For centuries, no wood was safe from the insatiable appetite of a chubby little clam known as a shipworm.

It was the bane of sailors, who rightly feared docks collapsing and ships taking on water, due to its ravages.

A bivalve mollusk, the shipworm was the last stowaway you wanted to find latched onto your boat in the middle of the Pacific Ocean.

Since then, ships have been built of sturdier stuff — iron and steel — and the menace of the wood-eating shipworm has mostly faded.

But back in 2006, scientists discovered a new shipworm on the block, with a very different kind of "sweet" tooth: Rocks.

Not Pop Rocks. Not rock lobster. Rock rocks.

The species, found in freshwater bodies in the Philippines, somehow eluded detailed study until last year when U.S. researchers cracked open a few rocks and took their strange occupants back to the lab. Their findings, published this week in Proceedings of the Royal Society B, suggest when it comes to weird, the rock-eating shipworm takes the cake — so long, of course, as that cake is made of limestone.

"It's almost mythical," lead researcher Reuben Shipway of Northeastern University, explains in a news release. "All the other species, for at least some part of their lives, actually require wood."

Not this little weirdo.

Researchers describe the species, Lithoredo abatanica, as essentially a 6-inch worm living in a toothy clam shell. Those teeth are big and flat — perfect for boring into stone and contrasting sharply with the saw-toothed smile of its wood-mulching cousin.

And while the wood-eating variety has a sac-like organ for storing and digesting wood, the rock eater forgoes the baggage in favor of a much more straightforward approach: The creature takes in a rock at one end, and expels sand at the other.

"There are a small number of animals that do ingest rock — for example, birds use gizzard stones to aid digestion," Shipway tells LiveScience. "But Lithoredo abatanica is the only known animal that eats rock through burrowing."

Fortunately, we don't build boats made from rocks. But these creatures do have an impact on the course that rivers take. And since these shipworms can turn rocks into sand, researchers suggest they could redirect rivers with potentially disastrous results.

Now, at some point in your life, someone — in a fit of ill humor — may have suggested you "suck rocks."

The idea is that a rock is about the least useful thing you could put in your mouth. And it's true, the nutritional value of a rock remains zero. That also applies, researchers suggest, to the rock-eating shipworm.

Without the digestive sack of its wood-loving cousin, the Lithoredo abatanica doesn't get any sustenance from its strange habit.

So why does this little white worm go through the trouble of eating rocks — and why is its body custom-made for the task?

Sure, some of that stone is eventually transformed into a protective burrow for the animal. And when the shipworm abandons its home, crabs and shrimp are happy to move in. But for the most part, the researchers have yet to find a motive for its rock-eating madness. And more to the point, how Lithoredo manges to get so ... chubby.

How do you get your protein, worm?

Wood-eating shipworms, for example, keep a little symbiotic bacteria around in their gills to help them digest wood. But scientists have yet to determine what kind of bacteria a rock eater needs to get its dinner down. It may be something completely new that's derived from bedrock at the bottom of rivers, a compound that could someday propel advances in human medicine.

"We know from previous shipworms that the symbiosis is really important for the nutrition of the animal," Shipway notes in the release. "We're going to be examining the symbiosis really closely for further clues about how they get their food."

This newly identified critter eats rocks and secretes them as sand
Researchers have found a kind of shipworm that eats its way through rocks.