Cryptic diversity within the rotifer Polyarthra dolichoptera along an altitudinal gradient

Cryptic species (species that are difficult or impossible to distinguish morphologically) are found worldwide and neglecting them may hamper our understanding of species-environment relationships. The rotifer Polyarthra dolichoptera occurs across wide environmental gradients, indicating the potential for a cryptic species complex. We used P. dolichoptera as a model to investigate spatial patterns and relationships of cryptic species with environmental variables along a broad altitudinal gradient. We sequenced 575 specimens of Polyarthra from 42 waterbodies from Germany, Hungary and Italy sampled in summer. Thirty-five lakes were distributed along an altitudinal gradient (250–2564 m a.s.l.) in a geographically restricted area, the Trentino–South Tyrol region, Italy, and one lake (Lake Tovel) was sampled in two basins and at different depths year-round to investigate temporal and spatial patterns of cryptic species. We used the Generalised Mixed Yule Coalescent model to delimit cryptic species (GMYC entities) based on a phylogenetic tree and related their presence/absence to space and the environment by applying Mantel tests and variance partitioning. We found twelve GMYC entities of P. dolichoptera. In variance partitioning, most explained variability was attributed to the interaction between space and altitude and by environmental variables and geographical space alone. However, by splitting the data into sets of low- and high-altitude lakes, the strong effect of altitude was removed; conductivity and silica played a major role in high-altitude lakes, whereas neither environment nor space were related to entity occurrence in low-altitude lakes. In Lake Tovel, three cryptic entities were found that did not show spatial segregation; however, two entities showed temporal segregation while the third one occurred throughout the year. Our study showed that P. dolichoptera is a complex of species which distribution is driven by geography and environmental parameters whose importance is different in low- and high-altitude lakes. Considerable unexplained variability points to a major contribution of other variables, potentially in-lake properties such as competition, predation and species behaviour