Aim: The Antarctic marine environment hosts diversified and highly endemic benthos owing to its unique geologic and climatic history. Current warming trends have increased the urgency of understanding Antarctic species history to predict how environmental changes will impact ecosystem functioning. Antarctic benthic lineages have traditionally been examined under three hypotheses: 1) high endemism and local radiation, 2) emergence of deep-sea taxa through thermohaline circulation, 3) species migrations across the Polar Front. In this study, we investigated which hypotheses best describe benthic invertebrate origins by examining Antarctic scale worms.

Location: Southern Ocean, Kerguelen archipelago, South American peninsula, Indian Ocean, New Zealand.

Taxon: Scale worm polychaetes (Polynoidae).

Methods: We amassed 670 polynoids from the Southern Ocean and neighbouring areas and performed phylogenetic reconstructions to identify lineages across geographic regions, aided by mitochondrial markers Cytochrome c oxidase subunit I (Cox1) and 16S ribosomal RNA (16S). Additionally, we produced haplotype networks at the species scale to examine genetic diversity, biogeographic separations, and past demography.

Results: The Cox1 dataset provided the most illuminating insights into the evolution of polynoids. Eunoe sp. was present at South America and Kerguelen, in favour of the latter acting as a migration crossroads. Harmothoe fuligineum, widespread around the Antarctic continent, was also present but isolated at Kerguelen, possibly resulting from historical freeze-thaw cycles. The genus Polyeunoa appears to have diversified prior to colonizing the continent, leading to the cooccurrence of at least three cryptic species around the Southern and Indian Oceans. Analyses identified that nearly all populations are presently expanding following a bottleneck event, possibly caused by habitat reduction from the last glacial episodes.

Main Conclusions: This study details the largest phylogenetic dataset assembled to date for Antarctic polynoids. These findings provide insight into past demographic events experienced by Antarctic marine benthos and identify multiple origin scenarios for contemporary polynoids. 

Polynoids were retrieved across several years aboard multiple research vessels, aided by an assortment of sampling equipment. Specimens processed within the context of this study were preserved either in 96° ethanol or at ‑80 °C. Each collected individual was identified to the lowest possible taxonomic level based on morphological characters. DNA was extracted from each specimen either by following a modified CTAB protocol (Doyle & Doyle 1987) or  shipped to the Canadian Center for DNA Barcoding (CCDB) at the University of Guelph to be processed following the CCDB automated standard protocols. PCR assays were performed using extracted DNA to amplify fragments of two mitochondrial genes, Cytochrome c oxidase subunit I (Cox1) and 16S ribosomal RNA (16S). Resulting PCR products were either submitted to Eurofins Scientific for purification and Sanger sequencing in both directions, using ABI BigDye® Terminator v3.1 Cycle sequencing kit (Applied Biosystems) or underwent amplification and sequencing at CCDB.

Sources: Doyle, J, Doyle, J (1987). Genomic plant DNA preparation from fresh tissue-CTAB method. Phytochem Bulletin, 19(11), 11-15.
Canadian Center for DNA Barcoding (CCDB – University of Guelph, Canada): http://ccdb.ca/resources/

Resulting sequences were visualized and edited using Codoncode Aligner 7.1.2 (CodonCode Corporation) and Geneious v.10.0.5 (Kearse et al. 2012).