Biogeographic barriers can set range limits for marine organisms by restricting migration, or because subsequent recruitment may be prevented by post-settlement selection. We used microsatellite and mtDNA data for adults and juveniles of the barnacle Catomerus polymerus to test the hypothesis that persistent differentiation of populations occurring to the northeast as compared with south and west of the southeast Australian biogeographic barrier (SEABB) is maintained by selection rather than dispersal limitation. We also explored the dispersal of C. polymerus along uninterrupted linear stretches of coastline (1440 km) to the east and west (120 km) of the SEABB. Within the regions flanking the SEABB, we found little genetic differentiation (F ST < 0.006), implying strong gene flow. In contrast, adults to the northeast were significantly differentiated from those to the south and southwest of the SEABB (F ST = 0.10), confirming some restriction of gene flow by the SEABB. Similarly, Bayesian analyses revealed eastern and western clusters of adults and juveniles, with the great majority assigning strongly to their region of collection. Nevertheless, 5 of 556 adults, and 10 of 537 juveniles, had genotypes that aligned more strongly with the genetic cluster of the opposite side of the SEABB, implying that migration does occur. Mitochondrial DNA sequence data (n = 71) revealed one additional eastern adult with an immigrant western haplotype. Our data imply that differentiation of eastern and western lineages reflects both restricted dispersal and possibly regional-scale selection acting on immigrant genotypes.

In southern New South Wales, a suite of widespread plant species exhibit short-statured ‘dwarf’ growth forms on coastal headlands. It is unclear whether such populations are genetically distinct or whether dwarfism is a plastic response to the environment. We used four microsatellite markers to assess genetic differentiation among populations from coastal and inland sites for Banksia spinulosa var. spinulosa. We sampled plants from six locations, including from three ‘dwarf’ and three ‘normal’ populations. Mean levels of genetic diversity were slightly higher in the forest (Na = 7.07 ± 0.25; He = 0.80 ± 0.09) than on the coast (Na = 5.92 ± 0.70; He = 0.72 ± 0.10). In general, populations displayed genotypic diversity expected for outcrossed sexual reproduction, with 161 of 172 individuals displaying unique genotypes and mean values of FIS close to zero. However, we found evidence of at least limited clonal replication in each of four populations and, within one coastal population, 11 of 27 individuals displayed one of three replicated genotypes, implying that the effective population size may be considerably smaller than would be inferred from the number of plants at this site. Relative to studies with other Proteaceae, this set of populations showed low, although significant, levels of differentiation (global FST = 0.061; P < 0.001), with extremely low, although significant, divergence of forest and coastal populations (FRT = 0.009; P < 0.001). There was no evidence of isolation by distance. These data imply that coastal dwarf populations are genetically similar to more extensive inland populations but in at least one case, may be at a greater risk of extinction because of low effective population size.

Phylogeographic and genetic studies have revealed complex variation in connectivity across the Southeast Australian Biogeographic Barrier (SEABB) for intertidal rocky‐shore invertebrates. The objective of this study was to use Connie2 to test whether differences between El Niño and La Niña years alter the probability of larvae crossing the SEABB. The SEABB occurs on the SE corner of Australia and is the convergence site of two major ocean currents (Zeehan and East Australian Currents) as well as an historical land bridge connecting Tasmania to the mainland. It includes extensive sandy shore lacking suitable substrate for rocky‐shore specialists. Study locations included coastal sites from New South Wales, Victoria, and Tasmania categorised into four regions (Barrier, Eastern, Western, and Southern). Connie2, an interactive hydrodynamic modelling tool available online, was used to evaluate connectivity via larval dispersal by comparing the effect of planktonic larval duration (PLD) from five to 120 days, season of spawning, and ENSO variability (comparing La Niña and El Niño years) on the cumulative probability of dispersal between and among populations (regions) arrayed either side of the SEABB. For all years, strong connections were detected among sites within regions, even for low to moderate (less than 20% predicted dispersal) PLDs and irrespective of season. Connectivity across SEABB (between regions) was also strongly influenced by all variables but occurred only at PLD of at least 30 days for summer spawning. Connectivity across the SEABB was strongest from west to east and south to east during La Niña events. Our findings support the importance of SEABB as a barrier to dispersal. Migration across SEABB is predicted only for summer spawners (PLD ≥30 days). Predicted dispersal is only weakly influenced by El Niño and La Niña extremes, but connectivity may be altered by projected changes to the relative strengths of the Zeehan and East Australian Currents.

The family Catophragmidae is one of the lower balanomorphs from traditional and recent multiple mitochondrial and nuclear markers molecular analysis. Here, we characterized the first mitogenome of the catophragmid barnacle Catomerus polymerus , which was 15,446 bp in length with a 68.3% AT content. The mitogenome had the typical pancrustacean gene arrangement, which was identical to the mitogenome configurations of the chthamalid Octomeris sp. and pachylasmatoid Eochionelasmus ohtai . On the mitogenomic tree, the catophragmid Catomerus polymerus formed an independent branch that was basal to the members of the superfamilies Tetraclitoidea and Balanoidea, which was inconsistent with previous findings.