However, even though the clam consumes limestone, its discoverers aren't sure if the clam is able to catch any real food from these rocks. For example, do the bacteria in the clam's gut help break down the rocks and release nutrients?" Study leader Reuben Shipway, a postdoctoral researcher at the Center for Marine Science at Northeastern University in Massachusetts, said, "We want to look at the s*** organisms that live inside these animals to see if they provide any nutrition, and that's an area of research we're focusing on right now." . [In photo: Spooky deep-sea creatures]

The newly discovered clams are a type of sea worm, which is the name of a group of clams called so because they swallow wood, especially from boats. Wood is tough to eat, but adaptations help these clams burrow into the material; these adaptations include "rows of small, sharp teeth on the shell and a specialized wood-storage and digestive organ called the cecum," Shipway told Live Science.

Every known boat worm eats wood, so when the 2018 Filipino locals in Bohol province told Shipway and his colleagues about a rock-eating freshwater worm, they were surprised. Locally, he said, it is known as "Antigua" and young mothers eat it because they think it will help them produce milk. (The newly discovered species was found during a recent expedition by French biologist Philippe Bouchet at the Museum of Natural History in Paris, but the researchers say it was locals who helped the new team find the mysterious sea worm.)

The rock, which has 12 burrows, was formed by limestone-eating sea worms. (Reuben Shipway; Marvin Altamia)

Unlike other shipworms, the newly-named Litoredo abatanica (roughly "stoneworm from the Abatan River") has lost all its wood-boring adaptations, including the appendix, Shipway said. appendix, Shipway says. More specifically, the clam's "shell has these very large spade-like projections that can be used to dig into the rock," he said.

The creature's shell crunches the rock, which the animal then eats, digests and expels as fine sand." Shipway said, "A small percentage of animals do swallow rocks; for example, birds use sand sac stones to help with digestion." But stromatolites are the only animals known to eat rocks by digging holes.

It's the newly described rock-eating shipworm known as the stromatolite. (Reuben Shipway; Marvin Altamia)

He described the rock-eating clams as "very strange - they're plump, translucent, worm-like clams." Most of the specimens the researchers collected were 4 inches (10 centimeters) long, but there were some individuals that were much larger.

When I was diving in the river, I saw burrows that were more than 2 feet (60 centimeters) long Shipway said So there may be some absolute monsters living deep in the rocks.

By eating the rocks, the Abatanika is literally changing the course of the river, Shipway added These burrows also provide habitat for a myriad of other species that live in the river, including crabs and fish This is a very rare, yet important process in freshwater environments.

The study was published online today (June 19) in the Journal of the Royal Society B: Biological Sciences.

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Originally published in Live Science.