THE SHIPWORM (Teredo, Xylotrya, etc.). This is a general name that applies to several species of mollusks of the genus Teredo, together with other species in other genera. The mol lusks known as shipworms are characterized by the fact that they bore in wood, and are represented, along the north-central Atlantic Coast, by species of the genus Xylotrya.
Shipworms are widely distributed throughout the waters of the tropics, and are present in smaller numbers in cooler waters of temperate regions. They inhabit European waters from Sweden to Sicily, and are also found in the vicinity of Bermuda, the West Indies, New Zealand, and Australia. In the United States they exist from Maine to Florida, and along the Pacific Coast as far north as Alaska. The United States Fish Commission reports their distribution in local waters to be as follows: Teredo navalis, between Florida and Cape Cod. Teredo norvegica, Cape Cod northward to Maine. Teredo megotara, New Bedford, south to South Carolina. Teredo dilatata, Massachusetts to South Carolina. Teredo thompsoni, Massachusetts. Xylophaga dorsalis, North Atlan tic. Xylotrya fimbriata; Long Island Sound to Florida.
The form of the shipworm is shown in the picture. It is a long, worm-like organism of which the posterior end s remains at the outer surface of the timber, while the other or anterior end B occupies the inner extremity of the tunnel.
The two horn-shaped structures s are the free extremities of other wise united tubes, known as siphons, that pass throughout the entire length of the ship worm to the vital organs and boring shell at B. These horn shaped extremities are the only parts of the shipworm that can extend outward beyond the wood, and are therefore the only parts that are evident to the casual observer.
A general idea of the form of the shipworm may be gained by examining the ordinary long, or soft-shelled, clam (Mya arenaria) so familiar to residents of New England. This clam possesses a very long worm-like neck penetrated by two parallel tubes or siphons through one of which water, oxygen, and food pass in ward, while through the other exhausted water and debris pass out. It is also helpful to examine the common razor clam (Ennis directus) which, besides siphons, possesses a powerful, muscular club-shaped foot or sucker that enables it to bore into the sand. The long and razor clams and the shipworm are all true mollusks, and each one suggests a worm only because the part that sur rounds the siphons is soft and cylindrical.
The parts of the shipworm that are important in the present connection are the body, siphons, collar, pallets, boring shell, foot, and lining shell. These parts will be considered separately.
The Body. The translucent substance of which the body is composed resembles the living substance in the body of the oyster. In some species, in addition to their normal functions of respiration, the gills perform the important office of sheltering the embryo. The nervous system is well developed. Vital organs, such as the liver, are protected by being enclosed within the boring shell. The stomach is not distin guished by any peculiarity. There is a long intestine.' The Siphons. The siphons extend through almost the entire length of the body. One of them conveys the oxygen, water, and infusorial food to the digestive organs; while the other conveys the ex hausted water, excretions, and wood particles from the excavation to the free water without.
The siphons are joined together for most of their length, but separate as they pass outward at their extremities, s, and are then capa ble of being thrust out and with drawn through the orifice in the wood. They are the only parts that can be seen from the outside of the wood. It will be noted that these extremities must always re main at the orifice to the tunnel.
When the conditions are favora ble, the extremities of the siphons are extended out to their full length beyond the surface of the wood. Other wise, they are withdrawn and there is then but little evidence that the shipworm is within the piece. The picture shows the siphons as they appeared fully extended after several consecutive days of warm weather.
The Collar. The collar C is a muscular, wrinkled membrane that ex tends around the posterior portion of the shipworm at the point of union between the siphon and the body proper, and forms a connection between the body and the calcareous lining of the tunnel. This is the only place at which the body of the shipworm is not free and separated from its surroundings. The collar in eludes several well-defined muscles and these act upon the small shells known as pallets by which the entrance to the perforation may be guarded.
The Palle Is.--The two shells or plates, located at p and called pallets, are broad, slightly curved and flattened at the top and contracted at the bottom where they pass under the collar. When the siphon ex tremities are withdrawn into the body, the tops of the pallets are brought together over them and protect them. These shells are sometimes con fused with the boring shells, which are quite distinct, and at the other end of the body. Details differ with species.
The Boring Shell. The principal or boring shell B is small and very beautifully formed. The two parts together are nearly as long as they are broad, and present an irregular triangular appear ance when observed front the side. They close tightly at the hinge and at the side opposite. As distinct from this, however, an open space at the top permits the body to emerge while a simile r opening at the bottom isfc the foot or sucker. The shells of young shipworms are much larger in proportion than those of older worms, and when the worm is very young, it is for short time entirely enclosed in its shell.
The Foot—The foot, which in form resembles a pestle, is a short, stout, muscular organ, broadly truncated or rounded at the end, and so ar ranged that it can exert a powerful suction upon anything to which it is attached. The extent to which this cupping action assists the exca vating has probably been underestimated.
The Calcareous Lining. Calcareous material deposited upon the woody surface of the tunnel forms a smooth lining along which the body of the shipworm can pass as it contracts or expands. This shell-like tube is distinct from the pallets and from the boring shell. Its thick ness, which varies with species, is sometimes so slight that the shell is detached by the slightest shock, and many specimens, exhibited in museums, do not show the lining for this reason. The lining is some times very thick. The shipworm can rarely advance through the wood very far in a straight line, but is forced to pass here and there so as to avoid obstacles such as cracks, knots, and the tunnels of its companions. In such cases, the linings are curved as they wind in and out, and often so many are present, that almost, the entire content of the wood is occupied. avoid scams and joints in wood, possibly because of their effect upon the calcareous linings.
Shipworms live principally, if not wholly, upon organic particles obtained from sea water. Particles of wood are sometimes found in their intestines, and it is not certain that these particles, cut from the burrows, do not serve in some minor way as food. It is certain, however, that the principal reason for the boring is to prepare a shelter.
A shipworm can live for a short time out of water. But, since it derives its sus tenance from the water, it must have access to it much of the time. It does not have to be submerged all of the time, and can live and work under such con ditions as exist between tide levels. It has been known to live for about two weeks in timbers that have been transferred from the sea to fresh water, and could possibly have lived longer than two weeks.
Many logs in a cargo of Central American woods recently received in New York, after a voyage of about two weeks, were found to be full of living shipworms that had gained entrance while the logs were waiting for shipment in the South. The shipworms were apparently in good condition when the timbers were removed from the hold of the ship. The wood itself, and the hold of the ship, contained considerable water, yet the logs were by no means submerged, and the fact exists that these particular specimens survived during a voyage of about two weeks. They were very numerous, so much so, that later the logs had to be removed from the yard because of the odor.
Reproduction and Development. Most mollusks reproduce by means of eggs, which, in the case of some shipworms, are spheri cal in shape and greenish yellow in color. Shipworms are very prolific, the eggs of a single specimen being numbered by the million. The eggs are very hardy and many survive and yield young shipworms. A shipworm can swim at the end of about three hours after hatching and has a well-developed shell before the end of the first day.
The shipworm passes through several stages before it assumes the character and form of the adult. It is first covered with fine hairs or cilia, which enable it to swim. Soon most of the cilia are lost and the rudiments of a small bivalve shell appear. At first, this shell is heart-shaped and very small, yet it is large enough to enclose almost the entire body. The portion of the body that protrudes from the cell is fringed with cilia, and these enable it to continue to swim until it finally encounters a piece of wood.
The results of some observations upon the shipworm (Xylotrya fimbriata) at Beaufort have been summarized by Professor Sigerfoos as follows:' "The free-swimming stage is reached in three hours, and a well developed shell is formed in a day. We have no direct observations as to the time the ship larva is free-swimming. We may assume, I think, that it is at least a month, or it may be two. Most of its energies are devoted to locomotion during this period, but, after it has attached itself, all of its energies are devoted to forming its burrow and securing its food. Coming into contact with the wood, the larva throws out a sin gle, long byssus thread for attachment and never again leaves its place. The newly attached larva is somewhat less than 0.25 mm. long. In twelve days it has attained a length of 3 mm.; in sixteen days, 6 mm.; in twenty days, 11 mm.; in thirty days, 63 mm.; and in thirty-six days, about 100 mm., when it bears ripe eggs or sperm." The time of reproduction is important. In the vicinity of New York, this takes place principally during the month of May; but it may continue, although to a less extent, throughout the greater part of the summer. In tropical countries, it probably goes forward throughout the entire year. Although the extreme life limit of a shipworm is unknown, it is thought that individuals can live for several years under favorable conditions. A ship worm may attain to a comparatively large size during a single season: Influence of Temperature and Water.
In most cases ship worms are more plentiful where the water is not cold, and, for this reason, wood is destroyed more continuously and more rapidly in the tropics and semi-tropics. In the United States destruction is most serious along the entire Pacific Coast, as well as along the coast of the South Atlantic and Gulf States. Some shipworms are found, although they are much less active, where it is often extremely cold, as in Maine and Alaska.
Some shipworms thrive in pure sea water, while others do well in brackish, impure, or comparatively fresh waters. Sometimes the parts of timbers that are near the surface of the water are injured, and sometimes the parts that are down near the bottom. These and other differences are accounted for by the facts that there are many species of shipworms, and that differences some times exist between the qualities of higher and lower layers of water. For example, when fresh water from a river meets the heavier water of the sea, shipworms may sometimes be found near the bottom where the water is actually salt.
Some shipworms (Xylotrya fimbriata) survive in the brackish, polluted waters of New York Harbor, while other species that do not exist in these waters are present in the nearby ocean. Shipworms are very active along the north Pacific Coast but are said to be absent at some points near the mouth of the Columbia River because fresh water pre dominates at these points. A vessel carrying hardwood logs was wrecked in the vicinity of the Gulf of Mexico. The logs were conveyed to the sheltered, but brackish, waters of a creek where they remained ror about six weeks. The pieces were attacked as soon as they were Owed in the creek and the results were so noticeable that some borings were measured and are said to have been six inches in length. The wood that remained in the outer ocean was not injured.' The dis crepancies between these incidents may be accounted for by the presence of different species of shipworms.
The belief that shipworms are influenced by impurities in water was expressed in Holland as early as 1733. It was noticed that comparatively little rain fell in years when shipworms were quite plentiful, and it was thought that the diminished volumes of river water during these years permitted larger quantities of salt to exist in the waters near the mouths of the rivers. Analyses proved that the proportions of salt did vary during the dry and rainy seasons.
While the shipworm is yet very small, it settles upon the surface of the wood and almost immediately begins to clear away a place through which to burrow. There is some controversy as to the method by which the burrow is excavated, but it is quite certain that the foot assists the shell. The details are not perfectly understood, but the facts are that the hardest woods are penetrated and that surfaces are cut as smoothly as though a sharp chisel had been employed.
A shipworm is very small when it enters a piece of wood, but once within develops rapidly and then never leaves its burrow. The perforation through which the shipworm enters is very small, but the diameter of the boring increases rapidly, the average being reached at a point quite near the perforation through which the shipworm entered. A shipworm grows principally in length and must therefore tunnel to secure space for the increasing length of its body.
The shipworm does not encroach upon other tunnels because most of these tunnels are occupied by shipworms. It instinc tively avoids knots, imperfections, bark, cracks, and lines of cleavage. Woods are not exempt from attack simply because they are hard.
Wood may appear to be sound and yet be so weak that it can be crushed by the hand. As much as fifty per cent. of the weight of a piece may be removed without much evidence upon the out side. Failures Often come suddenly and without warning. The tops of piles thought to be in good condition are seen floating away. A freight train on the Louisville and Nashville Railway crushed through a trestle that had been standing about ten months and that had been frequently inspected.
The size of a boring depends upon that of the shipworm that made it, and the size of the shipworm depends upon its age and species. Five inches and as many feet may be regarded as minimum and maximum lengths. One-quarter of an inch is a small diameter from which measurements have been made up to one and one-eighth inches.' It is safer to disregard minimum possibilities in such a connection.
Evidence upon this subject is seldom ac companied by statements of conditions under which the results were accomplished, so it is sometimes hard to associate the boring with the species that made it. The species of the woodborer, the location of the piece, the season, and the kind of wood in which the boring exists are all important.
Conditions that influence the growth of the shipworm influence the speed with which it works. Generally speaking, cold retards while heat expedites the work of excavation. A six-inch burrow may be driven in as many weeks so that a one-foot pile attacked on all sides can be destroyed in that length of time. On the contrary, other pieces remain practically intact for many years.
Wood has been found to contain shipworms after a submergence of eight days (United States Annual Report of Scientific Discovery of 1857). Six-inch piles were destroyed at Aransas Pass in six weeks; other piles in the same locality have lasted as long as three or four months (Report, Chief of Engineers, U. S. A., 1888, pp. 13, 14).
Field of Attack.The fact that a shipworm lives upon micro scopic life present in sea water outside its burrow, makes it necessary for its siphon extremities to be located at the entrance to its burrow. This end of the shipworm being fixed as to posi tion, the wood is removed inward from the surface to a distance measured by the increasing length of the shipworm.
The small portals to the burrows that are evident upon the outside of the timber (see Plate IX) do not necessarily mark the exact content of wood that is destroyed within; since, although one end of the shipworm must remain at the entrance to its burrow, the other can reach upward into the wood above the water, or downward into that below the mud. Shipworms have been known to work under pressures caused by twenty or twenty five feet of water.
Woods Subject to Attack. Immunity is sometimes claimed for some particular wood; but it is usually found that such a claim, based upon local conditions, is not generally substantiated. It is safer to assume that all ordinary woods may be attacked by these forms. Doubt may be felt with regard to some woods that contain repellent gums, resins! or bitter essences, and some palms that have open, porous structure; yet very few woods such as these are used in American constructions.
Woods are not safe simply because they are hard. Osage Orange, which is a very hard wood, has failed in several instances where it has been used for piles. A Commission appointed in Holland to investigate this question reported as follows: "Although we do not know with any certainty whether among exotic woods there may not be found those which resist the shipworm, we can affirm that hardness is not an obstacle that prevents the mollusk from perforating its galleries."