The rock-eating shipworm is quite different from its wood-eating counterpart, the team reports today in the Proceedings of the Royal Society B. Really clams, all shipworms have two shrunken shells that have been modified into drill heads. Hundreds of sharp invisible teeth cover the shells in the wood eater, but the rock-eating shipworm has just dozens of thicker, millimeter-size teeth that scrape away rock.
Marine shipworms store the wood they eat in a special digestive sack, where bacteria degrade it. Like other shipworms, the rock-eating shipworm still ingests what it scrapes away to make its protective burrow, but it lacks both the sack and its bacteria and likely doesn’t get much sustenance from the rock bits. Their ingestion may be a holdover from wood-eating ancestors. Instead, it seems to rely on other bacteria residing in its gills to produce nutrients or food sucked in by a siphon at the clam’s back end for nourishment.