biostudies-literatureStewart JANatural Environment Research Council13105https://www.ebi.ac.uk/biostudies/studies/S-EPMC9339005biostudies-literatureUnknown12(1)Anthropogenic carbon emissions are causing seawater pH to decline, yet the impact on marine calcifiers is uncertain. Scleractinian corals and coralline algae strongly elevate the pH of their calcifying fluid (CF) to promote calcification. Other organisms adopt less energetically demanding calcification approaches but restrict their habitat. Stylasterid corals occur widely (extending well below the carbonate saturation horizon) and precipitate both aragonite and high-Mg calcite, however, their mode of biocalcification and resilience to ocean acidification are unknown. Here we measure skeletal boron isotopes (δ¹¹B), B/Ca, and U/Ca to provide the first assessment of pH and rate of seawater flushing of stylasterid CF. Remarkably, both aragonitic and high-Mg calcitic stylasterids havScientific reportsStylasterid corals build aragonite skeletons in undersaturated water despite low pH at the site of calcification.PMC9339005NE/S001743/1NE/R005117/1NE/N003861/1Robinson LFKershaw JStewart JAStrawson IfalseStylasterid corals build aragonite skeletons in undersaturated water despite low pH at the site of calcification.Anthropogenic carbon emissions are causing seawater pH to decline, yet the impact on marine calcifiers is uncertain. Scleractinian corals and coralline algae strongly elevate the pH of their calcifying fluid (CF) to promote calcification. Other organisms adopt less energetically demanding calcification approaches but restrict their habitat. Stylasterid corals occur widely (extending well below the carbonate saturation horizon) and precipitate both aragonite and high-Mg calcite, however, their mode of biocalcification and resilience to ocean acidification are unknown. Here we measure skeletal boron isotopes (δ¹¹B), B/Ca, and U/Ca to provide the first assessment of pH and rate of seawater flushing of stylasterid CF. Remarkably, both aragonitic and high-Mg calcitic stylasterids hav2022-01-01T00:00:00Z2022 Jul2025-04-27T03:17:02.697Z2025-04-06T18:49:18.099ZS-EPMC93390053590792610.1038/s41598-022-16787-y