| Members of the freshwater Basommatophoran family, Physidae, are among the most abundant gastropods in North America. Highly successful, some members of this group are distributed world wide. Physids are found in a variety of freshwater habitats and make excellent model organisms for a variety of studies. Yet as a group, they are poorly understood taxonomically. Over 400 nominal species have been described with most original descriptions relying solely on shell characters. There have been two comprehensive monographs of the group in recent times: Te (1978), resulting in 36 described species with another 43 subspecies and morphs, and Taylor (2003), resulting in 81 described species. Both classifications rely heavily on the group's penial complex, but neither employ modern phylogenetic methods. Here I use mitochondrial DNA (portions of 16S rRNA and CO1 genes) to examine evolutionary relationships among 66 physid specimens representing 28 taxa and six penial morphological groups. A correspondence between penial morphology and the resulting molecular phylogeny was uncovered.; Physa are more commonly found in hot water habitats in both Europe and North America than any other freshwater mollusk, including thermal springs (a few in caves) and water heated artificially by thermal effluent. Thermal springs are localized and unique, providing their inhabitants with a potentially harsh, yet stable environment. I explore whether these specialized habitats provide effective barriers against gene flow from nearby physid populations using mitochondrial DNA (16S rRNA and CO1 gene portions). Very little sequence divergence was uncovered between thermal spring taxa and their nearest neighbors. The exception was Physa spelunca, having a genetic divergence of ∼11% from a nearby P. acuta population.; Finally, I present both mitochondrial sequence variation and allozyme variation in six populations of Physa acuta from both Europe and North America. This one biological species also comprises one phylogenetic species, representing three common and widespread nominal species: P. acuta, P. heterostropha, and P. integra . There was a remarkably high between and within population variation for both the allozyme and mitochondrial data, but neither corresponded with any measured difference in genetic diversity between European and North American populations. |
