Phylogeography and population structure of Antarctic ophiuroids: effects of life history, oceanography and paleoclimatology

Abstract

The Antarctic landmass and surrounding continental shelf have been isolated for over 24 million years. Geographic and thermal isolation have resulted in a highly endemic, diverse benthic marine fauna. This fauna has been relatively well characterized morphologically, but little is known about the evolutionary history, genetic diversity and population genetic structure of Antarctic benthic marine invertebrates. Several important questions remain largely unanswered, including 1) are circumpolar species genetically homogeneous throughout their range, 2) are non-endemic species maintaining connectivity between populations distributed in Antarctica and South America, and 3) how does early life history influence dispersal throughout Antarctica? This research examined phylogeographic patterns within four Antarctic brittle stars (ophiuroids). Ophiuroid species were chosen that differed in their mode of development (i.e., presence/absence of a pelagic larval stage) and geographic distribution. A non-endemic brooding species, Astrotoma agassizii, was evaluated in order to assess connectivity across a major oceanographic barrier separating Antarctic and South American populations. Three Antarctic endemics, Ophiurolepis gelida, O. brevirima and Ophionotus victoriae, all possessing some form of pelagic larvae, were obtained from various locations around Antarctica in order to characterize their population structure and levels of gene flow throughout the Antarctic continental shelf. Analysis of intraspecific mitochondrial sequence data revealed several interesting patterns. First, all species showed evidence of restricted connectivity between major geographic regions in the Antarctic, subantarctic and/or South America (depending on sampling), regardless of developmental mode. Additionally, cryptic divergence and/or speciation were recovered in all cases. Second, the brooding species, A. agassizii, displayed evidence of greater connectivity within geographic regions compared to species with pelagic larvae. These results suggest that phylogeographic patterns are not easily predicted for Antarctic brittle stars, and ophiuroid diversity is underestimated in the Southern Ocean. These generalizations likely apply to other Antarctic marine invertebrates, and suggest that much more research will be required to quantify Southern Ocean biodiversity and fully understand the contemporary and historical processes driving evolution in this region.