Members of the gastropod genus Ferrissia Walker, 1903, have a near-cosmopolitan distribution in freshwater ecosystems. Five North American species, distinguished by conchological, habitat, and distributional details, are generally recognized. However, they can be difficult to diagnose in practice, and we aimed to comprehensively revisit their systematics and taxonomy using molecular phylogenies. Our primary result was congruent for all three genetic markers: two highly distinctive Ferrissia clades, each encompassing multiple nominal species, were widely distributed across North American watersheds in both lentic and in lotic habitats. One clade was restricted to North America and was exclusively composed of lotic F. rivularis (Say, 1817) and lentic F. parallela (Haldemann, 1841). These two taxa exhibited geographic, rather than taxonomic, genetic structuring, strongly implying that they are conspecific and that F. parallela is a junior synonym of F. (Ferrissia) rivularis. The other clade contained three North American taxa, F. fragilis (Tryon, 1863) and putative specimens of F. mcneilli Walker, 1925, and F. walkeri (Pilsbry & Ferriss, 1907), in addition to the Hawaiian F. sharpi (Sykes, 1900), as well as multiple invasive Eurasian founder populations. All four taxa shared overlapping genotypes and shell phenotypes, prompting us to conclude that they are conspecific with F. fragilis having taxonomic priority. This New World species had robust phylogenetic links to Old World congeners, thereby enabling us to unite global septum-forming Ferrissia species under the subgenus Kincaidilla Hannibal, 1912. Ferrissia rivularis and F. fragilis differed in a number of conchological traits, including maximum body size, details of apex positioning and orientation, and the ability to form a septum. The two former traits were sufficient to distinguish sympatric populations.
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