Shipworm Teredo navalis
Despite its wormlike appearance, the shipworm is a type of clam that has become elongated as an adaptation to its burrowing lifestyle. Its bivalve shell, situated at the anterior end, is very small and ridged. The shipworm uses it with a rocking motion to bore into wooden objects. Outside the shell its body is unprotected, except for a calcareous tube it secretes to line the burrow. These worms damage wooden structures, such as piers, and in the past caused many ships to sink. The burrow entrance is only about the size of a pinhead, but the burrow itself may be over in (1 cm) wide, so the extent of an infestation is often underestimated until it is too late.
Shipworms change from male to female during their lifetime, and the female form produces many eggs, from which free-swimming larvae hatch. When they mature and settle on a suitable piece of wood, the larvae quickly metamorphose into the adult form and start burrowing.