Conservation of Ancient Timbers from the Sea 271
and the pleotelson – have ornamentations on the dorsal surface that are used
for identification.
The limnoriids have a world-wide distribution, but water temperature has
a major influence on the occurrence of particular species, their reproduction
and their rate of attack. The warm-water species Lygropia tripunctata is most
active at 22–26°C and reproduction occurs at temperatures above 14°C. The
adult females carry the fertilised eggs and developing juveniles in a ventral
brood pouch below the thorax before they are eventually released into the
excavated galleries. The cold-water species Limnoria lignorum is only active
below 20°C and begins breeding at 9–10°C. At temperatures below 10°C, the
rate of wood degradation declines significantly. Salinity is also a factor that
affects limnoriid activity and animals are able to tolerate only 40–50% normal
seawater concentrations for short periods of time.
Sphaeromatids. These are often referred to as pill bugs because of their
capacity to curl up into a ball when they are disturbed. Like the limnoriids, they
are isopods and have a comparable segmented body comprising a head, tho-
rax and abdomen. However, they are significantly larger than limnoriids with
mature sphaeromatids reaching 10–15 mm in length (Figure 5). Such relatively
large wood-boring animals are capable of causing rapid damage to timber
structures in seawater by burrowing across the grain, perpendicular to the
wood surface in the inter-tidal zone.
The burrows they produce are ca. 5 mm in diameter and 15 mm or more in
length, so each animal is thought to excavate its own burrow to more or less
fill the space it has created. Although it is thought that animals leave their bur-
rows to reproduce, it has been suggested that pairs can live together main-
taining their broods at the end of a shared burrow.
Figure 5Adult Sphaeroma triste
(Photo courtesy of Dr Simon Cragg)