The coralline algal genus Clathromorphum is a dominant calcifier in the rocky Subarctic biogeographic region, stretching through the lower Arctic from the Labrador Sea to the Bering Sea. Although commonly 2–10 cm in thickness, Clathromorphum can reach a thickness of up to 50 cm while forming an annually layered structure that can reach currently documented ages of up to 850 years. Geochemical and growth information archived in annual growth bands of Clathromorphum sp. has been used to provide long time series of past environmental conditions in regions that are poorly understood major drivers of Northern Hemisphere climate. However, information on Clathromorphum calcification, growth, and ecology that would allow interpretation of these records has previously been quite limited. Here we relate extensive field and laboratory data on the biology, physiology, and ecology of species of this genus and their controlling environmental parameters. We show that Clathromorphum has evolved a unique mode of double calcification, with high-magnesium calcite crystals, that enhances long life and leads to a multielement climate archive. Growth rates are controlled by temperature, and carbonate density is controlled by light, determined by both latitude and sea ice cover, whereas carbonate buildup and ultimate thickness are determined by local geomorphology and faunal interactions. Reproduction is complexly linked to vegetative anatomy. Precise paleoenvironmental information can be retrieved from Clathromorphum because of its unique cytological and anatomical structures, described and modeled for the first time in this volume.