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Deep-sea stylasterid corals in the Antarctic, sub-Antarctic and Patagonian Benthos : biogeography, phylogenetics, connectivity and conservation

posted on 2023-05-27, 20:42 authored by Bax, N
Large aggregations of sylasterid corals have been identified throughout the offshore waters of the Antarctic, Sub-Antarctic and South America. These biodiverse regions are interspersed by deep trenches, channels, sedimentary plains and isolated rocky habitat, which may facilitate or inhibit dispersal over evolutionary and ecological time scales. Deep-sea sampling has increased exponentially, across these benthic habitats, due to collaborative projects such as the Census of Antarctic Marine Life (CAML). Consequently, it is now possible to attempt to combine genetic and taxonomic expertise, explore evolutionary relationships and assess this data in relation to environmental change ‚Äö- both past and future. The biogeographic distribution of stylasterid corals is representative of population isolation, based on the discovery of dissimilar species aggregations throughout sampled regions. To further investigate this biogeographic pattern, I sampled all 33 of the known stylasterid species documented from the Antarctic, Sub-Antarctic, South West Atlantic and Patagonian fiord regions across depths (~10 m - > 2000 m), geographic spatial scales (~10 km ‚Äö- 10, 000 km), and habitat types (shelf, slope, seamount and fiords). Genetic relationships were investigated using DNA sequence data from multiple gene regions including: The mitochondrial ribosomal subunit (16S), cytochrome c oxidase subunit 1 (CO1), and the nuclear Internal Transcribed Spacer (ITS). This data was assigned to four research components to determine 1) the biogeographic distribution of Antarctic and Sub-Antarctic stylasterids (n = 33 species, 14 genera). 2) Phylogenetic relationships based on morphology and genetics (n = 12 species, 8 genera). 3) Phylogenetic relationships incorporating the fossil record, to assess the evolutionary history of stylasterid populations in the Drake Passage (n = 7 species, 6 genera), and lastly, 4) genetic and demographic connectivity between populations to inform conservation management regimes (n = 7 species, 4 genera). Morphological taxonomy combined with mitochondrial DNA sequence data produced a well aligned phylogenetic cladogram. The genetic variability seen in stylasterid 16S and CO1 sequences was comparatively higher than other coral and hydrozoan studies, offering potential for these gene regions in DNA barcoding. This has practical implications including the discovery of new species, cataloguing of Antarctic biodiversity and identification of specimens that are impossible to determine by taxonomic means. However, phylogenetic and taxonomic alignment was only achieved through the incorporation of systematic expertise in species identification, and inter-species relationships remain unresolved when compared to the nuclear ITS gene region. Therefore, the incorporation of more gene regions for study, and the use of molecular taxonomy as a complementary tool, rather than a replacement for traditional systematics is recommended for future studies. When the mitochondrial phylogeny was calibrated with the fossil record, phylogenetic topology represented an evolutionary scenario in which stylasterid ancestors' speciated in the Drake Passage during the Eocene/Oligocene transition boundary from calcite to aragonite sea conditions (~ 34 MYA). The phylogeny also suggests that skeletal bi-mineralogy may have played a central role in the speciation process. The presence of calcite in some genera and literature on the utility of either calcite or aragonite through oceanic time suggest a successional progression toward aragonite mineralogy in response to modern oceanic conditions (Oligocene => modern). Further research in this area may lead to the identification of acclimation states in stylasterid corals, and information on their ability to buffer impending ocean acidification, as the chemical state of the Southern Ocean shifts towards calcite sea conditions in the near future. When investigating genetic population connectivity in the Sub-Antarctic, and across the Polar Front into South America, estimates demonstrate limited to no gene-flow across spatial scales of 300 - > 1000 km. Large scale comparisons were clearly subdivided, and genetic subdivision was evident both among populations either side of, and north of the Polar Front based on CO1 data. However, disparate gene-flow estimates derieved from 16S signify that populations were connected through evolutionary linkages, and connectivity south of the Polar Front may be amplified by the presence of the Antarctic Circumpolar Current (ACC). For fine scale comparision, local estimates of connectivity (~ 200 km) between two Errina spp. fiord populations in Patagonia, Chile, showed no evidence of genetic subdivision (FST = 0, p = 0.6). Similarly, Errina spp in East Antarctica also showed no evidence of genetic subdivision (ITS-1 FST = 0.03 P = 0.165 and ITS-2 FST = 0.002, P = 0.27). However, despite a lack of genetic differentiation in ITS Errina population comparisons, haplotype networks typify a pattern of adaptive radiation from a common ancestor, and upon comparing nucleotide polymorphism in CO1 (˜ìvÑ =0.012 ‚Äö- 0.11), 16S (˜ìvÑ =0 ‚Äö- 0.05), ITS-1 (˜ìvÑ 0 - 0.002) and ITS-2 (˜ìvÑ 0.02 ‚Äö- 0.03) it was determined that relative variability in 16S and ITS represented historic connections, whilst CO1 being more variable, may also be more recent. Taken together, results suggest that a multitude of factors influence stylasterid coral populations, and temporal variation is particularly important in the context of this study. It is recommended that researchers focus on contemporary measures of connectivity, preserve specimens with genetic research in mind (> 90% ethanol preservation at the time of collection), and incorporate more loci to test connectivity across multiple spatial scales and species. The potential use of CO1 or 16S as barcoding genes will help in this process. However, until funding towards more deep-sea Antarctic sampling and molecular information emerges, the data presented in this thesis has ascribed a measure of localised geographic segregation, historic isolation and a limited capacity to recover following benthic disturbance. Substantiating that stylasterid corals congregate in diminutive and isolated populations. Therefore, to preempt anthropogenic damage to coral ecosystems, patterns of geographic isolation need to be incorporated into the design of Antarctic Marine Protected Areas (MPAs) - to preserve essential habitat, buffer climate change, mitigate the effects of ocean acidification, and combat localised impacts such as destructive fisheries which pose a direct threat to coral populations, and their associated taxa.


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