Historic divergence with contemporary connectivity in a catadromous fish, the estuary perch (Macquaria colonorum)

The estuary perch (Macquaria colonorum) represents an important model for assessing how historical changes in coastal geomorphology and current oceanographic and estuarine conditions may have impacted connectivity in a cata- dromous fish. A fragment of the mitochondrial control region and six microsatellite DNA markers were used to clarify connectivity in 17 populations (n = 354) of estuary perch from the southeast and southern coasts of Australia. The mtDNA data showed a latitudinal disjunction in haplotype frequencies that divided populations into two groups (FST = 0.419), in a pattern suggestive of isolation by geographic distance. However, no marked structure or correlation with distance was ap- parent within each group, a result consistent with microsatellite data that showed high contemporary population connectiv- ity across large distances. This was contrary to expectations that the species would exhibit moderate to strong genetic structure consistent with a one-dimensional stepping stone pattern. Coalescent phylogeographic and population genetic analyses provided support for a historical divergence probably due to the emergence of the Bassian Isthmus in southern Australia. Current connectivity appears to be maintained by both large- and fine-scale oceanographic currents and proc- esses, highlighting the important role of the marine environment for an estuarine resident species.