A history of sponge erosion: from past myths and hypotheses to recent approaches

Bioeroding sponges have historically been mystical beasts of the sea. Originally, they were classified as half cnidarian, half sponge. It required much time and scientific convincing to confirm their status as active bioeroders. The scientists Hancock and Nasonov were pioneers of their time. They recognised and defended the main concepts: the endolithic organisms are sponges, they produce the cavities they inhabit, and their activities are likely to involve chemical and mechanical processes. However, this viewpoint was often scathingly challenged, and the notion of actively bioeroding sponges was hotly disputed. Once the concept was firmly established in the mid 1900s, related research experienced a significant leap. Notably studies by Pomponi and Hatch on the ultrastructure of etching cells and their associated biochemical properties left no room for doubt: The enzymes carbonic anhydrase and acid phosphatase are associated with sponge bioerosion, providing means for mineral dissolution and digestion of organic components, thus enabling the removal of the so-called sponge chips. However, the exact etching agent remained undetected, and since 1980 research on this phenomenon significantly slowed down. Further studies predominantly focused on environmental control of bioerosion and the taxonomic value of sponge erosion traces. A recent study with electrochemical liquid ion exchange microsensors revisited the question of how sponge bioerosion is achieved and whether acid is involved. This study is presented here to conclude the summary of current knowledge in this context. Microgradients of pH and calcium ions in the tissues of Cliona celata and the non-eroding sponge Halichondria panicea from the North Sea were compared. The pH slightly decreased with distance into the tissue of C. celata, whereas after an initial drop it remained stable in H. panicea. Calcium concentrations in C. celata increased slightly more with tissue depth than in H. panicea. C. celata bioerosion may be periodic (fewhours cycle) as evidenced by oscillating pH values at the sponge-substrate interface, which may explain micro-terracing in sponge scars. Nevertheless, measured pH changes were too weak to prove beyond doubt that sponge bioerosion employs acid, and further studies will be necessary to confirm present preliminary findings.