A long-running debate over how coral skeletons are formed may be closer to resolution, as a new study reports that these structures form by a biologically controlled process, not one driven by chemical processes. Corals are threatened by ocean acidification and warming, among other factors, but being able to accurately project the magnitude of such threats requires understanding how corals form their carbonate skeletons. Today, there are two prevailing theories as to how corals deposit their calcium carbonate skeleton in the form of aragonite - by an inorganic pathway driven by environmental chemistry, or by a biologically controlled mechanism. To better understand the processes at play, Stanislas Von Euw and colleagues used a wide range of imaging techniques to study the skeletal development of Stylophora pistillata, a type of coral common in the Indo-Pacific ocean. Their detailed imaging reveals that these corals' skeletal fibers arise from organic matter. Along the surface of the organic matter are "immature" aragonite particles that act as a precursor for the coral structure, the authors report. Based on their results, they suggest that corals may retain greater metabolic capability to form aragonite skeletons in the face of decreasing ocean pH than commonly assumed.