Incipient speciation of Catostylus mosaicus (Scyphozoa, Rhizostomeae, Catostylidae), comparative phylogeography and biogeography in south-east Australia
Phylogeography provides a framework to explain and integrate patterns of marine biodiversity at infra- and supra-specific levels. As originally expounded, the phylogeographic hypotheses are generalities that have limited discriminatory power; the goal of this study is to generate and test specific instances of the hypotheses, thereby better elucidating both local patterns of evolution and the conditions under which the generalities do or do not apply. Location
Coastal south-east Australia (New South Wales, Tasmania and Victoria), and south-west North America (California and Baja California). Methods
Phylogeographic hypotheses specific to coastal south-east Australia were generated a priori, principally from existing detailed distributional analyses of echinoderms and decapods. The hypotheses are tested using mitochondrial cytochrome c oxidase subunit I (COI) and nuclear internal transcribed spacer 1 (ITS1) DNA sequence data describing population variation in the jellyfish Catostylus mosaicus, integrated with comparable data from the literature. Results
Mitochondrial COI distinguished two reciprocally monophyletic clades of C. mosaicus (mean ± SD: 3.61 ± 0.40% pairwise sequence divergence) that were also differentiated by ITS1 haplotype frequency differences; the boundary between the clades was geographically proximate to a provincial zoogeographic boundary in the vicinity of Bass Strait. There was also limited evidence of another genetic inhomogeneity, of considerably smaller magnitude, in close proximity to a second hypothesized zoogeographic discontinuity near Sydney. Other coastal marine species also show genetic divergences in the vicinity of Bass Strait, although they are not closely concordant with each other or with reported biogeographic discontinuities in the region, being up to several hundreds of kilometres apart. None of the species studied to date show a strong phylogeographic discontinuity across the biogeographic transition zone near Sydney. Main conclusions
Patterns of evolution in the Bass Strait and coastal New South Wales regions differ fundamentally because of long-term differences in extrinsic factors. Since the late Pliocene, periods of cold climate and low sea-level segregated warm temperate organisms east or west of an emergent Bassian Isthmus resulting in population divergence and speciation; during subsequent periods of warmer and higher seas, sister taxa expanded into the Bass Strait region leading to weakly correlated phylogeographic and biogeographic patterns. The Sydney region, by contrast, has been more consistently favourable to shifts in species’ ranges and long-distance movement, resulting in a lack of intra-specific and species-level diversification. Comparisons between the Sydney and Bass Strait regions and prior studies in North America suggest that vicariance plays a key role in generating coastal biodiversity and that dispersal explains many of the deviations from the phylogeographic hypotheses.