Would the real Ophioplocus incipiens please stand up? A Phylogeographic Study of the Antarctic Species Complex

Ophiuroids are a diverse group of echinoderms in the Southern Ocean, yet current records likely capture only part of their true diversity due to taxonomic complexity. Some species show highly variable or overlapping morphological characters, while others lack visible differences despite genetic and geographic isolation, complicating accurate identification. This study examines the distribution and genetic structure of Ophioplocus incipiens, a species with an unusual range. Verified specimens occur on the South Georgia and South Orkney shelves, around Elephant Island and King George Island, in the eastern Weddell Sea, and on the East Antarctic shelf near Dumont d’Urville. Despite this broad latitudinal distribution, the species is notably absent from the Antarctic Peninsula, Bouvetøya, and several other Southern Ocean islands. Where present, it occurs in high abundance, prompting questions about possible cryptic diversity and population subdivision. We tested whether O. incipiens forms a single, panmictic species by analysing mitochondrial cytochrome c oxidase subunit 1 (mtCOI), small subunit ribosomal 18S, and large subunit ribosomal 28S sequences from specimens collected during multiple research cruises across shelf and slope environments. MtCOI data reveal deep divergences between the South Georgia shelf samples and samples from both South Orkney and Antarctic shelf, with nuclear markers supporting this separation. South Orkney Island and South Shetland populations show very high nucleotide diversity, while Weddell Sea samples failed to amplify with standard primers, also suggesting further genetic differentiation. Our results indicate that O. incipiens, like other Southern Ocean ophiuroids, likely comprises cryptic species. Divergent lineages in the South Orkney and South Shetland Islands mirror patterns seen in Amphiura belgicae, highlighting shared evolutionary histories and the need for molecular tools to better assess Southern Ocean biodiversity.