Carbonate mineralogy of free-living bryozoans (Bryozoa: Otionellidae), Otago shelf, southern New Zealand

On the open temperate shelf off Otago Peninsula, eastern South Island, New Zealand, in water depths ranging from 90 to 140 m, free-living motile bryozoans of the genus Otionellina survive in shifting quartzofeldspathic sands. Unlike most bryozoans, these free-living colonies characteristically precipitate skeletons of aragonite. Here, we report on skeletal carbonate mineralogy of 104 specimens of Otionellina from four different species, across the shelf, which are also in different stages of development. All were dominantly composed of aragonite, with only five specimens containing 0–15 wt.% calcite (but with an overall mean of <1 wt.%), with low to intermediate MgCO3 content in the calcite (<5 wt.% MgCO3). Mineralogy in Otionellina varies, but is not related to species, colony size, or environmental factors such as water temperature, salinity, water depth, and substratum. Otionellinids may lack the environmentally influenced biomineralisation pathways found in other carbonate-producing taxa. It appears that most free-living (or “vagrant”) bryozoans are aragonitic, although some rare examples contain up to 5% calcite; there may be no free-living genera which are always entirely aragonite. While preservation of aragonitic skeletons is by no means certain, the presence of free-living bryozoans indicates a sandy outer-shelf environment and a diagenetic history that allows for preservation of aragonite.