An Estimation of the Global Diversity and Distribution of the Smallest Eukaryotes: Biogeography of Marine Benthic Heterotrophic Flagellates.

Protists are ubiquitous, but the factors influencing their diversity and biogeography remain unclear. We use a comprehensive database on the marine benthic heterotrophic flagellate (HF) morphospecies to explore the worldwide patterns in their diversity and distribution in comparison with predictions of the Ubiquity model (UM) and Moderate Endemicity model (MEM). The number of known HF morphospecies was limited (even when considering the rates of descriptions), and the local-to-global diversity ratio was relatively high (10-25%). Regional diversity was highly correlated with the investigative effort, indicating considerable under-exploration. Regional endemics were few (not over 19% of total richness), and many morphospecies were widespread or even cosmopolitan. No obvious latitudinal trend in HF diversity was detected. By species composition, the regions were distinctly arranged into three groups according to cold, temperate and warm waters, but not in accordance with geographical distances. This distribution pattern was most likely explained by contemporary climate (temperature) but did not suggest clear geographical barriers for dispersal. Therefore, the HF are less concordant with the MEM predictions but closer to the UM than other (larger) protists. Molecular studies reveal significantly higher HF diversity; the distributional patterns obtained from genetic- and morphology-based data, however, complement but not generally contradict each other.