Anatomical and genetic variation of western Oxyloma (Pulmonata: Succineidae) concerning the endangered Kanab ambersnail ( Oxyloma haydeni kanabense ) in Arizona and Utah

Abstract

The land snail genus Oxyloma (Pulmonata: Succineidae) includes the Federally endangered Kanab ambersnail (Oxyloma haydeni kanabense Pilsbry), which is known at the time of this study from only two locations in the United States: Three Lakes, Utah, and Vaseys Paradise, Arizona, on the Colorado River in Grand Canyon National Park. Since 1994, the Kanab ambersnail has received much attention because its presence at Vaseys Paradise has implications for the ecosystem-wide management of the Colorado River. This attention is primarily because an experimental high-flow release of water from Glen Canyon Dam in 1996 destroyed or degraded Kanab ambersnail habitat at Vaseys Paradise. This experimental high flow was designed to replicate natural flow regimes throughout the Grand Canyon river corridor. However, as a result of the habitat destruction at Vaseys Paradise, in 1996, the U.S. Fish and Wildlife Service ruled that no further experimental high-discharge floods could be carried out until additional Kanab ambersnail populations were discovered or established. This mandate created a situation where the management of a single endangered species conflicted directly with the management of an entire ecosystem. Although since 1996, the U.S. Fish and Wildlife Service has permitted the use of flows as high as stage heights equivalent to 44,000 cubic feet per second, higher flows were requested by various Grand Canyon stakeholders and scientists but were not possible owing to low storage of Lake Powell. Adding to the controversy about Oxyloma and the Kanab ambersnail were previous anatomical and genetic analyses of the genus, which showed that genetic characteristics of specimens did not correspond with their identifications based on traditional taxonomic criteria, raising questions about the validity of the taxonomy of Oxyloma and the protected status of Kanab ambersnails. Specifically, a previous study suggested that the endangered Kanab ambersnail population at Three Lakes was more closely related to other, non-endangered ambersnail populations across the Southwest. In contrast, the Kanab ambersnail population at Vaseys Paradise appeared to be genetically distinct from all other ambersnail populations studied. Management options for the ambersnail population at Vaseys Paradise, at the time of this study, conflict with ecosystem-wide measures proposed to benefit other natural resources in the Grand Canyon. The U.S. Fish and Wildlife Service will not revise the 1995 Kanab Ambersnail Recovery Plan until further genetic and anatomical analyses provide more fine-scale taxonomic resolution of the identity of Oxyloma populations on the Colorado Plateau and elsewhere in the American Southwest. Likewise, interagency cooperators cannot revise down-listing criteria for the Kanab ambersnail until substantial evidence is provided identifying distinct Oxyloma taxa or a larger group of conspecifics that reasonably could be managed as one species. Therefore, given the current controversy about the taxonomy of Oxyloma and the endangered Kanab ambersnail, new detailed analyses were completed of morphological and genetic variation from many Oxyloma specimens collected at 12 western North American locations. These new data have allowed us to evaluate many issues related to Kanab ambersnail taxonomy. Using this dataset, the study of shells and anatomy indicates that the holotype of Oxyloma haydeni kanabense plausibly can be regarded as a member of the same species as the populations of Oxyloma analyzed in this study. Additionally, the presence of gene flow among all populations is evidence that they are members of the same species. Almost all the observed genetic diversity can be accounted for by short-distance or long-distance dispersal events between populations in this study. Our major taxonomic conclusion is that all samples collected for this study were drawn from populations of the same species.