Evolution of the genus Sicydium (Gobiidae:sicydiinae)


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A dissertation submitted in partial fulfillment of the requirements for the degree of DOCTOR of PHILOSOPHY in MARINE BIOLOGY from Texas A&M University-Corpus Christi in Corpus Christi, Texas.
Gobies are one of the most speciose groups of fishes on the planet. Despite their prominence little is known about their evolution and diversity. Gobies of the genus Sicydium are abundant inhabitants of tropical streams in the Atlantic and eastern Pacific basins. The amphidromous life history of Sicydium presents challenges in understanding their diversity and the evolutionary relationships among species. With this life history, the adults live and breed in freshwater streams. Newly hatched larvae are carried out to the marine environment where they live as ichthyoplankton for a period of time. Upon completion of the marine larval phase juveniles will re-enter the freshwater environment. Additionally, poor species descriptions, parochial studies, and highly variable morphological characters have resulted in taxonomic uncertainty within the genus. This dissertation presents separate phylogenetic hypotheses of the relationships between species of Sicydium based upon morphology and DNA data. In the first chapter, I present a phylogenetic hypothesis of the relationships among the species of Sicydium based upon morphological characters. This chapter shows that changes in jaw morphology are important in the evolution of Sicydium. Three clades of Sicydium were recovered, however the relationship among these clades was unresolved. Different oral morphology characters were important in determining the relationships between species. This includes morphology of the premaxillary teeth, which has been used as a diagnostic character for species of Sicydium. This study presents the first phylogenetic hypothesis based on morphology for the species of Sicydium. The second chapter presents a molecular assessment of species diversity and a molecular phylogenetic hypothesis of Sicydium based upon two nuclear and two mtDNA genes. Previous molecular studies that have included Sicydium were at the population or subfamily level. Here I present the first molecular hypothesis of the evolutionary relationships among the species of Sicydium. The analysis recovered two clades of a monophyletic Sicydium, one less than morphological data, with most relationships among species well resolved. The species recovered in these two clades were similar to those recovered with the morphological data. However some species were not recovered as monophyletic in the molecular analysis, most likely due to incomplete lineage sorting. In the third chapter, I present a population level study of an eastern Pacific species, Sicydium salvini, Ogilvie-Grant, 1884. This chapter explores the diversity of a species recovered by the previous chapters. This study showed that S. salvini inhabits a wide geographic range including areas previously considered to be occupied by Sicydium multipunctatum. DNA samples from across the range of this species were used to test for population structure and population expansion. A lack of structure between river populations was found which is most easily explained by the amphidromous life history. The dispersion capabilities of the marine larvae allow free exchange of genes between distant populations. I also show that tectonic activity may explain a historic population expansion for Sicydium salvini. In this dissertation I present different views on the evolution of Sicydium. In the first two chapters, I present phylogenetic hypotheses of the species of Sicydium. The third chapter shows a population level view of Sicydium salvini. These data can be used as a stepping-stone for future work involving the evolution of sicydiine gobies as well as providing new characters for a much needed taxonomic revision of Sicydium. It also clarifies population level dynamics and historical environmental influences on those dynamics.
Life Sciences
College of Science and Engineering