feature for identifying species of shipworm. In addition, the delicate nature of
the pallets of some species demands extreme care when extracting animals
from wood for identification.
Experiments to measure the rate of growth of shipworms in wood have been
carried out by repeated X-ray analysis. Rates vary according to the exposure
conditions and the wood species tested, but values of 18 mm per month by
Lyrodus pedicellatus and 74 mm per month by Bankia setacea have been
recorded. However, where high levels of infestation occur, overcrowding in
the wood results in slower rates of tunnelling and smaller diameter worms.
Actively burrowing worms do not intrude into neighbouring tunnels, nor do
they emerge through the wood surface, even though the wood may be totally
destroyed internally. The wood fragments ingested by the worms may be broken
down in the gut by cellulolytic enzymes produced by specialised cells of the
digestive diverticulae. However, there is now strong evidence to show that
microbial endosymbionts play an important part in the digestive process.
Cellulolytic,nitrogen-fixing bacteria in the gills of shipworms are also thought
to contribute to the nutrition of animals that cannot be wholly dependent on
naturally low-nitrogen woody substrates. This aspect of shipworm biology will
be dealt with later.
Pholads. The family Pholadidae comprises bivalve molluscs that are wood-
borers, but also includes genera that bore into soft rock, firm mud and even
concrete. Although wood-boring pholads can be very destructive, the general
view has been that they do not digest the wood that they excavate and their
burrows simply afford protection allowing the animals to filter feed on plank-
ton. However, recent studies indicate that this may not be totally correct since
bacterial endosymbionts similar to those associated with teredinids have been
observed within the gills of members of the Xylophaga. The burrows of
mature pholads can be 5–10 mm in diameter and range in depth from 3 to
8 times the length of the animal (Figure 3).
There is no calcareous lining and apart from members of the Xylophaginae,
the majority of pholads lack the soft worm-like body that is so characteristic
of teredinids. However, the two calcareous valves and accessory calcareous
plates between the valves can be recognised on X-radiographs of damaged
wood. Indeed, the morphology of the valves and plates plus the siphons is the
main feature to aid identification. The pholads are typically found in warm
water situations, although species of Xylophagahave been recorded in cold,
as well as tropical, latitudes. They also show some tolerance to environmental
extremes and can be found in brackish estuarine waters and shallow waters
where high temperatures occur.
Gribble. Members of the Limnoriidae are dominated by the genus
Limnoriaand 51 species have been recognised recently. Two other genera of
Conservation of Ancient Timbers from the Sea 269