Submission note: Submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy [to the] Department of Ecology, Environment and Evolution, School of Life Sciences, La Trobe University, Victoria.
Benthic octopuses (family Octopodidae) are a highly valuable global fisheries resource (worth >$US1/2 billion per annum). However, taxonomic relationships within the Octopodidae remain unresolved, which impedes their appropriate management. The type species of the genus Octopus, Octopus vulgaris, has historically been considered cosmopolitan. Recent studies, however, suggest O. vulgaris may represent a complex of morphologically similar, yet genetically distinct, species (the “Octopus vulgaris complex”). My thesis aims to investigate the species-level relationships within the O. vulgaris complex. In chapter two, I found congruence between morphological and mitochondrial DNA-based evidence, suggesting that allopatric populations of O. tetricus from the east and west coasts of Australia are distinct species. In addition, Asian O. vulgaris formed a monophyletic clade with both Australian species, which was distinct from other O. vulgaris populations. In chapters three and four, I investigate the species-level diversity within the O. vulgaris complex using unprecedented levels of morphological and genomic data, respectively, resulting in the most comprehensive global-scale investigation of this group to date. Discrete differences in morphology successfully delimited three species within the O. vulgaris complex; (1) Mediterranean/NE Atlantic and South Africa (2) southern Brazil and (3) Asia. Genome-wide evidence obtained via next generation sequencing provided greater species-level resolution in comparison to those using mtDNA, supporting the distinction of South African O. vulgaris, and highlighting the limitations of mtDNA for resolving relationships within the group. In chapter five, I investigated the ancestral biogeography of the O. vulgaris group and found the ancestor was a widespread species. Furthermore, speciation of extant taxa was most likely driven by the sequential isolation of the eastern-most populations over the past 8 Ma. These results have significant taxonomic implications for the O. vulgaris group. As the majority of octopus fisheries worldwide are in decline, the findings of this thesis may inform the implementation of appropriate management strategies.
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