Graduation Year


Document Type




Degree Granting Department


Major Professor

Earl McCoy, Ph.D.

Co-Major Professor

Stephen Karl, Ph.D.


Phylogeny, Population genetics, Mitochondrial DNA, Nuclear DNA, Cladogenesis


Members of the family Placobranchidae have been the focus of numerous studies because of their interesting physiological adaptations (kleptochemistry and kleptoplasty) and ecological associations with marine algae. More than 100 species have been described since the 1800's, but no determination of phylogenetic relationships in the family has occurred. DNA sequences from two mitochondrial genes (large subunit ribosomal and cytochrome c oxidase subunit I) and one nuclear gene (Histone 3) were used to reconstruct relationships among 34 nominal species within the Placobranchidae. Strong support for terminal nodes was found using Bayesian phylogenetic methods; however, a "soft" polytomy was detected at internal nodes. In general, the results confirm the paraphyly of Elysia. Proposals for the designation of the genera, Thallepus Swainson, 1840 and Tridachia Deshayes, 1857 and delineation of species are presented.

How intrinsic factors affect speciation rates in groups of animals has been a central focus in evolutionary biology. Adaptive radiation associated with food choice has been postulated for the opisthobranch order Sacoglossa. An increase in the rate of cladogenesis was detected within the family Placobranchidae and intrinsic factors such as larval development, kleptoplasty and food choice were investigated as potential key innovations. Ancestral state reconstruction of characters suggests that food choice played a dominant role in influencing speciation rate within the family Placobranchidae. Life history strategy may play an important role in structuring genetic variation throughout the geographic range of a species. Members of the sacoglossan family Placobranchidae Gray, 1840, exhibit variation in life history strategies such as feeding preference and degree of kleptoplasty that could be influential in population structuring.

Three species, Elysia subornata Verrill, 1901, Elysia tuca Marcus and Marcus, 1967, and Elysia crispata Mörch, 1863 were sampled from several locations in an attempt to quantify genetic variation as indicated by cytochrome c oxidase subunit I gene sequencing. Elysia crispata exhibited the highest degree of population subdivision followed by Elysia tuca and Elysia subornata. Nucleotide and haplotype diversity also revealed the same general ranking of species. Preliminary conclusions suggest that diversity in feeding preference and degree of kleptoplasty play significant roles in population subdivision of these species.