Elucidating the evolution and diversity of Southern Ocean ophiuroids with reference to present day ecosystem management.

The evolution of life on our small world has a long and tumultuous history, beginning as self-replicating molecules and evolving into complex organisms able to consciously and deliberately alter their planet’s natural order. Over the epochs, periods of slow, evolutionary development have been punctuated by explosive diversification and catastrophic collapse. After 3.8 billion years the bewildering array of life, and its ability to persist through the most difficult of circumstances, is facing another period of lineage extinction out-pacing lineage divergence. However, this time, the difference is that the primary cause is a single species, humans. A question asked in this thesis is, “will we be able to capture the extent of diversity loss?”. Given the unknowns regarding the number of species on Earth, and the increasing evidence that many species are likely to harbour unappreciated genetic diversity, it is probable that many lineages and species will not be recorded before their demise. Nowhere is this more likely than in the least studied areas, one of which is also one of the biologically richest areas on Earth, the seafloor of the Southern Ocean. The studies presented here delve into the genetic diversity of Southern Ocean brittle stars (Class Ophiuroidea), based on two exemplar species, Amphiura belgicae and Ophioplinthus gelida, both common across the Southern Ocean shelf regions, that have been sampled over several expeditions across a wide spatial scale. Mitochondrial DNA sequences were used to identify patterns of divergence that differ from expectations of what is expected from a good “biological species” and are directly compared with two other similar published studies on the species Astrotoma agassizii and Ophiuroglypha lymani. Some interesting, and consistent, patterns of diversity emerge. Genetic divergences, possibly consistent with being unrecognised or cryptic species, are found in all four species, indicating that each has undergone a recent radiation. Many of the clades had discrete geographical distributions, often limited to the shelf regions of a single Southern Ocean island or archipelago. In each radiation, one or more clades identified a priori as a sister species, or, in the case of O. gelida, where Ophioplinthus martensi was included as a more distant outgroup, the sister species (and O. martensi) were found to be an element of the radiating clades. This indicates that the depth of divergence of a recognised morphospecies was equivalent to the depth of divergence of the other geographically explicit clades. I speculate early in the thesis that shared phylogeographic patterns indicate a shared evolutionary history and, therefore, that comparative phylogeography could provide a tool for large-scale conservation management, such as that required for the Southern Ocean. I further speculate that, if the shared phylogeographic patterns could be linked to a shared diversification event, then the patterns observed could potentially be extrapolated across the entire assemblage as each lineage present would track back to that same shared history. The thesis includes a test of this approach using a molecular clock method, with results strongly supporting a single diversification event. The key implications of these studies are that there is a large quantity of unappreciated diversity that may represent considerably higher species richness than recognised in the currently-available estimates. Furthermore, the current biogeographic paradigm of four Southern Ocean biogeographic regions – Antarctic, sub-Antarctic, shallow and deep – is a concerning simplification that has led to a generally unquestioned suggestion of large, well-connected populations in these regions with the implication of species resilience. The reality of much smaller populations being present in small, localised areas with little or no connectivity, implies vulnerability and the need for careful consideration from conservation managers and policy makers. Finally, the results of these studies question the current hypothesis explaining the unusually high diversity across the Southern Ocean benthic fauna. The biodiversity pump of the Milankovitch cycles implies that diversity increases with each glacial cycle, while the results presented here strongly suggest that the last diversification event predated the onset of Milankovitch cycles, and that there has likely been a net loss of lineages over the past one million year