Open‐ocean barriers to dispersal: a test case with the Antarctic Polar Front and the ribbon wormParborlasia corrugatus(Nemertea: Lineidae)

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Abstract
Open‐ocean environments provide few obvious barriers to the dispersal of marine organisms. Major currents and/or environmental gradients potentially impede gene flow. One system hypothesized to form an open‐ocean dispersal barrier is the Antarctic Polar Front, an area characterized by marked temperature change, deep water, and the high‐flow Antarctic Circumpolar current. Despite these potential isolating factors, several invertebrate species occur in both regions, including the broadcast‐spawning nemertean wormParborlasia corrugatus. To empirically test for the presence of an open‐ocean dispersal barrier, we sampledP. corrugatusand other nemerteans from southern South America, Antarctica, and the sub‐Antarctic islands. Diversity was assessed by analyzing mitochondrial 16S rRNA and cytochromecoxidase subunit I sequence data with Bayesian inference andtcshaplotype network analysis. Appropriate neutrality tests were also employed. Although our results indicate a single well‐mixed lineage in Antarctica and the sub‐Antarctic, no evidence for recent gene flow was detected between this population and South AmericanP. corrugatus. Thus, even thoughP. corrugatuscan disperse over large geographical distances, physical oceanographic barriers (i.e. Antarctic Polar Front and Antarctic Circumpolar Current) between continents have likely restricted dispersal over evolutionary time. Genetic distances and haplotype network analysis between South American and Antarctic/sub‐AntarcticP. corrugatussuggest that these two populations are possibly two cryptic species.

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