National Geographic : 1969 Mar
into the hole as it pulls itself in; the mud washes out and the hole gets deeper. The terrors among burrowing bivalves are the shipworms-Teredo navalis and its rela tives-which have been wreaking havoc on wooden ships and wharves since man first built them. Using the filelike surfaces of its two shells, a teredo can bore as far as two and a half feet into a ship's timbers.* The teredo creates such destruction that it forced naval builders to sheathe ships' bot toms with copper. Even in this century its invisible damage toppled the municipal wharf of Benicia, California. The shipworm lines its tunnel with a shelly coat, and trails behind it two tubes for feed ing and discharging waste. It spends its entire adult life secure in its burrow. One might consider this the ultimate in self-sufficiency. But that honor, to my mind, is reserved for the largest of the tridacnid clams of Australia's Great Barrier Reef.t I will never forget peering into the clear waters ringing Green Island and seeing giant clams (Tridacna gigas) three feet long and weighing several hundred pounds (pages 428-9). Popular belief tells of careless divers trapped in the stony grips of these clams, though in fact it seems never to have hap pened. But this much I can vouch for person *See "Shipworms, Saboteurs of the Sea," by F. G . Walton Smith, NATIONAL GEOGRAPHIC, October 1956. tThe author wrote of the reef in "On Australia's Coral Ramparts," NATIONAL GEOGRAPHIC, January 1957.