Life starts for Teredo navalis after internal fertilization as a typical bivalve larva inside its mothers gill chamber. Here it can after a „placental reaction“ be nourished by its parent and grows to the veliger stage and a size of about 70 µm. Together with thousands or up to 2 million siblings tha larva is expelled to find a new environment, i.e. a new piece of wood.
The trip takes two or three weeks and depends on the surrounding water currents. Like many other mollusc larvae Teredo lives on a diet of microalgae. In fact it is almost indistiguishable from other bivalves at this stage.
Once the larvae is „ripe“ for settlement, it can locate logs by „smelling“ decomposing wood and actively swimming the last few centimeters. It crawls with its foot on the surface to find a suitable location, where it attaches itself with a single byssus strand. Now starts a rather miraculous boring action. The larval shell is not calcified and therefore quite soft, so that it cannot be a functional borer. Instead it is speculated that the larva can soften wood with the aid of maternal enzymes and then excavate a small hole with its foot. In there Teredo metamorphoses to its adult shipworm form.
A shipworm cannot ever leave its hole, being connected to it close to the opening. It bores and grows into the wood. The opening is extended only slightly (up to ca. 1 mm diameter) during the lifetime of two to three years, but the tunnel grows in diameter from a millimeter to about pencil size. It is perfectly circular due to the circular rasping motion of the shell and usually extends to 20 or 30 cm length. However, the longest tunnel ever recorded was 62 cm long! And that was not in a warm water region, but in the Kattegat between the North Sea and the Baltic.