A simple method to calculate the volume of shipworm tunnels from radiographs

Shipworms (Bivalvia: Teredinidae) are considered one of the main agents of wood bioerosion in the marine environment. They dig tunnels inside the wood they feed on, causing its progressive weakening. X-rays are a widely used non-destructive technique to assess wood biodeterioration (e.g. EN 275). However, an X-ray is a two-dimensional projection, with the possibility that some of the objects overlap and obfuscate each other. The higher the density of the shipworms, the more difficult is to count and measure the single tunnel. The aim of this work was to find a straight-forward method for calculating the volume of shipworm tunnels based on the size of shipworm shells, which are always clearly identifiable on the X-rays. Some wood test panels attacked by shipworms were analysed, with two distinct geometric methods being applied to estimate the tunnels volume based on radiographs. An allometric model was then introduced linking the volume to the shell diameter, which allows an easy estimation of the actual bioeroded volume from conventional two-dimensional radiographs. This technique allows to evaluate the bioerosion after the first growing season, shortening the exposure time and reducing costs. The model can be applied even when the tunnels are difficult to distinguish from each other, i.e. overlapped, folded or covering the whole surface of the radiograph.