Abstract
Shallow‐water polychaetes are abundant and diverse components of the Southern Ocean benthic communities, and although they have been widely studied, new species that are relatively common are still discovered. Here, we report the discovery of Pterocirrus giribeti sp. n., a new and abundant intertidal and upper‐subtidal Antarctic phyllodocid. To establish the phylogenetic relationships of the new species, we sequenced two nuclear ( 18S and 28S ) and two mitochondrial ( COI and 16S ) markers. Although the phylogenetic relationships obtained for the family Phyllodocidae were not fully resolved, we assigned our new phyllodocid to the genus Pterocirrus based on both its phylogenetic position and its morphological characters. Using COI and 16S sequences of 126 and 118 individuals, respectively, from eight populations across the South Shetland Islands and the Antarctic Peninsula, we also investigated the genetic diversity and gene flow patterns of this new species. Our results suggested that all populations were panmictic, likely due to the presence of planktotrophic larvae allowing long‐distance dispersal. Interestingly, some genetic substructure was detected despite panmixis, and we identified a semipermeable barrier coinciding with an oceanic front produced by the intrusion into the Bransfield Strait of a tongue of water from the Weddell Sea. This front produced signatures of differentiation on populations at the tip of the West Antarctic Peninsula. Moreover, our results indicated a recent demographic expansion throughout the sampled area, in agreement with the “glacial refugium” hypothesis stated for other Antarctic shallow‐water invertebrates.