cavities. Attack is initiated at the lumen surface where bacteria attach them-
selves via extracellular slime. Following penetration of the S 3 layer, the under-
lying S 2 is attacked resulting in the formation of small cavities. It is assumed
that one bacterial cell or a small group enlarges the cavity and as cavities
increase in size they begin to coalesce. Although the bacterial population in
the cavities will increase, it is assumed that enzyme breakdown of the cell
wall is non-localised and that cells produce diffusible substances similar to
those observed with brown rot decay. It is clear from the literature that any
definitive diagnosis of cavitation attack by bacterial cells can only be achieved
with the aid of SEM and TEM facilities.
2.6 Bacterial Symbionts of Shipworm
Apart from recent observations of bacterial endosymbionts associated with
the pholad Xylophaga(see above), the most detailed studies of symbiotic asso-
ciations in marine-borers have focussed on the teredinids. Electron microscopy
pinpointed the gland of Deshayes, a specialised region of the shipworm gill,
as the site where bacterial symbionts reside. Unlike many symbiotic asso-
ciations, the bacteria can be isolated from the animal and cultivated in the
laboratory and so far symbionts from seven shipworm host species have
been maintained in culture. The bacteria produce cellulolytic and proteolytic
enzymes, plus nitrogenase for atmospheric nitrogen fixation allowing the ship-
worm to survive principally on a wood diet that is naturally nitrogen depleted.
Investigations into the identities of bacterial symbionts from four shipworm
species representing three different genera, revealed a phylogenetically common
bacterial strain Teredinibacter turnerae, a member of the gamma subdivision
of the Proteobacteria. However, later studies on the identity of the bacterial
symbiont from B. setacea, found that this teredinid harboured a different bac-
terial symbiont. This work also showed that acquisition of the symbiont by
successive generations of B. setaceawas through a vertical mode of transmis-
sion, the symbiont cells being present in the host ovary and offspring.
As a footnote to this work, the gut contents of Limnoriahave been exam-
ined using SEM but no microorganisms were associated with wood particles
even though the wood ingested by the limnoriids was decayed by soft rot fungi
and tunnelling bacteria. This suggests that ingested microbes were digested by
the animals.
2.7 Soft Rot Fungi
Although soft rot decay of wood was first described in the 1950s, prior to that
date numerous authors had recorded similar symptoms in decayed wood from
a range of aquatic and terrestrial habitats. The decay is superficial and below
Conservation of Ancient Timbers from the Sea 277