Lepas australis, Darwin, 1851

Schiffer, Philipp H. & Herbig, Hans-Georg, 2016, Endorsing Darwin: global biogeography of the epipelagic goose barnacles Lepas spp. (Cirripedia, Lepadomorpha) proves cryptic speciation, Zoological Journal of the Linnean Society 177 (3), pp. 507-525 : 516

LEPAS AUSTRALIS

It is most surprising to find a geographic pattern in L. australis that is even supported by the slowly evolving nuclear locus. The species in general is restricted to cold-water masses linked with Antarctica, an area where longitudinal landmasses that could impede gene flow, and thus lead to the evolution of geographically confined populations, are missing. In contrast, the westerly wind-driven Antarctic Circumpolar Current (ACC) effectively mediates the transport of organisms around Antarctica. Within the ACC the Antarctic Polar Front (APF, or Antarctic Convergence; U.S. Geological Survey 2010 /2012), which coincides approximately with the edge of the winter sea ice, and a sudden change in seawater surface temperature, acts as an effective latitudinal barrier between water masses, and also between marine organisms ( Thornhill et al., 2008). Further north, the Subantarctic Front (or Subtropical Convergence; U.S. Geological Survey, 2010 /2012), i.e. the northern boundary of the ACC, is a second major latitudinal water-mass barrier. The effectiveness of these fronts as barriers to gene flow has already been demonstrated, e.g. in the chaetognath Eukrohnia hamata (Mobius, 1875) ( Kulagin et al., 2014) .

All our sampling sites are situated north of the ACC. There, we recognize a common population of L. australis in the cool to temperate waters north of the Subantarctic Front, herein termed the ‘Southern Ocean subgroup’. This population of L. australis is obviously caught in the current system adjoining north of the Subtropical Convergence, albeit also circling eastwards around Antarctica, and in the northeast deviating current systems, which sweep up the coasts of Western Australia and New Zealand (South Indian Current s.l.), respectively, Argentina (Falkland Current), and South Africa (Benguela Current) ( Fig. 6).

The ‘coastal Chilean subgroup’ differs from all other sampling sites. It matches biogeographic results of a separate eastern Pacific/Chilean province from other pelagic organisms. Examples are the copepod Rhincalanus nasutus ( Goetze, 2003) , which has a sister group along the North American coast, the rafting bryozoan Membranipora ( Schwaninger, 2008) , the giant kelp Macrocystis ( Coyer, Smith & Andersen, 2001) , or, most evident, the bull kelp Durvillaea antarctica (Chamisso) Hariot 1892 ( Fraser et al., 2009). The latter shows distinct haplotypes along the central and northern Chilean coast. In analogy to results from New Zealand, they are most probably related to the continuous warming of seawater ( Fraser et al., 2009; Fraser, Nikula & Waters, 2011). The observations on D. antarctica are consistent with further zoogeographic studies of littoral benthic and pelagic organisms, which provide evidence for three distinct, most probably SSTrelated, zoogeographical regions along the Chilean coast ( Escribano, Fernandez & Aranıs, 2003; Hinojosa et al., 2006). As goose barnacles from the southern hemisphere are abundantly rafting on detached Macrocystis and D. antarctica ( Thiel & Gutow, 2004; Hinojosa et al., 2006), the coastal Chilean subgroup appears to be very plausible.

In addition to extreme genetic divergence between populations of the bull kelp D. antarctica from Chile and New Zealand, and further genetic differentiation within both regions, a genetically homogeneous population exists further south within the ACC ( Fraser et al., 2009, 2011). Unified Antarctic genotypes were also described for the chaetognath Eukrohnia hamata and the Antarctic krill Euphausia superba Dana, 1850 ( Bortolotto et al., 2011; Kulagin et al., 2014). Therefore, it might be hypothesized that a third, truly subantarctic L. australis subtype might exist south of the Subtropical Convergence. One tiny L. australis specimen collected from floating Durvillaea kelp at around 50 ° S off the Chilean coast does not belong to the postulated Subantarctic subgroup, nor to the coastal Chilean subgroup, but to the Southern Ocean subgroup.