MEPS 179:297-300 (1999)  -  doi:10.3354/meps179297

ABSTRACT: A constant amount of calcium oxalates, as weddellite and whewellite, has been detected by X ray diffraction analysis in the NaClO resistant fraction of the tissue of the demosponge Chondrosia reniformis. SEM observations showed that oxalate is present on the surface of living sponges, suggesting that the crystals are produced inside and/or on the sponge surface. The output of oxalate crystals through the sponge ectosome represents a significant percentage of sediments collected by sediment traps placed in a C. reniformis population. HPLC analyses allowed the detection of high levels of ascorbic acid in the sponge's tissue; the spontaneous conversion of the ascorbic acid to oxalic acid, well known in plants, may explain the oxalate occurrence. Although the ascorbic acid is involved in the biosynthesis of collagen, the release of ascorbic/oxalic acid through the sponge surface may be utilised to ward off other benthic organisms during competition for space and as an antifeeding mechanism against possible predators.

KEY WORDS: Oxalate · Ascorbic acid · Porifera · Biomineralization