Exploring Species Boundaries in the Diatom Genus Rhoicosphenia Using Morphology, Phylogeny, Ecology, and Biogeography

Certain taxonomic groups within the American flora and fauna are relatively unexplored in terms of their biodiversity – one of these groups are the diatoms. The diversity, phylogeny, and ecology of the common freshwater diatom genus Rhoicosphenia are explored. While determining the diversity of Rhoicosphenia in American streams, several new taxa have been discovered, both from fossil and recent collections. These discoveries are discussed in the context of the history of the genus, its diversity in other parts of the world, and the morphological characters used to identify species distinctions. Prior to this dissertation, one taxon (R. abbreviata) was commonly reported from the United States, and after these studies, eight morphologically distinct taxa were found. Presently, no published molecular studies have sequenced any species of Rhoicosphenia for use in phylogenetic analyses. In the literature, four historical hypotheses (dating back to the erection of the genus in 1860) about its position in the diatom tree of life remain untested by molecular data. This dissertation used morphological and multi-marker molecular data to test the four hypotheses and place Rhoicosphenia in the phylogeny of diatoms. The results did not fully support any of the four hypotheses, but did offer insight into parts of the diatom tree that have been underexplored. R. abbreviata was reported from nearly all floristic treatments of diatoms of the US, but little quantitative data was provided in regards to its niche. Due to its presence in a large percentage of studies, it has often been referred to as a geographically cosmopolitan species with broad tolerance of ecological parameters. Large water quality monitoring datasets were analyzed to understand the biogeographical patterns of the eight new taxa as well as their ecological niches. Results suggest that none of the taxa are cosmopolitan and none are broadly tolerant of all ecological conditions, but there is variation in both range size and ecological tolerance among the eight taxa. Traditionally, diatom species have been described based largely on morphological variation of their silica cell walls, but the results of the many aspects of this dissertation provide evidence for a more robust, unified species concept for diatoms that relies on many different types of data in addition to morphology, including geographical distributions, ecological preferences, and phylogenetics