Phylogeography and biodiversity of ophiuroids in the Southern Ocean

Abstract

The Southern Ocean surrounding Antarctica is home to a highly endemic benthic fauna, often attributed to the strong oceanic barrier, the Antarctic Polar front. The Antarctic Polar Front and Antarctic Circumpolar Current formed over 24 million years ago when the northern tip of the Antarctic Peninsula separated from the southern tip of South America, thus forming the Drake Passage. Although the Antarctic Circumpolar Current is often attributed as a vector for organismal dispersal in and around the Southern Ocean, the temperature and salinity change of the Antarctic Polar Front has helped to isolate the Southern Ocean from the warmer waters of more northerly latitudes. These two defining forces of the Southern Ocean ecosystem provide important questions, 1) are circumpolar organisms homogenous through their range, 2) does the Antarctic Polar Front act as an absolute barrier to dispersal for non-endemic species? This research focused on one of the Southern Ocean’s benthic ecosystems most conspicuous members, the class Ophiuroidea. Ophiuroidea is the most speciose of all echinoderm classes and represents a rich biodiversity within the Southern Ocean. This biodiversity represents 219 species, 126 are endemic, and many are considered to have circumpolar distributions. Due to the ecological importance of this class, this dissertation has focused on the biodiversity and phylogeography of both endemic and non-endemic species of Southern Ocean ophiuroids. The work presented here has revealed distinct geographic structure of two circumpolar ophiuroid species, specifically Ophionotus victoriae and Astrotoma agassizii. Ophionotus victoriae was revealed to have four geographically distinct populations based on two mitochondrial markers and a whole genome single nucleotide polymorphism based approach, specifically 2b-RAD. The first population occurs in the Bellingshausen and Amundsen Sea region, the second population occurs in the Ross Sea and on the western side of the Antarctic Peninsula. The final two populations both occur on the eastern side of the Antarctic Peninsula in the Weddell Sea. This first chapter on Ophionotus victoriae follows the formatting of Ecology and Evolution. Additionally, in the non-endemic species A. agassizii, genetic connectivity across the Antarctic Polar Front was recovered and is the first benthic invertebrate to have shown this pattern. Although a possible migrant was revealed from the mitochondrial data, the inclusion of whole genome bi-allelic SNP markers, allowed for the recovery of five admixed individuals. Four of the admixed individuals were recovered in South America while one was recovered in the Southern Ocean, providing evidence for bi-directional migration. The work presented in Chapter two on Astrotoma agassizii follows the formatting of Biological Bulletin. Evolutionary relationships were analyzed within Ophiuroidea using the mitochondrial genome from 10 new ophiuroid species, 9 of which were from the Southern Ocean and 1 from South America. These 10 new mitochondrial genomes more than doubled the number of what was publically available, which allowed our comparison of phylogenetic relationships and comparative gene orders comprising 17 different ophiuroid mitochondrial genomes. The 17 mitochondrial genomes represent a wide taxonomic sampling of the currently accepted evolutionary relationships within Ophiuroidea. These relationships revealed three conserved gene orders for the 13 coding genes and 2 ribosomal genes within Ophiuroidea. Both the conserved gene orders and Maximum Likelihood phylogenetic analyses support Ophiuroidea being comprised of the two recently suggested superorders, specifically Euryophiurida and Ophintegrida. The third chapter follows the formatting of Molecular Phylogenetics and Evolution.