It’s a conundrum anyone who comes across a tangle of slimy worms on the sidewalk after a rainstorm might ponder: Which side is the tail and which side is the head?
The question seems simple enough, but two newly described marine-dwelling, worm-like creatures reported in separate studies this week sort of complicate the matter. One—a marine worm that lives off the coast of Scotland—has a pair of eyes on its butt. The other, which can be found in a freshwater river in the Philippines, is a bivalve shipworm with a unique talent: it eats rocks and poops out sand.
Brandon Specktor at LiveScience reports that the Scottish worm was spotted during a survey of an unexplored area of the West Shetland Shelf Marine Protected Area to the north of Scotland. In sand pulled from the seafloor just 400 feet below the surface, researchers found 80 of the new quarter-inch-long worms. Most of the marine worm’s body wasn’t particularly unusual. But there was something peculiar about its hind-end. They discovered it has a pair of little tentacles sprouting from its rump, with a beady little black eye at the end of each stalk. The species, named Ampharete oculicirrata, is fully described in the European Journal of Taxonomy.
So why does the worm have eyes on its derriere? Specktor reports that’s not unusual for marine worms to have eyes both on their head and other places on their bodies to keep tabs on predators while they search for dinner on the seafloor. Finding eyes on their butt, however, is unusual.
The worm is an excellent case study in showing just how little we know about the seafloor. “The fact that it was found in relatively shallow depths, relatively close to the Scottish coastline, shows just how much more there is to understand about the creatures that live in our waters,” Jessica Taylor, marine evidence advisor for the United Kingdom’s Joint Nature Conservation Committee, says in a press release.
Further exemplifying that point is a different, equally odd worm-like critter that lives in the shallows on the other side of the planet. In another paper recently published in the Proceedings of the Royal Society B, researchers describe a new shipworm, Lithoredo abatanica, that eats rocks instead of wood—and then excretes sand from its backside.
Despite their name, shipworms aren’t really worms at all. They’re a type of bivalve with a tiny shell at one end and a long, worm-like body on the other. Long the bane of sailors, the animals specialize in digesting wood, with some causing significant damage to wooden sailing vessels and docks. Unique enzymes and other substances in the shipworm's gut allows the creature to digest the wood, making them particularly interesting to researchers studying new antibiotics.
In 2006, a French expedition to the Abatan River on Bohol Island in the Philippines first noted the existence of an unusual shipworm in the freshwater ecosystem. But it wasn’t the focus of a research expedition until a multinational group of scientists in the Philippine Mollusk Symbiont project sought out the strange worm in August 2018.
Veronique Greenwood at The New York Times reports that when they reached the river, locals—who actually eat the worms—suggested they search the bottom for the bivalve. While diving, the researchers noticed large chunks of sandstone dotted with holes. Upon closer look, researchers noticed the telltale twin siphons of the six-inch, sausagey shipworms protruded out of some of the holes. “That was when we knew we’d struck shipworm gold,” the study’s first author Reuben Shipway, a marine biologist at Northeastern University in Boston, says.
According to a press release, the worms and their abandoned holes had completely reshaped the riverbed, with tons of little fish and crustaceans taking up residence in the cavities.
After collecting and dissecting some specimens, they found that the worms had flat teeth good for boring through rock and were missing the cecum, an organ in other shipworms that digests wood. Instead, their gut was full of stone fragments, the same type of stone they lived in, and they excreted sandy particles of stone as well. The researchers were able to observe the process by watching some of the creatures in an aquarium.
The differences between abatanica and other shipworms means it likely split off from an ancestor of traditional shipworms long ago, and the species is not closely related to its wood-eating brethren.
So, what’s the point of eating rock? The researchers don’t think the worms derive nutrition from the rock. Instead, they may get nutrition from unique bacteria living in their oversized gills or pull in food from their siphon. The rocky particles in their gut may help to grind up things like krill, similar to the way grit in a bird’s gizzard works.
The team tells Greenwood that they hope to soon sequence the strange worm’s DNA to understand how its metabolism works, and they especially hope to learn more about the symbiotic bacteria in their gills.
“We know from previous shipworms that the symbiosis is really important for the nutrition of the animal,” Shipway says in the press release. “We’re going to be examining the symbiosis really closely for further clues about how they get their food.”