Natural timber carried into the sea from rivers in winter flood, or processed timber in the form of mans ships, boats and lost cargo, form a very special habitat for certain groups of wood-boring bivalve molluscs, of which the most well-known are the ship-worms of the family Teredinidae. A closely related group of molluscs are the wood piddocks, members of the Xylophagidae, whose wood-boring capabilities are just as devastating as those of the shipworms.
Like the shipworms, the shells of the mollusc have lost their function of protecting the soft body, and have become instead, specialised cutting tools with which the post-larval animal rapidly penetrates the timber, and which continue to grow through the life of the animal. The front margin of the shells are cut away to enable the round disc-like foot to grip the head of the boring, while the projecting, curved shell lobes above the foot bear rows of minute cutting teeth that cut a concave path deeper into the timber. Immediately behind these lobes, the sides of the rounded shell also bears lines of teeth, by which the walls of the excavation are simultaneously cut away, leaving, as the animal grows and advances, a perfectly smooth-walled boring through the timber.
Two species of Xylophaga occur in Yorkshire waters, X. dorsalis, and X. praestans.
The most prolific of all the wood boring molluscs is the pea-sized Xylophaga dorsalis, whose erratic borings, running in all directions and cutting across each other, completely demolish the timber, filling their surroundings with a paste of wood particles; it is found at all depths, from the deepest off-shore pits to just a couple of fathoms of water off the coast, and infests even the thin bottoms of traditional crab pots.
A much larger species, Xylophaga praestans, is a very special member of our coastal waters in a number of ways. First discovered and described as recently as 1912, it is rarely seen by most biologists and occurs only in the North Sea, and is usually quoted as being a rare species occurring only off the coasts of Northumberland and Durham, but in fact its distribution extends over a much greater area, both north and south. The first, and largest specimen that I examined came from just twelve miles off Scarborough in 1967, but I have, over the years, also seen its borings from many locations, including timber trawled up in the Norwegian sector of the North Sea.
This species can grow to a large size, cutting enormous, beautifully smooth borings up to 30 mm in diameter and over 300mm long, although usually it is much smaller. This first Yorkshire specimen was a solitary individual living in a blackened, sulphide rich piece of tree trunk that was so soft it could be shredded between the fingers, and on its body were bonded the traces of an egg capsule of some other organism that had at one time also occupied the boring, but which defied identification.
The mystery was solved several years later when a large colony of X. praestans was found in a log hauled up from the deep water around Bruceys Garden, near the edge of the Dogger, and where this wood-borer is common. Scattered through the borings and over the wood piddocks themselves were bonded hundreds of tiny brown domes, the egg capsules of a species of flatworm that was finally found by flushing the borings through with preservative. This is a remarkable and previously unknown example of commensalism; the flatworms Taenioplana teredini and Stylochoplana affinisare recorded from empty burrows of Teredo elsewhere in the world, but this appears to be the only such association known in British waters, or withXylophaga.
Other organisms inhabit the Xylophaga borings including a specialised polychaete that ingests the finely ground wood particles that partially fill the borings.
Xylophaga species, unlike the shipworms, do not digest the wood that they file away, but like related bivalves, filter minute planktonic organisms and detritus out of the surrounding water.
Although sometimes found in dense aggregations, they may also be found in low numbers or even as solitary isolated individuals, and frequently as isolated units at great depth, and the wood piddocks have therefore evolved a different reproductive strategy to other bivalve molluscs. Like the shipworms, smaller younger individuals are male, which as they grow, change to female. Unlike the shipworms however, sperm produced by the males is stored in a special receptacle, and when the animal eventually changes sex, this sperm is used to fertilise the eggs.