Selective attack of waterlogged archaeological wood by the shipworm, Teredo navalis and its implications for in-situ preservation

When exposed to open seawater, waterlogged wood can be subject to rapid degradation by Teredo navalis (commonly known as shipworm). However, in certain instances archaeological wood is not always attacked by shipworm. This does not appear to be related to wood species alone but more to the state of preservation of the wood. The aim of the current paper was to assess what effect the state of preservation of the wood has on attack by shipworm. To address this, sections of 6000 year-old waterlogged oak wood (Quercus sp.) that were heavily microbially degraded, were submerged at Lynæs in the southern part of Kattegat, Denmark, where shipworm are known to be prolific. Following four months exposure the wood samples were assessed visually, physically and chemically. Visually it was immediately apparent that only the heartwood was attacked by shipworm, whilst the surrounding sapwood was left intact. There was a significant difference in the state of preservation (density) of the sapwood (153 kg/m3) when compared with the heartwood (370 kg/m3). Further characterisation of the sapwood and heartwood by wet chemical methods and Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy showed that the amount of cellulose in the non-attacked sapwood was very low when compared to the attacked heartwood. This suggests that a certain amount of cellulose is needed before there is enough nutrition for the shipworm to attack. These initial results indicate that the state of preservation of timbers has an effect on the attack of wood by shipworm and therefore should be taken into consideration when planning any in-situ preservation/stabilisation of timbers.