Characterization of the calcium biomineral in the radular teeth of Chiton pelliserpentis.

The radula in a group of molluscan invertebrates, the chitons (Polyplacophora), is a ribbon-like apparatus used for feeding and which bears a series of distinctive mineralized teeth called the major lateral teeth. While some chiton species deposit only iron biominerals in these teeth, many others deposit both iron and calcium. In this study, the calcium biomineral in the teeth of one of the latter types of species, the Australian east-coast chiton, Chiton pelliserpentis, has been isolated and examined for the first time. Spectroscopic and crystallographic techniques have identified the biomineral as a carbonate-substituted apatite with significant fluoride substitution also likely. Fourier-transform infrared and laser Raman spectroscopy indicated that the carbonate content was less than that of either bovine tibia cortical bone or human tooth enamel. X-ray diffraction analysis showed the biomineral to be poorly crystalline due to small crystal size and appreciable anionic substitution. The lattice parameters were calculated to be a = 9.382 A and c = 6.883 A, which are suggestive of a fluorapatite material. It is postulated that structural and biochemical differences in the tooth organic matrix of different chiton species will ultimately determine if the teeth become partly calcified or iron mineralized only.