Crossing the Continental Divide: the Columbia drainage species Juga hemphilli (Henderson, 1935) is a cryptic member of the eastern North American genus Elimia (Cerithioidea: Pleuroceridae)

Abstract

Pleurocerid gastropods are ecologically prominent and diverse members of the North American freshwater malacofauna. However, their systematic study has been greatly complicated by pronounced conchological variability. Almost 1,000 taxa have been described north of Mexico, although this inflated estimate has been reduced by serial synonymization to 152 or 153 nominal species, comprising seven genera. Pleurocerid diversity is not evenly spread across continental watersheds. Only nine nominal species, placed in the genus Juga, are found in the drainages west of the Rocky Mountains. Juga was expressly erected to house the western taxa, but there are no diagnostic conchological characters to separate them from eastern con-familials and they had previously been regarded as western lineages of Elimia (formerly known as Goniobasis ). Consequently, identification keys have used biogeographic criteria to distinguish Juga (west of the Continental Divide) from Elimia (east of the Continental Divide) taxa. Until recently, the systematic validity of the genus Juga was rather tenuous, resting primarily on hints of oviductal groove structural distinctions. However, molecular phylogenetic data have now clearly demonstrated Juga’s separate evolutionary history. In a study of North American pleurocerid relationships, three species of Juga, including the type species Juga silicula (Gould, 1847), formed a robust clade sister to all of the genotyped eastern taxa. A subsequent study incorporating a global sampling of Cerithioidea emphasized the phylogenetic distinctiveness of Juga, finding it to be sister to the eastern Asian pleurocerid genera Hua and Semisulcospira. However, not all Juga species have been incorporated into molecular phylogenies and there are morphological and karyological lines of evidence to suggest that some nominal taxa occurring in the Columbia River drainage may actually be geographically disjunct members of the eastern genus Elimia. A century ago, a species of pleurocerid, Goniobasis columbiensis Whiteaves, 1905, was described from material collected in 1883 by J.B. Tyrrell at the headwaters of the Columbia River that represented ‘the first non-plicate Goniobasis, of the type of G. livescens, that has been found in the Pacific drainage system.’ Goodrich examined Whiteave’s type material and found that the ‘shell characters and opercula are identical with those of livescens of Lake Erie,’ but concluded that they were more likely to represent mislabeled eastern specimens rather than a geographically disjunct western population. More recently, a karyological study of North American pleurocerid taxa included a single Juga species, J. hemphilli (Henderson, 1935), sampled from Benton County, Oregon that exhibited an identical ideogram to E. livescens (Menke, 1830). Though constrained by the lack of karyological data for other Juga taxa, this surprising result suggested to us that J. hemphilli might well represent a cryptic western Elimia lineage. Henderson described Juga hemphilli, together with two subspecies, from lower Columbia River drainage locations, although Goodrich subsequently listed it, together with Whiteave’s G. columbiensis, as a species incerta. The goal of our study was to phylogenetically test, using gene trees, the hypothesis that Juga hemphilli is a cryptic western member of the genus Elimia. Specimens of J. hemphilli dallesensis (Henderson, 1935), sampled from the type locality, were kindly provided by F.G. Thompson (Fig. 1) and, together with the representatives of all extant eastern North American pleurocerid genera except for Lithasia (Table 1), they were sequenced for a portion of the nuclear large subunit ribosomal gene (28S). The choice of this gene fragment was driven by the need to avoid a potentially confounding issue – within-species mitochondrial (mt) polyphyly – that has complicated freshwater cerithioidean molecular phylogenetic studies using exclusively mt markers. –24 This is especially pertinent for North American pleurocerid taxa where pronounced within-population mt heterogeneity may occur and the only available case of mt/ nuclear genome comparison is surprisingly incongruent. Mobile basin Leptoxis picta (Lea, 1841) and L. taeniata (Tyron, 1873) were indistinguishable in terms of allozyme frequencies at nine loci but exhibited highly divergent mt genotypes, both in terms of sequence divergence [19% for mt large RNA subunit (16S)] and phylogenetic placement. In addition to the target nuclear rDNA marker, we also took advantage of the extensive GenBank submissions of pleurocerid mt 16S