Shipworm

Shipworms, or teredo worms, are a family of marine bivalve mollusks (Teredinidae) that eat wood submerged in saltwater. Despite their long, worm-like shape, which makes their classification as bivalves unclear—sailors have long called them "worms"—they are actually closer relatives of clams. These animals are infamous for tunneling into and eventually wrecking any wooden structure in seawater, such as piers, docks, and ships, earning them the nickname "termites of the sea."

The best-known genus, *Teredo*, was named by Carl Linnaeus in his 1758 *Systema Naturæ*. One species, *Kuphus polythalamia*, can grow over 1.6 meters long and 7 centimeters wide, making it one of the largest bivalves on Earth. It eats wood until maturity, then settles into marine sediment and relies on symbiotic chemoautotrophic bacteria that feed on sulfur for nutrition. Another species, *Lithoredo abatanica*, burrows into and feeds on carbonated limestone in freshwater.

Adult shipworms range from a few centimeters to about a meter in length, depending on the species. The body is cylindrical, slender, and naked, with a worm-like exterior, but they still have recognizable bivalve features: valves, gills (ctinidia), a mantle, and siphons. The gills sit mainly inside the branchial siphon, which pumps water over them. Two long siphons extend from the posterior end, passing between a pair of calcareous plates called pallets. When alarmed, the animal withdraws its siphons, and the pallets block the tunnel opening.

The two valves of the main shell are at the anterior end, used for boring. They are roughly triangular, concave inside, and convex outside, often deeply sculpted into sharp grinding surfaces. The valves are separated, with the mantle aperture between them, and a small foot can protrude through this opening. As shipworms bore into wood, bacterial symbionts in a gut organ called the typhlosole help digest wood particles. *Alteromonas* or related bacteria there digest lignin, breaking down tough molecular layers so enzymes can access cellulose. Another bacterium, *Teredinibacter turnerae*, lives in the gills and secretes cellulose-digesting enzymes, which may be sent to the gut via the gland of Deshayes. These secretions work alongside the shipworm’s own carbohydrate-active enzymes.

The burrow is usually lined with a calcareous tube. The small, separate valves at the anterior end are used only for excavation, as the animal spends its entire life surrounded by wood. *Teredo navalis* develops from eggs to metamorphosing larvae in about five weeks, half of that time in the mother’s gill chamber before being released as free-swimming larvae. Their sexes alternate: young are hermaphrodites, adults can be male or female, typically male first then female. A second male-to-female phase may occur, but shipworms rarely live long enough to complete it. Their lifespan is 1 to 3 years.

Shipworm anatomy includes typical bivalve organs, though shaped by the animal’s thin, long body. The gills are split into a small anterior half and a much larger posterior half, linked by the alimentary tract. The heart-kidney system is tilted, positioning the kidneys in a distinctive way.