Genetic structure, diversity, and population ecology of Antarctic benthic amphipods

With increasing anthropogenic threats to the marine environment, it has become a priority to improve our understanding and conservation of marine fauna. In Antarctic waters, a rich and diverse benthic fauna thrives. However, the relative isolation of these organisms and their adaptation to the unique Antarctic environment potentially heightens their vulnerability to environmental change. Thorough research on the genetic and ecological structure of Antarctic benthic invertebrate populations is lacking, particularly for some of the most dominant taxa, such as the Amphipoda. This study investigated genetic structure, diversity and population ecology in some common Antarctic benthic amphipod species, to build a more rigorous understanding of the Antarctic benthos that will aid in future management planning. Genetic structure was explored over a circum-Antarctic scale in the widespread amphipod species Eusirus perdentatus and Eusirus giganteus from the continental shelf, using DNA sequences of two mitochondrial regions (COI and CytB) and one nuclear region (ITS2). Phylogenies and haplotype networks provided strong evidence that E. perdentatus harbours two previously undetected cryptic species, and E. giganteus harbours at least three, highlighting our current misunderstanding of Antarctic benthic diversity. There were clear differences in the distribution, genetic diversity and connectivity of populations within each cryptic species, and it is proposed that this reflects different modes of post-glacial recolonisation of the continental shelf. Within one cryptic species, high genetic population differentiation (FST > 0.47, p < 0.01) suggested a potential allopatric speciation process at play. Genetic connectivity was explored over large (1000km) to very fine (100m) distances in the ubiquitous nearshore amphipod Orchomenella franklini, using seven highly polymorphic microsatellite markers. Genetic diversity differed significantly among populations, potentially reflecting local environmental conditions including anthropogenic pollution. Hierarchical AMOVA revealed marked genetic subdivision (FST = 0.16, p < 0.001) across the largest geographical scale and evolutionary isolation of these populations was inferred. Furthermore, three loci showed signs of selection across this scale, providing evidence of locally adapted populations. Gene flow was also restricted at smaller scales, indicating a stepping-stone mode of dispersal consistent with the brooded development of amphipods. The ecology of O. franklini was investigated through length measurements, sex and reproductive status of > 6000 individuals, spatial and temporal patterns in abundance, and corresponding relationships with environmental data. The life history of O. franklini revealed several traits that exemplify adaptation to the polar environment, including delayed reproduction, longevity (> 2 years), and seasonal breeding linked to the summer phytoplankton bloom. There was preliminary evidence of inter-annual and spatial fluctuations in reproductive timing, potentially reflecting local sea-ice conditions. O. franklini was found to reach astounding densities (> 65,000/m2) and abundance was highly heterogeneous in space and time. The distribution of O. franklini was related to various sediment properties although the relationship differed with geographic region, highlighting a close association to the local environment as well as broader Antarctic conditions. This study has provided significant insight into the dynamics of Antarctic benthic amphipod populations over a range of scales. Together the results emphasise a considerable degree of heterogeneity largely overlooked in Antarctic benthic organisms (from the taxonomy of entire species down to local-scale intraspecific population dynamics), and thereby support predictions of their vulnerability to anthropogenicinduced change. Results also shed light on speciation processes in Antarctic waters, and will ultimately help inform future planning decisions regarding spatial management of the Antarctic benthic ecosystem.