Species-specific hierarchical architecture and functional morphology in recent calcifying Australian bryozoans

Two recent marine calcifying bryozoans; Celleporaria cristata (Gymnolemata, Cheilotomata) and Densipora corrugata (Cyclostomata), from the South Australian coast show similar spindle-shaped colony forms with prominent ridges despite belonging to evolutionary separate classes. Accordingly, they differ in zooid shape, exhibiting box-shaped and tubular zooids, yet share many commonalities in their wall micro- and nanostructures. We correlate in-situ observations from electron microscopy, micro-computed tomography, atomic force microscopy, micro-Raman spectroscopy and electron probe microanalyzer to explore their building plans at high spatial resolution. We show that the calcitic colonies of both species are comprised of low-Mg calcite with 1.23–3.2% m/m Mg and detectable Na/Ca (22.8 vs 31.5 mmol/mol and Sr/Ca (3.1 vs 3.3 mmol/mol). Higher magnesium concentrations are localised in the ridges of D. corrugata (up to 3.2% m/m) while C. cristata skeletons show irregular Mg-rich bands. Similarities in skeletal microstructures are evident in the basal, vertical and frontal walls, with granular and platy microstructures. Minor, secondary aragonite formation with less microstructural textures in C. cristata suggests biologically induced mineralization. Fluid dynamic simulations used to investigate the functional morphology of the ridges in these two unrelated bryozoan species, showed that the ridges efficiently control water flow around the colonies by reducing incoming flow by 80% down to the optimal feeding velocity of 0.02–0.04 m s⁻¹, thus providing a functional advantage beyond structural stability.