Understanding a bimineralic bryozoan: Skeletal structure and carbonate mineralogy of Odontionella cyclops (Foveolariidae: Cheilostomata: Bryozoa) in New Zealand

Skeletal carbonate mineralogy of the bryozoan Odontionella cyclops (Busk, 1854) (family Foveolariidae) is extremely variable, with calcite:aragonite ratio ranging from 27 to 100 wt.% calcite (mean = 57 wt.% calcite, SD = 15, n = 118), and Mg content in calcite varying from 3.6 to 8.8 wt.% MgCO3 (mean = 6.2 wt.% MgCO3, SD = 1.1, n = 118). This study examines the sources of this wide variability and the possible effects of ocean acidification on bimineral invertebrates. Variation in calcite:aragonite ratio in O. cyclops is neither environmental nor related to colonial growth form, but appears to be astogenetic. Primary calcification of the zooecial 'box' is all calcite, followed by progressive construction of a secondary aragonitic superstructure which includes avicularia. Consequently, young parts of the colony are dominated by calcite, with increasing amounts of aragonite with age. Very old parts of the colony may have the aragonite eroded or chipped away to become again entirely calcitic. In contrast with many other bryozoans that are entirely calcitic or mainly aragonitic, this bipartite structure may result in increased vulnerability to ocean acidification. Given the southern-temperate shelf-to-slope distribution of this species, O. cyclops (and others like it) will begin to be subjected to decreasing pH in only a few decades. The consequence could be a modern sediment assemblage similar to a diagenetically-altered fossil assemblage - missing aragonitic skeletal parts and species.