Conservation of Ancient Timbers from the Sea 267
The wood-boring molluscs comprise the teredinids or shipworms (family
Teredinidae), and the pholads or piddocks (family Pholadidae). Both groups are
bivalves and burrow into wood through the rasping action of the two valves
or shells at the anterior end of the animal. Although the teredinid family is the
larger of the two groups and is distributed world wide, the occurrence of indi-
vidual species can be restricted within a range of water temperatures. In con-
trast, the pholads are found mainly in the warmer waters of the tropics and
subtropics, although some members do occur in cold-water situations, either
at depth or in higher latitudes. Most of the wood-boring crustaceans are
members of the Isopoda – the family Limnoridae or gribble and the family
Sphaeormatidae or pill bugs. A third group, the family Cheluridae are mem-
bers of the Amphipoda. The wood-boring crustaceans have segmented bodies
and are able to move over the surface of wood, unlike the molluscs that
remain in their burrows for life. Gribble attack of wood is superficial and the
animals excavate a network of narrow galleries on the wood surface to produce
an hourglass shape in the inter-tidal portion of vertical structural timbers. The
chelurids excavate wider galleries, often enlarging those formed by limnori-
ids, and are known to ingest the faecal pellets of limnoriids. The sphaeromatids
are larger in size than the limnoriids or chelurids and usually burrow into wood
by tunnellingacross the grain, sometimes producing a honeycomb of tunnels
in softened timber. Species of limnoriid, and to a great extent the chelurids,
have a world-wide distribution from cold temperate to tropical zones, whereas
the sphaeromatids occur in tropical, especially brackish waters. In short, the
main hazard toarchaeological timbers in cooler temperate waters exists from
shipworm, gribble and chelurids, while in warm temperate–tropical zones, pho-
lads and sphaeromatids pose an aggressive additional threat. Only a marked
reduction in the salinity of major bodies of water, such as the northern part of
the Baltic Sea, offers any natural defence against wood-boring animals in
non-polar regions.
Teredinids. The family Teredinidae is made up of many genera. When
mature, the animals have a soft, worm-like body that in extreme cases can be
up to 2 m long. The animals excavate tunnels within the wood by the grinding
action of the anterior valves of the worm (Figure 1) and a calcareous deposit
is laid down over the surface of the tunnel. This calcareous lining is clearly
visible in X-radiographs of timber specimens infested with shipworm and is
a useful way of assessing the extent of internal damage to a piece of wood which
may otherwise appear superficially intact (see Figure 2). The posterior end of
the worm supports two siphons that protrude through a narrow hole (1–2 mm
in diameter) at the wood surface. The incurrent siphon draws in oxygenated
seawater to support respiration by the animal and feeding on particulate organic
matter and microorganisms, while the excurrent siphon releases waste products
and reproductive units.