title: The ecology and evolution of Antarctic Pycnogonida
Creators: name: Maxwell, Jamie
Publisher: name: University of Galway
Year: 2024
Subjects: subject: Antarctic Pycnogonida
subject: Natural Sciences
subject: Zoology
subject: FOS: Biological sciences
subject: Biogeography
subject: sea spiders
subject: mitogenomes
subject: cryptic species
subject: genetics

description: Pycnogonida (sea spiders) are a class of globally distributed arthropods, that are particularly biodiverse within the Southern Ocean. The number of pycnogonid species and increased levels of endemism in the Southern Ocean form the basis of the hypothesis that Antarctica is the evolutionary centre of origin for extant pycnogonids. Despite a long history of investigation into Antarctic pycnogonids, little is known about their phylogeny, ecology, and biogeography. For example, until recently their phylogeny was based exclusively on morphology and low-resolution single gene barcodes, which was unable to resolve interfamilial relationshipsSimilarly, inferences about their ecology have been based on assumptions, interpretation of morphological characters such as their feeding apparatus, low resolution distribution patterns, and rare in-situ observations. Chapter 2 uses taxonomic keys and literature to identify 5704 sea spiders to the lowest taxonomic rank possible. I identified 81 species, across 15 genera and eight families. All records were uploaded to Global Biodiversity Information Facility (GBIF) which increased the number of stations where pycnogonids have been recorded in the Southern Ocean, southern South America, and New Zealand waters by 11% and I have increased, by 18%, the number of stations above 60 degrees latitude. Any observed epifauna were also documented. Chapter 3 used the newly generated data from chapter 2 combined with publicly available occurrence records to investigate the diversity and connectivity of pycnogonid communities either side of the Antarctic Polar Front, with an emphasis on the role of water depth. I found that at depths shallower than 1000 m, communities north and south of the front were distinct, whereas below this depth geographic structure disintegrated, providing evidence that, for pycnogonids, Antarctica becomes less isolated from other Oceans with depth. Chapter 4 used a subset of the dataset in chapter 3 to investigate whether water mass properties had a role in structuring pycnogonid communities. Multivariant analyses were run on the species data set that had been categorised according to environmental parameters. Six distinct species assemblages were identified, each inhabiting a particular potential temperature-salinity which is inferred as a distinct water masses. Colder waters were more species-rich and had a higher proportion of unique species than warmer waters. This is the first evidence that water masses have a structuring effect on a mobile benthic taxon. Chapter 5 investigates the genetic structure within the species Pentanymphon antarcticum using Single Nuclecotide Polymorphisms (SNP). Discriminant Analysis of Principle Components, F-statistics and an AMOVA suggested the presence of multiple cryptic species. Single-locus species delimitation methods were congruent with the analyses of the genome-wide SNP data providing strong support for the presence of between two and twelve cryptic species within this hitherto monotypic genus. Chapter 6 tests the hypothesis that the Southern Ocean is the evolutionary centre of origin for extant pycnogonids. Phylogenetic trees were created from a global mitochondrial dataset containing 197 specimens (including 18 newly sequenced Antarctic species). The trees inferred that only the family Pycnogonidae originated in the Antarctic, and the region is a centre of radiation for Nymphonidae and Colossendeidae. The presence of novel gene rearrangements and conserved gene overlaps were also investigated, with the loss of overlaps in the superfamily Nymphonoidea suggested to have facilitated unique gene rearrangements in this group. In the final chapter (Chapter 7), I place my findings in a broader context and consider future avenues for research.