| Summary: | The biogeographic pattern of single‐celled eukaryotes (protists), including ciliates, is poorly understood. Most marine species are believed to have a relatively high dispersal potential, such that both globally distributed and geographically isolated taxa exist. Primary occurrence data for three large, easily identified ciliate species, Parafavella gigantea, Schmidingerella serrata, and Zoothamnium pelagicum, and environmental data drawn from the National Oceanic and Atmospheric Administration's World Ocean Atlas were used to estimate each species’ spatial and environmental distributions using Maxent v3.3.3k. The predictive power of the models was tested with a series of spatial stratification studies, which were evaluated using partial receiver operating characteristic (ROC) statistics. Differences between niches occupied by each taxon were evaluated using background similarity tests. All predictions showed significant ability to anticipate test points. The null hypotheses of niche similarity were rejected in all background similarity tests comparing the niches among the three species. This article provides the first quantitative assessment of environmental conditions associated with three species of ciliates and a first estimate of their spatial distributions in the North Atlantic, which can serve as a benchmark against which to document distributional shifts. These species follow consistent, predictable patterns related to climate and environmental biochemistry; the importance of climatic conditions as regards protist distributions is noteworthy considering the effects of global climate change. We dedicate this paper to John Clamp, who was an integral part of the research and writing of this paper. Clamp passed while this manuscript was under review. He was a professor at North Carolina Central University for more than 30 yr, devoting his outstanding research to evolutionary biology, and particularly the study of ciliated protists. He was a larger‐than‐life character and will be greatly missed as friend and mentor. This work was supported by the Ministerio de Economía y Competitividad, Española (MINECO, grant number CGL2013‐40851‐P/ BOS) to MMC, and travel support from the Research Coordination Network for Biodiversity of Ciliates (National Science Foundation grant number DEB 1136580) from the U.S. National Science Foundation to JC. RAJW was supported by the Crafoord Foundation, Sweden (grant numbers 20160971 and 20170671), while completing this manuscript.
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