in understanding how symbiotic, or beneficial, bacteria avoid making animals whose cells they inhabit sick,” says Distel.
“Can you learn anything from this about the treatment of pathogenic infections?”
As for the shipworms themselves, mysteries remain. According to Distel, no one knows how long they can live, but scientists do know that they become fertile within a month or two of birth.
“Then they just keep growing and growing and growing,” Distel says, adding that shipworms can eat through the hull of a wooden ship in a matter of months.
Shipworms even eat ancient cypress logs from an underwater forest exposed by hurricanes eight miles off the Alabama coast. The forest is featured in a video produced by the Marine Science Center and released in October.
“They have what we call indeterminate growth,” says Distel. “That means they just keep growing as long as there is wood to eat. Or something will kill her.”
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