National Geographic : 1956 Oct
Shipworms, Saboteurs of the Sea 559 Scientists Seek Ways to Combat Teredos, Tiny Wood-boring Mollusks That Topple Wharves and Sink Ships BY F. G. WALTON SMITH Director, The Marine Laboratory, University of Miami SHIPS sunk in battle, docks wrecked by aerial bombs, or vessels lost in storms or collisions-all these are visible, au dible, and entirely understandable forms of destruction. But ever since man launched his first primitive wooden craft, there has been an equally destructive force at work unseen, silent, and often unsuspected. Wherever a wharf or piling stands in salt water or a wooden ship lies in the sea, secret enemies wait to attack. Their activities ac count for many millions of dollars in damage each year. The principal villain is an insignificant looking, soft-bodied creature called the teredo. The shipworm, as it is also termed, enters sub merged timbers while very small and grows rapidly inside the wood as it works. Myriads of the creatures riddle the interior until the whole structure has been honeycombed. Thus, without damage noticeable on the outside, an entire structure may suddenly collapse in as little as three months from the time it was built. Ship Departs with Dock Attached The fate of a fishing vessel tied to a San Francisco wharf dramatizes the unseen, un suspected nature of the shipworm. While its crew went ashore for lunch, not only the ship but part of the wharf drifted away in a heavy wind. Weakened by shipworms, the wharf's underwater timbers had simply broken off. In tropic seas especially, shipworms multi ply so rapidly that the effective life of un treated timber may be little more than six months. A few years ago a long wooden trestle was built off Sand Island opposite Honolulu Harbor to carry the equipment used in laying a concrete sewer outfall. Since the trestle was needed only temporarily, it was not treated for protection against marine borers. But the shipworms won the race; in less than 70 days large sections collapsed, drop ping heavy machinery into the sea. What kind of creature is this marine sub- versive? To begin with, it is not even re motely related to a worm. Despite outward appearances, the teredo is a mollusk related to the oyster, clam, and mussel. At first sight the creature appears to be simply a long, soft, unprotected body. Closer examination reveals that the paired shells common to bivalve mollusks have here be come reduced to two small curved plates that lie alongside the head. Most of the shell surface is equipped with rows of fine teeth for rasping away the wood as the animal excavates its tunnel (page 563). Why structures collapse and boats sink without warning is easy to understand if we look at the burrow itself. Although the borer's home is about a quarter-inch wide throughout its meanderings in the wood, the entrance appears as scarcely more than a pinhole. At the Marine Laboratory, University of Miami, where marine borers are being in tensively studied, it is possible to watch the living shipworm in sea-water tanks. All over the surface of an infested piece of wood tiny tubes, or siphons, stick out. Each pair belongs to a single shipworm. It draws in water for its breathing through one tube and expels the exhausted water with waste products through the other (page 565). Shipworm Lays 100,000,000 Eggs Research at Miami has shown that ship worms may become sexually mature within two months of entering the wood. During the course of a year some shipworms release into the water the amazing total of 100,000, 000 eggs. Thus the infestation may spread quickly, once unprotected timber provides a home for borers. In certain kinds of shipworms the eggs de velop into larvae inside the burrow of the female. Whichever happens, sooner or later the little borer is swimming about in the water. It is very small, perhaps 1/200 of an inch in diameter. This is the reason why, when it finally enters the wood, only a small burrow need be made.