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

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 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.

Nuclear and Mitochondrial Gene Data Support Recent Radiation within the Sea Spider Species Complex Pallenopsis patagonica

“…Also, the radiation of the many Antarctic P. patagonica species very likely started in the late Pliocene and increased during the Pleistocene (last 2.5 myr). Such patterns have been reported before (e.g., Held, 2000;Page and Linse, 2002;Thornhill et al, 2008;Krabbe et al, 2010;Leese et al, 2010;Hemery et al, 2012;Dietz et al, 2015a,b) suggesting that over evolutionary time scales the Polar Front has not been an impermeable barrier to gene flow in general, though we did not observe across Drake Passage exchange in more recent clades of P. patagonica s.l. However, in view of the above-mentioned limitations of molecular clock calculations, we advise to use the divergence estimates made here with caution.…”

“…Such a pattern has also been observed for many other benthic invertebrates (e.g., Page and Linse, 2002;Thornhill et al, 2008;Krabbe et al, 2010). The fact that individuals from South Georgia, i.e., a Subantarctic island south of the Polar Front, form one clade with individuals reported from the northernmost tip of the Antarctic Peninsula (ANT_D) and cluster within the Antarctic super-clade of P. patagonica s.l.…”

“…clade that crossed the deep sea between the continental shelf and the Subantarctic islands, but stayed within the Polar Front. Gene flow between the Antarctic continental shelf and South Georgia has already been reported in a few studies on other benthic invertebrates (Thornhill et al, 2008;Wilson et al, 2009;Dietz et al, 2015a,b).…”

A new member of the genus Antarctonemertes (Hoplonemertea, Nemertea) from Antarctic waters

“…A few well-known nemertean species commonly occur in the shallow Antarctic waters. One of the most conspicuous examples is Parborlasia corrugatus (McIntosh, 1876), a frequent and relatively large heteronemertean with a circumpolar distribution, also reported from sub-Antarctic Islands (Thornhill et al 2008). Other examples include the congeneric Antarctonemertes valida (Bürger, 1893) and Antarctonemertes riesgoae Taboada et al, 2013, two relatively small brooding hoplonemerteans whose females lay eggs inside cocoons, which commonly occur in the intertidal and shallow subtidal Southern Ocean (Taboada et al 2013).…”

“…Other examples include the congeneric Antarctonemertes valida (Bürger, 1893) and Antarctonemertes riesgoae Taboada et al, 2013, two relatively small brooding hoplonemerteans whose females lay eggs inside cocoons, which commonly occur in the intertidal and shallow subtidal Southern Ocean (Taboada et al 2013). The two Antarctonemertes and P. corrugatus should be considered as exceptional cases in the Southern Ocean in the sense that they have been extensively characterised either under morphological (including both external and internal features) or genetic approaches (Gibson 1983;Thollesson and Norenburg 2003;Thornhill et al 2008;Taboada et al 2013). …”