Investigations of estuarine taxa can provide a perspective on phylogeography that complements studies of marine littoral organisms. For example, reductions in gene flow between populations and increased genetic structuring would be expected in estuarine species. The substantial amount of information about marine species and the habitat diversity along long latitudinal spans makes south-eastern Australia an excellent potential location for comparing marine and estuarine taxa. To investigate this potential, we studied the phylogeography of the two species in the estuarine gastropod genus Tatea. These have extensive and broadly overlapping distributions that encompass known marine phylogeographic boundaries. Against expectation, both Tatea species showed a remarkable lack of geographic and inter-specific variability in mitochondrial 12S rRNA (107 specimens) and cytochrome c oxidase subunit I (39) DNA sequences. No major phylogeographic discontinuities were revealed in either species and there was minimal haplotype divergence between them for either 12S rRNA or COI. The patterns of mitochondrial DNA variation discovered in Tatea may be due to a recent selective sweep or range expansion from a population in which there was little variability. Both possibilities are complicated by having to explain the similarity of the patterns in the two species.
Keywords: cytochrome c oxidase, Gastropoda, phylogeography, 12S rRNA, south-eastern Australia.