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The Australian gummy shark Mustelus antarcticus is distributed across southern and eastern Australian waters and is the main target of a large shark fishery. Commercial harvest of the species is considered sustainable based on biomass estimates that show recovery from past overexploitation. However, the effective population size and genetic-based assessments of stock structure remain unresolved. We evaluated the genetic structure and effective population size (Ne) of the gummy shark using genome-wide single nucleotide polymorphisms. We found genetic divergence between individuals from the east coast and those along the south coast, resulting in 2 discrete populations with signatures of adaptive divergence. Spatial analyses revealed widespread gene flow within each of these populations, with some evidence for mild isolation-by-distance observed using individual-based tests. Demographic modelling of each population showed a comparatively rapid decline of Ne in the most recent past compared to more historical projections, with evidence that such declines have occurred to the point where genetic variation could be at risk. The identification of 2 divergent populations of gummy shark suggests that management should consider each individually to ensure the long-term sustainability of the species.

The European shore crab Carcinus maenas is a highly successful marine invader, and has displayed rapid range expansion following its introduction to many parts of the world. In Australia, it was first reported in Port Phillip Bay, Victoria in the late 1800s. Despite predictions that it would expand its range northward, its distribution has remained limited to the southeast coast of the mainland and to Tasmania. Using microsatellite loci and mitochondrial DNA, we assessed whether low connectivity among southeastern Australian estuaries might be contribut- ing to the limited distribution of this invasive species. Under this hypothesis, we expected that sampling of C. maenas from 6 estuaries, roughly evenly spaced along the southeast coast, would reveal: (1) greater genetic variability among than within estuaries; (2) increasing genetic dissimi- larity with distance from Port Phillip Bay; and (3) in the absence of human-mediated dispersal, declining genetic variation with distance from Port Phillip Bay. Contrary to these predictions, we found that genetic variability was no greater among than within mainland southeast Australian estuaries — indicating significant gene flow. Some slight genetic differentiation was, however, evident between Tasmania and the mainland. Multiple introductions appear to have contributed to the Australian population. The high connectivity of populations among southeast Australian estuaries suggest that management strategies focused on eradication of C. maenas from individ- ual localities will not be effective. Factors other than limited connectivity appear to be responsible for the slow range expansion up the east Australian coast.