The interconnected nature of the oceans, the small size and passive nature of larvae has resulted in the presumption that marine populations are large and open. However, this paradigm has been challenged in the last few of decades, where genetic studies have found significant structure at very fine-scales (of the order of 10 km). Given that many sedentary marine organisms can only disperse via a larval phase, it is intuitive that the duration of this phase (pelagic larval duration, PLD) should correspond with a species dispersal capacity and hence predict population genetic structure. Furthermore, if PLD predicts dispersal capacity then PLD should correlate with a species distribution. In this thesis, the population genetic structure of three sedentary marine organisms, endemic to the south-east Australia Pacific region, was examined to explore the relationships between dispersal capacity, life-history and geographical distribution. In chapter 2 the influence of an oceanic frontal system, the Subtropical Front (STF), on the genetic structure (mitochondrial region cytochrome c oxidase subunit I), trophic status (stable isotopes I'15N and I'13C) and condition (RNA:DNA, protein:DNA, DNA:dry weight) of the benthic gastropod Fusitriton magellanicus was examined across the Chatham Rise, New Zealand. The front acted as barrier to the transport of material between flanks, such that populations of F. magellanicus from the northern flank were in poorer condition than those from the southern flank. In addition, I'15N signatures were different between flanks, the northern flank being more enriched than the southern flank, indicating that F. magellanicus on the north flank where either starving or occupying a higher trophic level. Despite the STF shaping the trophic ecology of F. magellanicus, no differences in genetic structure were found. This indicates that there is larval exchange between or around the STF and hence larval biology may override environmental factors. To explore if a limited dispersal capacity, in contrast to its PLD (~30 days), was responsible for driving the unusual and abbreviated distribution of the sea urchin Heliocidaris tuberculata, its population genetic structure was characterised in chapter 3. Examination of the genetic structure using three different genetic markers (COI, 16S and 12 microsatellites) across the geographical distribution of the species (New South Wales coast and across the Tasman Sea to Lord Howe, Norfolk and the Kermadec Islands) was made, revealing a complete absence of structure (microsatellites FST = 0.003, COI PhiPT = 0.0021, and 16S PhiPT = 0.029). To elucidate the role of other important environmental factors in shaping genetic patterns, correlations between genetic measures and key environmental variables were explored, but no significant relationships were found. Therefore, H. tuberculata is panmictic across its range and its unusual restricted latitudinal distribution, despite it wide longitudinal distribution, is not explained by a limited dispersal capacity. The population genetics of H. erythrogramma was examined across south-eastern Australia, to explore the influence of historical versus contemporary forces on the structure of a species with a short PLD (3-5 days), in chapter 4. Using two different markers (cytochrome c oxidase subunit 1 (COI) and microsatellites), the population genetic structure of H. erythrogramma was examined from New South Wales to South Australia inclusive of Victoria and Tasmania. Analysis of the COI data revealed two strong eastern and southern genetic groups, diverging across a known phylogeographical barrier, the Bassian Isthmus. Within the two genetic groups, the signature of contemporary processes was evident with the direction of migration consistent with the influence of the ocean currents on larval dispersal. A combination of contemporary oceanography, and habitat restrictions appear to maintain these signatures in the present day. Whilst PLD places some restrictions on dispersal capacity, it does not prevent an extensive geographical range and/or high levels of gene flow at extensive spatial scales. In chapter 5, it was postulated that temperature may restrict the southern range limit of H. tuberculata. Examination of the thermal tolerance of the two species revealed that the upper temperature tolerance for both species was the same, however, H. erythrogramma had a lower cold thermal tolerance than H. tuberculata. Furthermore, the temperature tolerances of both species matched sea surface temperatures that their larvae would normally encounter. Hence, an inability to tolerate cold temperatures explains why H. tuberculata has a restricted southern range limit, whereas the wider thermal envelope of H. erythrogramma ensures its distribution around temperate Australia. In summary, the thesis reveals that whilst life history considerations can shape the genetic structure of a species, oceanography and environmental considerations (e.g. temperature) can play a more important role in determining how populations are connected and species range limits.
| Date of Award | 2016 |
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| Original language | English |
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