Transient calcite precursor phases in Bryozoa: Smittina cervicornis and Myriapora truncata from the Mediterranean

Myriapora truncata and Smittina cervicornis, two common Bryozoa in the Mediterranean Sea form erect calcitic colonies with similar ultrastructures characterised by platy and granular calcite grains. At the submicron scale, the mineralized parts of the colonies show abundant organic inclusions and irregular calcite grains with undulating diffraction contrasts, indicating that crystallization progressed via particle attachment. High Resolution Electron Diffraction analyses show areas consisting of amorphous calcium carbonate (ACC) in Myriapora truncata colony walls. In Smittina cervicornis colony walls we identified calcite III in micrometre-sized areas adjacent to the organic cuticula in otherwise calcitic walls. Calcite III is a CaCO₃ polymorph stable at high pressures and its occurrence at ambient conditions here suggests that it is a precursor to calcite along a stepwise crystallization pathway, stabilized by organic molecules. Our results establish that Bryozoa, like many other marine calcifiers, form their colonies via stepwise crystallization pathways rather than crystallizing calcite directly from seawater. Mg/Ca and U/Ca ratios measured in situ along the growth axis of the colony display regular peaks and troughs which could be interpreted as seasonal temperature signals and translate to growth rates of 1 to 2 mm per year for the measured branch. We suggest that these data represent a proof of concept and illustrate that Mg/Ca and U/Ca ratios could be developed into a reliable method to be added to the toolkit for measuring colony growth rates in Bryozoa.