pallets. The two shells are just boring 

 tools. The edges of these shells have 

 small teeth that rasp wood efficiently. 

 The long, soft body extends in length 

 from inches to feet, depending on 

 age, environment and species. At the 

 end nearest the outside of the hull, 

 each teredo has a pair of calcified 

 pallets that act as a plug in times of 

 distress. 



Otherwise, two siphons, sort of 

 biological straws, extend into the 

 outside water; one sucks in water for 

 breathing and feeding on microscopic 

 plankton, and the other flushes 

 wastes. Contrary to popular belief, 



Scientists and 

 engineers are 

 trying to devise 

 new preventive 

 treatments that do 

 not degrade the 

 marine environment. 

 Meanwhile, wooden 

 boats continue 

 to be vulnerable 

 to borers. 



wood dust is not the major part of 

 their diet. 



Shipworms vary in their repro- 

 ductive patterns. In some species, 

 fertilization of eggs and sperm occur 

 in the water; in others, eggs are 

 fertilized in the body. Either way, 

 shipworms produce up to 100 million 

 eggs per female. 



The fertilized egg develops into a 

 free-swimming larva, moving weakly 

 in the currents with tiny beating hairs 

 called cilia. After two or three weeks, 

 it assumes an adult form, loses its 

 swimming ability and finds a wooden 

 home. 



As with oyster larvae, luck plays 

 a major role in finding suitable 

 settlement. Tiny shipworm larvae 

 could land on mud or rock or drift 

 away in ocean currents, in which case 



they will not survive. Landing on 

 chemically treated wood is just as 

 lethal. Or the wood can be too 

 crowded with other fouling organisms 

 to provide a secure spot for shipworm 

 larvae. Strong currents can carry 

 larvae away before attachment; 

 likewise, ship movement makes 

 attachment difficult. Shipworm 

 infestation occurs when ships are 

 moored. Thus, the transformation 

 period from swimmer to burrower is 

 the weak link in the life of the 

 shipworm. 



When the larva or young adult 

 shipworm finds a suitable spot, its 

 shape continues to change. The shells 

 become rasping tools, and the ciliated 

 swimming parts disappear. It digs a 

 tiny hole, just large enough to admit 

 its body. As the shipworm lengthens 



and enlarges its internal burrow as it 

 grows, the outside hole through 

 which it entered remains the same 

 size — about the diameter of a 

 pinhead. 



The shipworm lines its burrow 

 with a calcareous coating. Bankia 

 setacea, the major shipworm north of 

 San Francisco, can grind and line a 

 burrow up to 6 feet long. The burrows 

 of Teredo navalis, common in the 

 Chesapeake and warmer waters of the 

 Atlantic and Pacific, measure 1 to 2 

 feet in length. If the lining is worn or 

 the outside hole enlarges beyond the 

 capacity of the pallets to plug it, then 

 shipworms are exposed to predation 

 and the vagaries of water quality. 



Shipworms are found worldwide. 

 Wooden ships, floating logs and 

 ocean currents have carried the adults 

 and larvae to all oceans, seas and 

 estuaries. Temperature, salinity and 

 the availability of wood are the 



limiting factors to their spread. 

 Larvae are more susceptible to 

 extremes in temperature and salinity, 

 but adults are considered tolerant. 

 Shipworm damage is more common 

 and more quickly accomplished in 

 warm, tropical waters. 



No one knows the limits of depth 

 distribution. An unknown species of 

 boring clam digested the Thank's 

 elaborate woodwork on the ocean 

 floor at a depth of 13,000 feet. 



Gribbles (Limnoria Ugnorwn) are 

 different from shipworms in their life 

 biology. Gribbles reproduce more 

 like crabs. The female broods 

 hundreds of fertilized eggs and then 

 releases well-developed young into 

 the water. They are able to start 

 burrowing into suitable wood 

 surfaces immediately. They actually 



use wood as food. As with ship- 

 worms, the critical period for gribbles 

 is their search for hospitable habitat. 

 Coated, covered or painted surfaces 

 deter the young from boring that 

 initial hole and entering the wood. 



Research continues on ship- 

 worms and gribbles. The economic 

 impact on wooden structures such as 

 boats, pilings and bulkheads is 

 enormous. Borer-resistant woods, 

 such as the trees from Fraser Island, 

 Australia, are available and were used 

 in pilings for the Suez Canal. 



Scientists and engineers are 

 trying to devise new preventive 

 treatments that do not degrade the 

 marine environment. Meanwhile, 

 wooden boats continue to be vulner- 

 able to borers. Regular inspection and 

 use of an anti-fouling paint on the 

 boat's bottom can prevent infestation. 



(This story was originally printed 

 in WoodenBoat Magazine.) 



COASTWATCH 19 



