Lithoredo abatanica is an organism with an unusual appetite: This creature eats stone. And when it excretes, what comes out is sand, the leftovers of a still-mysterious digestion process.
The mollusk, unearthed from the bottom of a river in the Philippines, was introduced this week by an international group of scientists in Proceedings of the Royal Society B. It is a shipworm, a group of burrowing animals related to clams, but so different from known examples that it is both a new species and genus.
Shipworms are usually known for their habit of eating wood. It’s right there in the name: They use their shells, attached to one end of their bodies, as chewing devices to burrow into and consume ship bottoms, docks and any other submerged wood. The behavior has made them the plague of mariners past and present, and in recent times, they have even sampled the delights of at least one New York City pier.
Wood-eating shipworms fascinate scientists because they digest pulverized wood with the help of symbiotic bacteria that live in their gills. The bacteria manufacture an array of enzymes and other substances, and studying them and finding new shipworms may prove helpful in the search for new antibiotics, a subject of interest to the scientists behind the new paper.
They first heard reports of a mysterious shipworm in the Abatan River on Bohol Island some years ago, from an expedition organized by the French National Museum of Natural History. Reuben Shipway and Daniel Distel of Northeastern University, members of the Philippine Mollusk Symbiont International Collaborative Biodiversity Group, went in search of the creatures with snorkeling masks and chisels in tow.
A local resident suggested they check the bottom of the river; underwater, they spotted huge chunks of sandstone peppered with holes. Protruding from some of the rocks were the twin flags of shipworm siphons, organs that the creatures use to excrete.
“That was when we knew we’d struck shipworm gold,” said Shipway.
The researchers extracted the translucent white creatures from their burrows and performed various tests and dissections. That was when they learned more details about L. abatanica’s intriguing lifestyle.
The cecum, a large organ used in digesting wood that is common across shipworms, was missing in the new species. The shipworms’ guts, however, were full of fragments of stone, which chemical analysis showed to be the same stone that the animals were living in. What came out was stone, too.
“We had a few animals in a makeshift aquarium,” said Shipway, “and you could put the animals in the aquarium and basically watch them excreting fine particles of sand out of their siphon.”
Exactly what the shipworms are getting from the stone is not yet clear. One possibility, the researchers speculate, is that the granules could be helping them grind up plankton and other creatures floating in the water, much the way gizzard stones help birds break down food they’ve swallowed whole.
But it may be that the worms are able to extract nutrients from the stone in a way not yet understood. The gills of the stone-eating shipworms are much larger than other shipworms’, suggesting that the organ’s tiny inhabitants may be particularly important to the creature’s survival. The team is now working on sequencing the genomes of L. abatanica’s symbionts to identify them and get a glimpse of how their metabolism works.
“It’s already looking very, very interesting,” said Distel. “What we can say is that the bacteria we find in the gills are not related to the bacteria we’ve found in any other shipworm to date.”