DNA uncovers Antarctic nemertean biodiversity and exposes a decades-old cold case of asymmetric inventory

With threats to biodiversity posed by anthropogenic impacts and global climate change, characterization of existing flora and fauna is increasingly important, but continues to focus predominantly on easily studied taxa. In the Southern Ocean, levels of species richness remain relatively unexplored due to remoteness and difficulties of sampling the region. Nemerteans (proboscis worms; ribbon worms) are unusually abundant and occasionally conspicuous in the Antarctic region. Despite being routinely collected, difficulties in preserving voucher material, morphological limitations, and shortage of taxonomic expertise have hindered our understanding of nemertean diversity. To assess patterns of diversity, we examined a fragment of the mitochondrial 16S rRNA gene from larval and adult nemerteans (n = 192) from 53 sites along the western Antarctic Peninsula. We found 20 distinct lineages having an uncorrected genetic distance (p) greater than 5% to the nearest sister taxon or group, 19 of which have not been genetically characterized in previous studies. Additionally, the putatively dominant adult species in the region, Parborlasia corrugatus, was found to comprise only 4. 3% of larvae sampled (n = 3 out of 69 samples from 12 locations). Of 47 nemertean species recorded from Antarctic waters, 20 are heteronemerteans and therefore could have a pelagic pilidium larval phase. These results suggest that Antarctic biodiversity is underestimated, and that unknown species of nemerteans await description from Southern Ocean waters.