Shell features and anatomy of the springsnail genus Radomaniola (Caenogastropoda: Hydrobiidae) show a different pace and mode of evolution over five million years.

Shell features are key factors for systematics and evolutionary biology studies of freshwater gastropods. Based mostly on shell morphology, 19 species of the springsnail genus Radomaniola (family Hydrobiidae) have been described. Although the scarce differentiation of various shell dimensions suggests morphological stasis, the evolutionary dynamics of shell and other anatomical structures have not yet been explored fully. By inferring a phylogeny and a time-calibrated species tree from mitochondrial (COI and 16S) and nuclear (28S) sequences of 15 recognized species, we initially examine the species diversity of the Radomaniola dataset and then, through phylogenetic comparative methods, assess the evolutionary correlation, pace and mode of 40 continuous shell and anatomical characters. By synthesizing the results of four species delimitation methods and through morphological examinations, we recognize 21 groups, for which taxonomy is discussed here. Seven new species are described. We reveal a high degree of correlated evolution between characters of the shell, which are constrained by a single morphological optimum, consistent with the morphological stasis model. Anatomical traits diverged rapidly in an unconstrained manner or toward multiple optima. These findings indicate that in order to understand the morphological evolution of springsnails, it is essential to examine different organs in detail. [ABSTRACT FROM AUTHOR]