TEREDO. From time immemorial seafaring men have known and dreaded the shipworm, which bores into and destroys the stoutest timbers in the sea. Nowadays, although the danger to ships is no longer to be feared, the timber of harbour works is still exposed to damage by the same pest. How serious the danger may become is shown, for example, by the outbreak of shipworm in San Francisco bay during the four years 1917-21 when the destruction of wharves and jetties was estimated to have cost the enormous sum of 25 million dollars.
The writers of antiquity did not distinguish clearly between the marine timber-worm and the wood-boring insect larvae of the land and they sometimes applied the name Teredo to both, but it was later restricted to the marine borer and is still in use as a general term for the shipworms, although in the nomenclature of zoological taxonomy it is the name of only one of the genera forming the family Teredinidae.
It was Godfrey Sellius who, in 1733, first showed that Teredo is a bivalve mollusc, although it differs widely in structure from the more familiar members of that group. When its burrow in the wood is laid open it is seen to have a long worm-like body bearing at its inner end a pair of small shelly plates which repre sent the valves of the shell. At the outer end, where the burrow narrows to a small opening, the body ends in a pair of siphons which can be extended from the surface of the wood and serve for the entrance and exit of the respiratory current of water.
When the siphons are withdrawn the opening of the burrow is blocked by a pair of paddle-shaped plates of shell known as the pallets. The interior of the burrow is lined with shelly material which is usually merely a thin film but when exposed by the decay of the wood may be considerably thickened. The valves of the shell are nearly hemispherical, with a deep right-angled notch occupying the ventral half of the anterior margin. In the gap left by these notches is the round sucker-like foot with which the animal adheres now to one part and now to another of the interior of its burrow. Parallel with the edges of the notch, the surface of each valve is marked with two sets of fine ridges which are seen under the microscope to be rows of minute teeth like those of a file. It is by means of these teeth that the excavation of the wood is effected. While the foot is adhering to the wood a rocking movement is imparted to the valves by alternate contractions of the anterior and posterior adductor muscles and the wood is rasped away as a fine dust. This is swept into the mouth and passes into the stomach. It has been shown beyond doubt that the wood is acted on by the digestive ferments and utilized as food, but in addition the stomach is usually found to contain minute floating organisms drawn in with the respiratory current.
In some species of Teredo the eggs are discharged and float freely in the sea, in others they are retained in the gill-cavity and pass through the early stages of their development before they are set free. In either case, the earliest stage is a free-swimming ciliated larva which soon develops a bivalve shell. After a time the larva settles on the surface of the wood, creeping about by means of a large tongue-shaped muscular foot. Settling on a suit
able spot it begins to scrape away the wood with the edges of the valves and soon disappears below the surface. As the burrow deepens the body lengthens so that the siphons can protrude from the surface while the valves are working at the inner end. In temperate seas shipworms rarely exceed a foot in length but some tropical species may reach six feet.
Many species of shipworms have been described and grouped in a number of genera forming the family Teredinidae, but the taxonomy is not in a satisfactory condition and much more work is required before the limits of species or genera can be properly defined. What is of importance to the engineer is that the differ ent species differ considerably in their physiological requirements. Thus while many species are only able to flourish in places where the salinity of the water is nearly the same as that of the open sea, the typical species Teredo navalis can survive a considerable reduction in the salinity and is enabled to invade estuaries and harbours where the influx of fresh water excludes other species. It happens from time to time, as in the case of the San Francisco outbreak alluded to above, that a temporary increase of salinity resulting from reduced rainfall allows T. navalis to gain a f not ing in places where it had not previously existed.
Numerous methods have been tried for protecting timber against the attacks of shipworms. Metal sheathing for the hulls of wooden ships was employed in Greek and Roman times and copper sheathing came into extensive use in the 18th century. At the present day the piles of harbour works are sometimes sheathed with metal or enclosed in concrete "jackets." These methods, however, are only effective when the surface is com pletely covered. A small space left unprotected may admit enough shipworm larvae to destroy completely the interior of the timber. Certain kinds of timber, such as the South American greenheart, are comparatively resistant to shipworm attacks but it appears that this resistance is only temporary and that no kind of wood is permanently immune. Many methods for impregnating the wood with poisonous substances have been tried, but the only agent that has come into general use for this purpose is creosote. Heavy impregnation with creosote greatly lengthens the "life" of timbers exposed to attack, but lasting immunity is not attained owing to the gradual washing out of the creosote from the wood.
Besides the shipworms several species of Crustacea are the cause of serious damage to timber in the sea. The best known of these is the "gribble," a tiny isopod which burrows in the superficial layers of the wood, reducing them to a spongy mass which is easily washed away, exposing fresh layers to attack. It is common in European waters and has been found in many other parts of the world. The damage done by it is visible on the surface and is less likely to attain serious proportions without being noticed than is that caused by the shipworm. (W. T. C.)