Some Notes on American Ship Worms

October 12th, 2023

July 9, 1898. Forest and Stream Pg. 25

Some Notes on American Ship-Worms.

[Read before the American Fishes Congress at Tampa.]

While we wish to preserve and protect most of the products of our waters, these creatures we would gladly obliterate from the realm of living things. For we have been studying and combatting them for a century and more, but we have found no adequate means of counteracting their depredations. During the summer of 1893, while engaged in observations on the oyster at Beaufort, North Carolina, for the United States Fish Commissioner, I became interested in the various ship-worms which are found so abundantly in the waters of North Carolina. During the summer I made some observations on their natural history, and returned for periods during the two succeeding seasons to continue them. The results have been incorporated in a paper on “The Natural History, Organization and Late Development of the Teredinidæ,” which is almost ready for publication.

The ship-worms were favorite objects of study during the eighteenth century, on account of their great damage to the dykes of Holland in 1733 and subsequent years. The contemporaneous observers seem to have been unaware of the observations of Pliny and others in ancient times, and supposed the ship-worms were natives of India, whence they had been brought by shipping in modern times. During these times they were considered true worms, and it was not till the time of Cuvier that their molluscan characters were recognized.

Even if the ship-worms were not recognized to be bivalve-molluscs from their adult organization, it would be easy to determine this fact from a study of the development.

The ship-worm starts in its development as an egg, which none but a specialist could distinguish from the eggs of most bivalves. In American forms that seem most abundant, at least in our Southern waters, the eggs are cast freely into the water and soon fertilized by the male element. As soon as fertilized the eggs begin to develop, and in our warm Southern climate become little free-swimming creatures in from three to four hours. It is true that these little creatures have as yet none of the distinctive features of the ship-worms, or even of bivalve-molluscs. But within a day the bivalve shell is acquired. For a few clays one can rear the larvre in aquaria, but after a time the conditions become unfavorable and they disappear. For perhaps three weeks more, in a state of nature, they lead a free-swimming life and are gradually transformed into a little free-swimming bivalve almost exactly like the little clam or oyster. But how and where, in nature, this transitional period is passed has not been observed.

The next stage which I found was that of the little bivalves, about a hundredth of an inch in diameter, crawling over the surface of the wood in quest of places for their future homes. Once they have found appropriate places they begin to change. One by one the bivalve characters are lost, and the little bivalves are transformed into the very long, worm-like creatures which are found in wooden structures in salt water the world over.

But along with the transformation the bivalve shell is preserved, though it is much modified as compared with other’ bivalve shells, and covers only a small part of the head end of the body. With it the ship-worm excavates the burrow in the wood in which it lives, and seems able to penetrate the hardest or softest kinds of wood with equal facility. As the wood is grated away by the shell, the small particles are taken into the digestive canal, and the debris is extruded, but whether it serves for food in any way is a question in dispute. During its life in the wood at least the larger portion of the worm’s nutrition is taken in through the tube which hangs at rest in the water, and consists of small animal and especially vegetable organisms.

In thinking of sp worms then, it should be remembered that the wood in which they form their burrows is primarily for their own protection: their long, naked, delicate bodies are perfectly defenseless.

At Beaufort all kinds of unprotected wood becomes literally riddled in a very short time. There are two kinds of worms found there in great and about equal abundance.

These are Teredo norvegica and Xylotrya fimbriata whose mode of spawning has been already described. However, a very small proportion of specimens were of Teredo navalis. one of the common European forms. In this species the eggs are retained in the trills of the t mother during a considerable period of their development perhaps almost till time for them to set into the wood. It is apparently this last species which I have many Xylotrya fimbriata were also found.

The breeding season in North Carolina, so far as determined, lasts at least till the middle of August, and perhaps throughout the summer. That the latter is the true period is indicated by two sets of facts. In the first place individuals are found with ripe sexual products during the early part of August, and the young derived from eggs laid at this time must continue to set till September or later. In the second place the young were silting in the wood abundantly till the middle of August a fact which indicates that the same condition continues to some degree for some time longer. Of course, from an economic standpoint the period during which the wood is attacked is one of the most vital joints to discover.

The number of young produced is amazing, being estimated in one case, from a single very large female, at a hundred million, and while the greater part are lost before the setting stage is reached, yet the number that set is very great, and this is one of the most discouraging features in dealing with ship-worms in a practical way. If the spat were of fairly appreciable she and set in but moderate numbers, it might be feasible, by the careful removal of all old piles and other old timbers, to sufficiently reduce the number to a minimum. But when under favorable conditions, over a hundred to a square inch set where there is not room for more than one cr two to reach maturity, it is easily seen what an excess is always present, and how futile it is to try to combat the larva; before they enter the wood.

The practical way of course, is to prevent their entrance into the wood by protecting the wood with cop per paint and sheathing. With small piles and timbers it would seem to be worth while to try various means of keeping the bark off the word, which so far as I know, has net been done; for it is well known that as long as the hark is on timbers they are not attacked by ship worms.

Once the ship-worm has set into the wood it grows with amazing rapidity in our Southern waters. In twelve days it lies grown to be an 1/8in. long; in twenty days about 1/8in. and in thirty-six days 4in., when it is thousands of times as large in volume as when it sets. It has become sexually mature and is ready to produce a new generation. Hew long ship-worms may live has never been observed, though it is probable for several years, and that during this time they keep growing if there be room in the wood for growth, though when crowded the individuals become dwarfed. I have found specimens of great size of T. norvegica some 3 to 4ft. long: and it is easily seen how destructive may be a few of these individuals which may become almost an inch in diameter. The age of such specimens I have not been able to determine, but it is estimated to be less than two years.

In the colder waters of Long Island I have found specimens of both T. navalis (?) and Xylotrya fimbriata. the former the more abundant. They seem to set most abundantly after the 1st of July, though observations for one season cannot be conclusive. The rate of growth is much slower, and it would seem to rake twice as long to attain the same sizes as in the warmer Southern waters.

Observations, to be of any considerable economic value, must cover a variety of localities under different conditions, and must extend through a period of years—observations which I have not had sufficient opportunity to make, and which for our American forms have unfortunately never been made.
Chas. P. Sigerfoos.

Teredolites borings in a modern wharf piling; the work of bivalves known as “shipworms”. Photograph taken by Mark A. Wilson (Department of Geology, The College of Wooster).

Some Notes on American Ship Worms

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