A shipworm begins as a conventional bivalve larva that settles on submerged wood. It then grows into a form that looks nothing like a clam: a soft tube of a body up to tens of centimetres long, with the two valves reduced to small ridged plates at the anterior end. Those plates are worked against the timber to rasp out a tunnel, which the animal lines with a smooth calcareous coating. Two siphons remain at the entrance for water exchange, closed by paddle-shaped pallets when conditions turn bad.
Wood is a poor food, mostly cellulose and lignin with almost no nitrogen. Shipworms house symbiotic bacteria in a specialised gland of the gills, which supply cellulose-degrading enzymes and fix nitrogen, closing the nutritional gap.
Historically the consequences were severe. Shipworm damage weakened wooden hulls badly enough to drive the adoption of copper sheathing, and in 1731 to 1732 it prompted a crisis in Dutch sea defences when infested timber dyke revetments failed. The animal is destructive precisely because it is a mollusc doing a job usually left to insects and fungi.