Electron diffraction studies of the calcareous skeletons of bryozoans

Electron microscopy and electron diffraction were used to investigate mineral crystallites dissociated from the skeletal walls of six species belonging to the Bryozoa, a phylum of predominantly marine colony-forming invertebrate animals. Four cheilostome bryozoans (Flustra foliacea, Membranipora membranacea, Thalamoporella novaehollandiae and Cellarinella foveolata) and two cyclostomes (Fasciculipora ramosa and Hornera robusta) were analysed. In each case, an attempt was made to relate the crystal morphology imaged in situ by scanning electron microscopy with the crystallographic orientation of isolated crystals determined by electron diffraction analysis in the transmission electron microscope. The results showed that the calcitic cheilostome and cyclostome skeletons consisted of closely packed arrays of plate-like Mg-containing calcite crystallites, and that the crystallographic a-axis was preferentially aligned perpendicular to the top and bottom surfaces of the flattened particles. The results suggest that calcite biomineralization occurs under similar crystallographic constraints in the five species studied even though the origins of cheilostomes and cyclostomes are separated by over 300 million years in the fossil record of the bryozoans. Similar studies for the aragonite crystallites in skeletons of M. membranacea indicated that the crystallographic b-axis was preferentially oriented perpendicular to the basal surfaces of irregular plate-like particles.