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

摘要

Large aggregations of sylasterid corals have been identified throughout the offshoreudwaters of the Antarctic, Sub-Antarctic and South America. These biodiverse regions are interspersedudby deep trenches, channels, sedimentary plains and isolated rocky habitat, whichudmay facilitate or inhibit dispersal over evolutionary and ecological time scales. Deep-seaudsampling has increased exponentially, across these benthic habitats, due to collaborative projectsudsuch as the Census of Antarctic Marine Life (CAML). Consequently, it is now possibleudto attempt to combine genetic and taxonomic expertise, explore evolutionary relationshipsudand assess this data in relation to environmental change – both past and future.udThe biogeographic distribution of stylasterid corals is representative of populationudisolation, based on the discovery of dissimilar species aggregations throughout sampled regions.udTo further investigate this biogeographic pattern, I sampled all 33 of the known stylasteridudspecies documented from the Antarctic, Sub-Antarctic, South West Atlantic and Patagonianudfiord regions across depths (~10 m - > 2000 m), geographic spatial scales (~10 km –ud10, 000 km), and habitat types (shelf, slope, seamount and fiords). Genetic relationshipsudwere investigated using DNA sequence data from multiple gene regions including: The mitochondrialudribosomal subunit (16S), cytochrome c oxidase subunit 1 (CO1), and the nuclearudInternal Transcribed Spacer (ITS). This data was assigned to four research components touddetermine 1) the biogeographic distribution of Antarctic and Sub-Antarctic stylasterids (n =ud33 species, 14 genera). 2) Phylogenetic relationships based on morphology and genetics (n =ud12 species, 8 genera). 3) Phylogenetic relationships incorporating the fossil record, to assessudthe evolutionary history of stylasterid populations in the Drake Passage (n = 7 species, 6udgenera), and lastly, 4) genetic and demographic connectivity between populations to informudconservation management regimes (n = 7 species, 4 genera).udMorphological taxonomy combined with mitochondrial DNA sequence data produceduda well aligned phylogenetic cladogram. The genetic variability seen in stylasterid 16S andudCO1 sequences was comparatively higher than other coral and hydrozoan studies, offeringudpotential for these gene regions in DNA barcoding. This has practical implications includingudthe discovery of new species, cataloguing of Antarctic biodiversity and identification of specimensudthat are impossible to determine by taxonomic means. However, phylogenetic and taxonomicudalignment was only achieved through the incorporation of systematic expertise inudspecies identification, and inter-species relationships remain unresolved when compared to the nuclear ITS gene region. Therefore, the incorporation of more gene regions for study, andudthe use of molecular taxonomy as a complementary tool, rather than a replacement for traditionaludsystematics is recommended for future studies.udWhen the mitochondrial phylogeny was calibrated with the fossil record, phylogeneticudtopology represented an evolutionary scenario in which stylasterid ancestors’ speciated in theudDrake Passage during the Eocene/Oligocene transition boundary from calcite to aragonite seaudconditions (~ 34 MYA). The phylogeny also suggests that skeletal bi-mineralogy may haveudplayed a central role in the speciation process. The presence of calcite in some genera andudliterature on the utility of either calcite or aragonite through oceanic time suggest a successionaludprogression toward aragonite mineralogy in response to modern oceanic conditionsud(Oligocene => modern). Further research in this area may lead to the identification of acclimationudstates in stylasterid corals, and information on their ability to buffer impending oceanudacidification, as the chemical state of the Southern Ocean shifts towards calcite sea conditionsudin the near future.udWhen investigating genetic population connectivity in the Sub-Antarctic, and across theudPolar Front into South America, estimates demonstrate limited to no gene-flow across spatialudscales of 300 - > 1000 km. Large scale comparisons were clearly subdivided, and geneticudsubdivision was evident both among populations either side of, and north of the Polar Frontudbased on CO1 data. However, disparate gene-flow estimates derieved from 16S signify thatudpopulations were connected through evolutionary linkages, and connectivity south of the PolarudFront may be amplified by the presence of the Antarctic Circumpolar Current (ACC). Forudfine scale comparision, local estimates of connectivity (~ 200 km) between two Errina spp.udfiord populations in Patagonia, Chile, showed no evidence of genetic subdivision (FST = 0, pud= 0.6). Similarly, Errina spp in East Antarctica also showed no evidence of genetic subdivisionud(ITS-1 FST = 0.03 P = 0.165 and ITS-2 FST = 0.002, P = 0.27). However, despite a lack ofudgenetic differentiation in ITS Errina population comparisons, haplotype networks typify audpattern of adaptive radiation from a common ancestor, and upon comparing nucleotide polymorphismudin CO1 (π =0.012 – 0.11), 16S (π =0 – 0.05), ITS-1 (π 0 - 0.002) and ITS-2 (π 0.02ud– 0.03) it was determined that relative variability in 16S and ITS represented historic connections,udwhilst CO1 being more variable, may also be more recent.udTaken together, results suggest that a multitude of factors influence stylasterid coraludpopulations, and temporal variation is particularly important in the context of this study. It isudrecommended that researchers focus on contemporary measures of connectivity, preserveudspecimens 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.udThe potential use of CO1 or 16S as barcoding genes will help in this process. However, untiludfunding towards more deep-sea Antarctic sampling and molecular information emerges, theuddata presented in this thesis has ascribed a measure of localised geographic segregation, historicudisolation and a limited capacity to recover following benthic disturbance. Substantiatingudthat stylasterid corals congregate in diminutive and isolated populations. Therefore, to preemptudanthropogenic damage to coral ecosystems, patterns of geographic isolation need to beudincorporated into the design of Antarctic Marine Protected Areas (MPAs) - to preserve essentialudhabitat, buffer climate change, mitigate the effects of ocean acidification, and combat localisedudimpacts such as destructive fisheries which pose a direct threat to coral populations,udand their associated taxa.