The rock-eating clam is, as advertised, a clam that eats rock. Lithoredo abatanica lives in a stretch of the Abatan River on Bohol Island in the Philippines, and instead of chewing wood the way every other shipworm does, it chews limestone. It swallows the limestone. It then defecates the limestone, as a slurry of fine calcite sand, out the back of its siphon. The whole operation takes place in fresh water, well upstream of the river's brackish mouth.
The species was named in 2019, in Proceedings of the Royal Society B, by a group led by J. Reuben Shipway, Marvin Altamia and Daniel Distel out of Northeastern University and the University of the Philippines. The companion paper, in PLoS ONE, lays out what the boring does to the river and to a century's worth of palaeontological assumption. Both papers are open access; both are the same story told twice, in two different keys.
What makes the animal interesting isn't that it bores. Plenty of bivalves bore — the piddocks, the angelwings, a small guild of saltwater specialists that rasp their way into clay, coral or wood. What's interesting is that Lithoredo does it by ingestion. Wood-boring shipworms rasp with their shell valves, swallow the shavings, and digest the cellulose with help from bacterial symbionts housed in a special gut pouch called the caecum. Lithoredo has no caecum. Its valves are not the fine, saw-toothed drills of its cousins; they are short, broad, blunt, more like a tiny stone chisel than a saw. The animal rasps limestone off the working face of its burrow, swallows it, and the rock passes through. X-ray diffraction of the gut contents comes back as calcite. The composition of what's inside the clam matches the composition of the bedrock outside it.
How the animal feeds is the open question. Nobody has published a definitive answer. The most likely candidates are filter-feeding on river plankton (the gills are unusually large), scraping algae and microbes off the burrow walls, or — as with the giant Indo-Pacific shipworm Kuphus polythalamius, which lives in mud and houses sulfur-oxidising bacteria in its gills — running on some kind of bacterial symbiosis. The Shipway group's 2019 description says so plainly: "the trophic strategy of this species remains obscure and possibly unique." That sentence is, in a quiet way, the most interesting one in the paper.
| Lithoredo abatanica at a glance | |
|---|---|
| Habitat | Freshwater (less than 0.5 ppt) limestone bed and banks of the Abatan River, Bohol |
| Length | 5.5 to 105.4 mm in collected specimens; larger individuals likely exist in the subtidal bed |
| Substrate | Moderately indurated Neogene foraminiferal packstone (Anda Limestone) |
| Diet | Limestone, ingested; energy source undetermined |
| Distribution | A roughly 2-km stretch of the Abatan River, bracketed by two known sites |
| First described | Shipway, Altamia, Rosenberg, Concepcion, Haygood & Distel, Proc. R. Soc. B, June 2019 |
The thing I find genuinely satisfying is the second-order consequence. Shipworm borings in fossil wood — long, sinuous, calcite-lined tunnels — have an ichnological name, Teredolites longissimus, and for roughly a century that name has functioned as a kind of fossil ID card: Teredolites in the stratigraphic column meant the sea was here. Beach, estuary, shallow shelf, marine transgression. The 2019 papers don't overturn that, but they do pry the door open. Lithoredo makes shipworm-shaped borings in rock, in fresh water. The tunnels look, at first glance, like a typical wood-bored assemblage. If a palaeontologist pulled one out of an outcrop tomorrow and didn't know about Lithoredo, they would almost certainly read it as evidence of an ancient shore.
The honest version of the story is that there is now a freshwater clam quietly chewing a riverbed into sand, and the only reason this is not a bigger deal in geomorphology is that nobody has yet found a fossil analogue. The substrate is there. The animals are there. The borings are dense enough to count as ecosystem engineering — abandoned tunnels host crabs, shrimp, limpets and polychaetes, and the clasts of bored limestone are already being mechanically reworked downstream into the Bohol reef catchment. The 2019 papers are the discovery notice. The interesting results will be the ones that come after.
Sources
- Shipway, J.R., Altamia, M.A., Rosenberg, G., Concepcion, G.P., Haygood, M.G., & Distel, D.L. (2019). "A rock-boring and rock-ingesting freshwater bivalve (shipworm) from the Philippines." Proceedings of the Royal Society B 286(1905): 20190434 — doi:10.1098/rspb.2019.0434 (open access at PMC6599978)
- Shipway, J.R., Rosenberg, G., Concepcion, G.P., Haygood, M.G., Savrda, C., & Distel, D.L. (2019). "Shipworm bioerosion of lithic substrates in a freshwater setting, Abatan River, Philippines: Ichnologic, paleoenvironmental and biogeomorphical implications." PLoS ONE 14(10): e0224551 — doi:10.1371/journal.pone.0224551
- Wikipedia, Lithoredo abatanica — en.wikipedia.org/wiki/Lithoredo
- Wikimedia Commons, "Morphology of Lithoredo abatanica" (Figure 3 of the 2019 Proc. R. Soc. B description, CC BY 4.0) — commons.wikimedia.org/wiki/Category:Lithoredo_abatanica