Taxonomic re-evaluation of the Brachionus calyciflorus species complex (Rotifera) in Japanese freshwaters

Abstract Recently, the Brachionus calyciflorus species complex was divided into four nominal species: B. calyciflorus sensu stricto (s.s.) Pallas, 1766; Brachionus dorcas Gosse, 1851, Brachionus elevatus Michaloudi et al., 2018, and Brachionus fernandoi Michaloudi et al., 2018 based on the results of both morphological and molecular investigations. We applied this systematic approach to B. “ calyciflorus ” collected from lakes and ponds in various parts of Japan to examine spatial and temporal distributions of these species. We found that all four species indeed distributed in Japan . Among the species, B. fernandoi exhibited a somewhat different occurrence pattern and morphology. Generally, B. fernandoi appeared to occur in the winter–spring, while the other three species seemed to occur in the early summer–late autumn. A seasonal succession from B. fernandoi -dominant community to other species-dominant communities was observed in the same lakes/ponds. A statistical analysis showed that B. fernandoi -dominant species composition was also related to low temperatures. It was also found that the body of B. fernandoi was more elongated compared to those of other species. These results suggest that B. fernandoi is a distinct species in terms of morphology and phenology. Among the other three species, we observed a variation in the length of posterolateral spines. However, it was not clear if this variation is a trait that distinguish these species morphologically. In addition, some of these species co-occurred in the same lakes and ponds. The mechanism by which they coexist is a topic for future study.