Skeletal microstructures of cheilostome bryozoans (phylum Bryozoa, class Gymnolaemata): crystallography and secretion patterns

Gymnolaemata bryozoans produce CaCO₃ skeletons of either calcite, aragonite, or both. Despite extensive research, their crystallography and biomineralization patterns remain unclear. We present a detailed study of the microstructures, mineralogy, and crystallography of eight extant cheilostome species using scanning electron microscopy, electron backscatter diffraction, atomic force microscopy, and micro-computed tomography. We distinguished five basic microstructures, three calcitic (tabular, irregularly platy, and granular), and two aragonitic (granular-platy and fibrous). The calcitic microstructures consist of crystal aggregates that transition from tabular or irregularly platy to granular assemblies. Fibrous aragonite consists of fibers arranged into spherulites. In all cases, the crystallographic textures are axial, and stronger in aragonite than in calcite, with the c-axis as the fiber axis. We reconstruct the biomineralization sequence in the different species by considering the distribution and morphology of the growth fronts of crystals and the location of the secretory epithelium. In bimineralic species, calcite formation always predates aragonite formation. In interior compound walls, growth proceeds from the cuticle toward the zooecium interior. We conclude that, with the exception of tabular calcite, biomineralization is remote and occurs within a relatively wide extrapallial space, which is consistent with the inorganic-like appearance of the microstructures. This biomineralization mode is rare among invertebrates.