Home >> Encyclopedia Americana, Volume 26 >> Analysis to Or The Me Chanical >> or Teredo

or Teredo

wood, burrow, water, timber, body, piece, bivalve, animal, shell and cilia

TEREDO, or a small marine bivalve boring mollusk (Teredo navalis), which excavates burrows in wood under (salt) water, attacking the timbers of piers and ves sels in immense numbers, and riddling them to such an extent that they are rendered utterly useless, in a surprisingly short time, if left un protected. It abounds destructively throughout the Mediterranean and Baltic seas, and on both shores of the Atlantic. Its steady burrowings once almost caused the inundation of a large part of Holland. Along the sea-front had been built a system of dikes, made principally of timber. In three years breaks were being patched up; in five, whole sections gave way. Only the heroic efforts of the whole seaside population saved the Dutch from one of the worst catastrophes in their history. The tim bers were completely honeycombed, so rotten that the wood could be crushed in the hand.

North America suffers as much as Europe from this pest. All down the New England Coast piles are attacked and destroyed. In this region two years forms the average life of a piece of submerged timber. Channel buoys are left in the water only six months in the year, then a new set is put in and the old one dried. The zone of the ship-worm's devastation is comparatively large. Wood is attacked between points well above low-water mark and points 10 or more feet below it. The hardest oalc offers no more difficulty than the softest pine, and the toughest knots are traversed. Teak alone resists the attack.

.0- The agent of this vast amougt of damage much resembles a worm, but is a true mollusk. Its long, whitish body, tapering toward the posterior end, is found imbedded in a shell-lined burrow. Individuals of this species sometimes attain the length of 10 inches, are one-quarter inch in diameter. Such size, however, is rare, four inches being the average length.

The "head" end of the animal is covered with a white bivalve shell. This protects the vital organs of the little creature, and from its interior opening projects a short °foot" which is probably the instrument by which the burrow is dug and lined with its pearly coating. Two pallets shaped and fastened to the pos terior end of the body, much as leaves are fastened to the stem, close the teredo's hole, and protect from attacks the soft portions of the animal. Between these two plates lie the siphon tubes — used for inhaling and exhaling water. Through the lower of these (bronchial) is,drawn the water breathed by the animal, and likewise those minute animakules which serve it for food. The dorsal tube serves as the or gan of excretion. Through it passes a stream of vitiated water carrying along the fmces and the wood excavated. Surrounding both the pallets and the siphon tubes is a much wrinkled muscular band, by which the teredo adheres to its "burrow.° The appearance of the teredo burrow is very peculiar. Outwardly the piece of timber in-. feskd shows a number of very small, holes. Inwardly it resembles nothing more than a Swiss cheese. The channels run in all direc tions, sometimes so close to each otber that the wood separating them is as thin as paper. But between the holes there is always a partition, for the animals never interfere with each other.

Their sense of hearing seems to enable them to tell when they are approaching the outside of the wood or are nearing another burrow and they turn aside. The holes' are always lined with irregularly laid shell and they generally go with the grain. Like many other mollusca the teredo passes through a long series of com plicated metamorphoses before arriving at full maturity. The eggs, from the beginning of the breeding season in May, are confined in the gill cavity. Here they have their first period of growth. From the gill cavity the embryos arc discharged in the form of free-swimming ani .mals covered with vibrating cilia or hairs, by which they swim. In this stage they are almost exactly like ciliated infusoria. Next they lose these locomotive filaments and devekip a rudi "mentary bivalve shell. In the third stage their relation to other bivalves is apparent in their resemblance to the common mussel. They have a mantle and shell covering their entire body and another sort of cilia replaces those lost. This bivalve character is further accentuated by the development of a long foot used for creeping and by the appearance of eyes and organs for bearing. These eyes, however, dis appear as the ammal elongates and the loco motive cilia are lost. In this stage the young teredo, settling on some convenient piece of wood and starting veith a hole about the size of a pin-head, be,gms his burrow, and enlarges it as he goes on, until he has reached his full growth.

The fact that the ship-worm does not use as food the wood it excavates, but simply passes it through its body, has much to do with the. failure of many attempts to make wood teredo proof by poisons. Up to date creosote and dead oils are the remedies which have given the best results. The piece of lumber to be so treated is first steamed. Next the air is ex hausted. and the .poisonous or noxious com pound is forced in under a pressure of 400 pounds to the square inch. Usually, however, this system fails of the desired result. At Christiania, timbers poisoned in this manner w.ere found to be, three years later, quite riddled with teredo, In some instances, how ever, piles so treated have been known to re main free frorn for as many as 15 to 20 years.

Although poisoned timbers are often used for such structures as government docks (which must be as permanent as possible), for ordinary piers and for submerged work, the expense of so treating the wood is generally greater than the cost of periodical renewal. Of course the most thorough defense would be one which pre-, vented the entrance of the young animal. Cop per-sheathed vessels are quite free from its at tacks, while copper paint, creosote or coal tar frequently applied has the same effect. Piles may be defended by broad-headed nails closely driven, for the ship-worm seems to avoid enter ing any wood impregnated with iron rust.

A large species of teredo (T. gigantea), from Sumatra, has been found to measure from four to six feet, and to have a diameter of about three inches, It bores into the solid mud, and' does not appear to destroy timber like its smaller neighbor. Consult Cooke, 'Molluscs' (London 1898); Verrill 'Invertebrates of Vineyard Sound' (Washington 1875).