A Systematic Study of North American Freshwater Limpets (Gastropoda: Hygrophila: Ancylidae)

Abstract

The freshwater limpet family Ancylidae, comprised exclusively of patelliform taxa, has a near-cosmopolitan distribution in freshwater ecosystems. Paul Basch was the last to thoroughly study North American ancylid systematics, and I used his 1963 monograph as a guide in sampling nominal North American ancylid species and constructing representative nuclear and mitochondrial (mt) gene trees. My objectives were to a) assess the monophyly of the family Ancylidae, b) analyze intergeneric and interspecific ancylid relationships, and c) address the validity of nominal North American species. In the context of global samples, I recovered a monophyletic Ancylidae and a pronounced sub-familial dichotomy separating the New World genera (Laevapex, Hebetancylus, Uncancylus, Gundlachia) from a Holarctic sister clade (Ferrissia, Rhodacmea, Ancylus). While support for the two ancylid subfamilies was robust, support for specific intergeneric relationships within the subfamilies was lacking. In my gene trees, all nominal Laevapex species emerged as a single lineage, suggesting that the North American Laevapex is monotypic, comprised of only L. fuscus. This finding was corroborated by a geometric morphometric analysis of shell vouchers that indicated no difference in shell shape among the nominal species. Rare, highly divergent mt lineages in some Laevapex populations likely originated from either introgression or from persistent ancestral polymorphisms. Only two of 36 L. fuscus mt haplotypes occurred in multiple populations, indicating that long distance dispersal is rare, and field observations of phoresy on the giant water bug Belostoma flumineum identified it as a potential agent of local dispersal. North American Ferrissia samples formed two major clades, corresponding to the type species F. rivularis and the widespread F. fragilis. The former exhibited pronounced east-west geographic structure, with evidence of secondary transcontinental movement from the east to the west but no indication of introgression; thus, these lineages may be speciating. North American F. fragilis has cryptically invaded Europe and Asia, providing striking evidence of human-mediated intercontinental dispersal in ancylids, a feature that has further complicated systematic studies of this group.