Micro-computed tomography for studies on Entobia: Transparent substrate versus modern technology

Endolithic bioerosion is difficult to analyse and to describe, and it usually requires damaging of the sample material. Sponge erosion (Entobia) may be one of the most difficult to evaluate, as it is simultaneously macroscopically inhomogeneous and microstructurally intricate. We studied the bioerosion traces of the two Australian sponges Cliona celata and Cliona orientalis with modern technology: high resolution X-ray micro-computed tomography. Micro-CT allows non-destructive visualisation of live and dead structures in three dimensions and was compared to traditional microscopic methods. Micro-CT and microscopy showed that C. celata bioerosion was more intense in the centre and branched out in the periphery (21 vs. 9% substrate removed). In contrast, C. orientalis produced a dense, even meshwork and caused an overall more intense erosion pattern than C. celata (48 central vs. 42% marginal substrate removed). Extended pioneering filaments were not usually found at the margins of the studied sponge erosion, but branches ended abruptly or tapered to points. Results obtained with micro-CT were similar in quality to observations from transparent optical spar under the dissecting microscope. Microstructures could not be resolved as well with micro-CT as anticipated. Even though sponge scars and sponge chips were easily recognisable on maximum magnification micro-CT images, they lacked the detail that is available from SEM. Other drawbacks of micro-CT involve high costs and presently limited access. Even though micro-CT cannot presently replace traditional techniques such as epoxy resin casts viewed by SEM, we obtained valuable information. Especially for the possibility to measure endolithic pore volumes, we regard micro-CT as a very promising tool that will continue to be optimised. A combination of different methods will produce the best results in the study of Entobia.