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 worm Parborlasia
corrugatus. To empirically test for the presence of an open-ocean dispersal barrier, we
sampled P. corrugatus and other nemerteans from southern South America, Antarctica, and
the sub-Antarctic islands. Diversity was assessed by analyzing mitochondrial 16S rRNA
and cytochrome c oxidase subunit I sequence data with Bayesian inference and TCS haplotype
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 American P. corrugatus.
Thus, even though P. corrugatus can 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-Antarctic P. corrugatus suggest that these two populations are possibly two cryptic species.