Abstract
High-energy freshwater environments such as rapids and waterfalls in the Upper-Paraná Atlantic Forest are home to highly endemic minute freshwater snails of the genus Acrorbis. Only one species, Acrorbis petricola, is currently included within this genus, whose geographical distribution is restricted to three known populations, one in Brazil and the other two in Argentina. Because of habitat specificity and limited geographical distribution, the species is considered vulnerable in Argentina and endangered in Brazil. In this work, we identify five new populations of A. petricola in southern Upper-Paraná Atlantic Forest, exclusively found on waterfalls from the Misiones Province, Argentina. Based on these populations and on specimens of one of the two historical populations from the Misiones Province, we explored the morphological features of shells and reproductive system of specimens from each location and provide the first molecular data on the species. We used DNA sequences from cytochrome c oxidase subunit I (COI) and 16S-rRNA genes to investigate the molecular diversity, genetic distances and genealogical relationships among populations. We verified the existence of intra- and interpopulation morphological variability, with the greatest variation being found in spire, spiral sculpture, penis sheath, flagella, prostatic diverticula and bursa copulatrix. We found interpopulation genetic diversity, with no intrapopulation variation, and identified six geographically structured genetic lineages with maximum genetic distances of up to 2.3%. Different combinations of morphological characters with the same genetic background within each locality were observed. The finding of new populations genetically differentiated not only broadens the known distribution of the species, but also illustrates that waterfall environments in the Atlantic Forest harbour a hidden diversity of Acrorbis that still remains to be discovered. This scenario suggests a complex evolutionary history that needs to be unveiled and taken into account for future development of conservation strategies in this endemic genus.
Highlights
Waterfalls and river rapids are natural laboratories for evolutionary research as they usually host a highly endemic flora and fauna, especially adapted to the constant humid conditions and the force of water, including several molluscan species [1,2,3,4]
Subsequent studies based on shell, anatomy and radular morphology of the species from Salto Encantado, verified that Argentinian specimens were indistinguishable from the Brazilian congeners, and A. odhneri was regarded as a junior synonym of A. petricola [22]
Living specimens of Acrorbis were recorded in the six waterfall environments surveyed in central and southern Misiones Province, five of these being new localities for the genus (Figs 2 and 3 and Table 1)
Summary
Waterfalls and river rapids are natural laboratories for evolutionary research as they usually host a highly endemic flora and fauna, especially adapted to the constant humid conditions and the force of water, including several molluscan species [1,2,3,4]. Such high-energy freshwater environments are abundant in the Atlantic Forest, which extends across southeastern Brazil, eastern Paraguay and northeastern Argentina [5,6]. Based on the Iguazuspecimens and on a reexamination of the type-series of A. odhneri, that study suggested that both nominal species were conspecific under the name A. petricola [24]
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