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

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Bax, N (2014) Deep-sea stylasterid corals in the Antarctic, sub-Antarctic and Patagonian Benthos : biogeography, phylogenetics, connectivity and conservation. PhD thesis, University of Tasmania.

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Abstract

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 – 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 –
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 (π =0.012 – 0.11), 16S (π =0 – 0.05), ITS-1 (π 0 - 0.002) and ITS-2 (π 0.02
– 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.

Item Type: Thesis (PhD)
Keywords: VME, Stylasteridae, Antarctica, Patagonia, Biogeography, Phylogenetics, Connectivity, Conservation
Copyright Holders: The Author
Copyright Information:

Copyright 2014 the author

Date Deposited: 02 Jun 2016 07:33
Last Modified: 02 Jun 2016 07:33
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