Diversity and endemism in Rotifera: a review, and Keratella Bory de St Vincent

We confront patterns in the chorology and diversity of freshwater and limnoterrestrial Rotifera with predictions following from the recently revived ubiquity theorem on the distribution of microscopic organisms. Notwithstanding a strong taxonomic impediment and lack of data, both bdelloid and monogonont rotifers appear to conform to the hypothesis’ predictions that local diversity is relatively high compared to global diversity and that cosmopolitism is important. To the contrary, however, a latitudinal diversity gradient is obvious, and endemicity is present, and exhibits diverse patterns. This is illustrated by the case of Keratella rotifers, in which we identify purported relict endemicity hotspots in the east Palaearctic (China) and in temperate and cold regions of the southern hemisphere, and a recent radiation in North America. The apparent paradox may result from an antagonism between rotifer’s high population sizes and presence of potentially highly efficient propagules, versus pre-emption of habitats and local adaptation by resident populations, specific dispersal ability, and ecological and geographical factors. We conclude that distribution patterns of microscopic organisms, as represented by rotifers, most likely span the whole range of alternatives, from full cosmopolitanism to local endemism, and suggest that studying this diversity is more productive to come to an understanding of their chorology and diversity.