rather than excavate their own tunnels. The close association between these
two groups of wood-borers not only offers the chelurids a suitable habitat in
which to live, but is also a factor in promoting sexual maturity in the chelurid
population.
Chelurids are widely distributed, and Chelura terebransis found in wood
in temperate and subtropical waters. Wood infested by both limnoriids and
chelurids normally has the chelurid population closer to the surface while the
limnoriids inhabit the deeper region. Chelurids are also less tolerant of envir-
onmental extremes and do not survive long in conditions of low salinity or
low oxygen.
2.2 Lignolytic Marine Microorganisms
As mentioned earlier, the major agents of damage to archaeological timbers
in oxygenated marine environments are the marine wood-boring animals that
destroy the wood over relatively short periods of time. In contrast, microbial
decay of wood in these situations is slow and progressive and in anoxic envir-
onments such as sediments, it may take centuries for degradation of the poly-
saccharide components of the wood cell walls to occur in large timber
structures. As with attack by animals, the speed of microbial degradation will
depend on the natural durability of different wood species, the greater resist-
ance of heartwood versus sapwood, plus environmental factors including water
temperature, oxygen availability and salinity. A further consideration in this
regard is the role of microorganisms in the digestion of wood fragments in the
gut of marine-boring animals and the symbiotic relationship that exists between
some members of these two groups of organisms. This will be dealt with in
more detail later, but suffice to say that the accepted view that wood biodete-
rioration in the sea is a marine-borer problem may have underestimated the
importance of microorganisms in the overall process.
For many years, bacteria have been recognised as early colonisers of damp
and wet wood. In terrestrial situations, they are regarded as the first phase of a
consortium of decayed microorganisms most of which have greater lignolytic
capabilities and include the soft rot fungi (Ascomycota and mitosporic fungi)
and the brown and white rot fungi (Basidiomycota). In the marine environment,
submerged wood is colonised quite rapidly by bacteria and soft rot fungi, but
colonisation by basidiomycetes is less common.
Bacteria and soft rot fungi are more tolerant of the low oxygen conditions
in saturated wood, and in wood that is buried in sediments bacterial decay
tends to predominate. Early reports identified unicellular bacteria in founda-
tion piling and shipwreck timbers, but more recently three bacterial decay types
in wood cell walls are now recognised – erosion, tunnelling and cavitation
bacterial decay.
Conservation of Ancient Timbers from the Sea 273