Abstract:
Freshwater gastropods of the family Pleuroceridae are incredibly important to the health of freshwater ecosystems in the southeastern United States. Surprisingly, however, they are one of the most understudied groups of mollusks in North America. Recent data suggest that pleurocerids have an imperilment rate of 79% and many species are under immediate threat of extinction. As such, the time is now to better understand their biology, taxonomy, and phylogeny. Any study that wishes to understand a species group like pleurocerids must start with extensive field work and taxon sampling. As a part of that endeavor I rediscovered Leptoxis compacta , a snail that had not been collected live in 76 years. In chapter two, I report on this discovery and I propose a captive propagation plan and potential reintroduction sites for L. compacta in an attempt to prevent its extinction. In chapter three, I explore the often reported species level polyphyly on mitochondrial gene trees for gastropods in the family Pleuroceridae and its sister family Semisulcospiridae. Explanations for this paraphyly have ranged from unsatisfying (e.g. that species-level polyphyly is caused by historical introgression) to absurd (e.g. the same species on earth today were here over 65 million years ago). I conclusively demonstrate that the "divergent" haplotypes that have caused this phylogenetic pattern in previous studies are paralogous nuclear copies of mitochondrial genes as that were included in phylogenetic inference as mitochondrial homologs. In chapter four, I present a detailed analysis on egg-laying behaviors in Leptoxis species. I also demonstrate that, despite claims to the contrary, shell variation among species is a result of genetic differences, not ecophenotypic plasticity. I also show that three different egg-laying behaviors exist within the genus, single egg-laying, laying eggs in a line, and clutch formation egg-laying. Finally, I tackle the chaotic Leptoxis taxonomy by inferring phylogenetic hypotheses for Leptoxis and other pleurocerids outgroups using three genes. Leptoxis is resolved as para- and polyphyletic, and I elevate one genus and describe a new genus. This dissertation is an important first step for better understanding pleurocerid biology, and I hope it will stimulate much future study on this fascinating family of freshwater snails.
