Abstract
Pairs of closely related species on either side of a barrier provide an opportunity to test evolutionary hypotheses. In southern Australian seas, many species pairs occur on either side of the Bass Strait, which temporarily becomes an isthmus during glacial stages. We used mitochondrial and nuclear approaches to examine recent demographic changes in one such species pair, Arripis trutta (east of Bass Strait) and A. truttaceus (west of Bass Strait). Data from the two remaining members of the family Arripidae, A. xylabion and A. georgianus, were included to provide context for interpreting patterns in the focal species pair. All Arripis species were characterised by low genetic diversity and there was evidence for a more recent and/or severe population contraction in A. truttaceus compared to A. trutta. These different demographic signals support the a priori hypothesis that environmental conditions to the west of the Bass Strait were more severe than to the east during glacial stages and, in particular, the last glacial maximum. The results are considered in the context of the biology of the species and the historical marine biogeography of temperate Australia.