Dueñas et al. (2016) provided the first phylogeographic assessment of deep-sea octocorals across the Antarctic Circumpolar Current (ACC), sequencing two nuclear ribosomal DNA loci (the internal transcribed spacer 2, ITS2, and the 28S ribosomal-subunit gene) for 80 individuals of a bottlebrush primnoid complex (the nominal species Tokoprymno maia plus a closely allied Thouarella-like sub-Antarctic morph) from four regions: Antarctica, New Zealand, the Macquarie Ridge, and Tasmania. They found a phylogenetic break at the ACC, significant structure among the four regional populations, shared haplotypes implying recent asymmetrical long-distance migration (probably younger than about 100 thousand years), a Middle Miocene (about 12.6 Ma) divergence between populations on opposite sides of the ACC (an amphi-ACC divergence) post-dating ACC formation, and evidence that each regional population could constitute a different species. They explicitly left open the genome-wide quantification of gene flow and admixture among these populations, the coalescent dating of the recent trans-ACC dispersal events, integrative species delimitation and taxonomic revision of the complex, and the comparative test across brooding and broadcast-spawning octocorals of whether the ACC generally acts as a semi-permeable ('soft'/'filter') barrier. These threads remain open: Thouarella systematics and the Primnoidae phylogeny have advanced and SNP-based studies on other Southern Ocean benthos corroborate a leaky-barrier regime, but no multilocus or genomic study of the Tokoprymno/Thouarella system itself exists.

Background

The Antarctic Circumpolar Current (ACC) is the strongest current in the world ocean, flowing eastward around Antarctica and connecting the Atlantic, Indian, and Pacific basins; its northern limit is the Antarctic Polar Front, a sharp temperature gradient. The ACC became established after the opening of the Drake Passage, with its onset commonly dated near the Oligocene–Miocene boundary around 25 million years ago. For Southern Ocean bottom-dwelling (benthic) organisms the ACC therefore acts simultaneously as a corridor for eastward dispersal and as a potential barrier separating Antarctic from sub-Antarctic and temperate faunas. Whether that barrier is absolute ('hard') or leaky ('soft' or 'filter', permitting occasional gene flow while maintaining structure) is a central question of Southern Ocean biogeography, and one whose answer may depend on reproductive mode: broadcast spawners release larvae that can disperse in the plankton for weeks to months, whereas brooders incubate their larvae and settle them close to the parent, an expectation of limited dispersal.

Octocorals (Anthozoa: Octocorallia) are colonial cnidarians that include soft corals and sea fans; the family Primnoidae is a species-rich, mostly deep-sea group of sea fans found commonly on seamounts and continental slopes at several hundred to over a thousand meters depth, where they contribute to vulnerable marine ecosystems; the taxa in this study are 'bottlebrush' primnoids whose colonies bear their polyps on side branches like a bottle brush. Phylogeography — the joint analysis of geographic and genetic structure within species — is usually powered in octocorals by mitochondrial and nuclear ribosomal DNA (rDNA) markers, because mitochondrial genes evolve unusually slowly in anthozoans. Microsatellites (highly variable short tandem-repeat nuclear loci) and single nucleotide polymorphisms (SNPs, genome-wide single-base variants) give far finer resolution of gene flow, of admixture (the mixed ancestry expected when divergent populations interbreed), and of the ages of recent events via multilocus coalescent analysis, which models gene genealogies backward in time; species delimitation is the formal testing of whether populations or lineages deserve species rank.

Dueñas et al. (2016) applied the two-locus rDNA approach to the bottlebrush complex spanning both sides of the ACC and concluded that the ACC is a semi-permeable barrier for these corals, separating and structuring populations while allowing short periods of gene flow, with Miocene latitudinal fluctuations of the ACC plausibly driving their diversification. Their inference was, however, bounded by the markers available: two nuclear rDNA loci cannot finely quantify gene flow or admixture, cannot date the postulated recent migrations, and cannot by themselves delimit species. Adjacent results sharpen the stakes: a 2b-RAD study of the brittle star Astrotoma agassizii (2b-RAD is a reduced-representation genome-sequencing method that scores many thousands of scattered single-base markers) found strongly divergent populations on either side of the Drake Passage together with admixed individuals on both sides of the Polar Front, i.e. a 'strong but imperfect' barrier (Galaska et al. 2017); 3RAD SNPs (a related reduced-representation method) in the brooding sea spider Nymphon australe revealed strong regional structuring and a possible species complex (Zehnpfennig et al. 2024); the sea star Glabraster antarctica is a single circumpolar species yet strongly structured across its range, rejecting panmixia (random mating throughout the population) (Moore et al. 2018); seven cryptic (morphologically indistinguishable but genetically distinct) periwinkle species were uncovered within one nominal Southern Ocean snail species (González-Wevar et al. 2022); and the octocoral Alcyonium spp. shows a clear genetic break and four putative species across the Drake Passage, though from mitochondrial and 28S sequences only (Bruning et al. 2024). Recent Thouarella systematics has also revealed extensive cryptic diversity in the Southern Ocean (Núñez-Flores et al. 2020, 2021). None of these substitutes for a multilocus, genome-wide analysis of the bottlebrush octocoral complex across the ACC, which is what the source study called for.

Problem Statement

The overarching objective is to determine, for deep-sea Southern Ocean octocorals, whether the Antarctic Circumpolar Current (ACC) acts as a semi-permeable ('soft'/'filter') barrier — separating and structuring populations while permitting episodic gene flow — and what that barrier regime implies for the species limits of the bottlebrush-octocoral complex that spans both sides of the ACC (Primnoidae: the nominal species Tokoprymno maia together with the closely allied Thouarella-like sub-Antarctic morph). All requested results below serve this single objective.

Specifically, the objective comprises the following four items:

  1. A multilocus, preferably genome-wide (microsatellite- or SNP-based), quantification of gene flow among the four regional populations (Antarctica, New Zealand, Macquarie Ridge, Tasmania) — its magnitude, direction, and admixture structure — replacing the two-locus nuclear rDNA inference of Dueñas et al. (2016).
  2. Coalescent-based estimates, from the multilocus data, of the timing of gene flow across the ACC — in particular, testing the source's inference from shared haplotypes that the migration events between these localities are probably more recent than roughly 100 thousand years — with credible or confidence intervals for the estimated ages.
  3. An integrative species delimitation deciding whether the four regional populations constitute distinct species — combining the multilocus genetic evidence with morphology (colony form, polyp arrangement, and sclerites, the calcareous microscopic skeletal elements embedded in octocoral tissue) and whatever ecological information is obtainable at sampling depths — and, where the evidence supports it, a formal taxonomic revision of the complex.
  4. A comparative phylogeographic assessment across additional Southern Ocean octocoral species that includes both brooding and broadcast-spawning taxa, testing whether the ACC's soft/filter-barrier behavior generalizes across octocorals as the source study proposed.

All four items are part of the requested answer: items 1–3 address the bottlebrush complex itself, while item 4 extends the same objective to additional octocorals.

Two auxiliary aspects that the source flags as unresolved may be addressed within the same framework where the evidence allows: the deeper divergence-time estimates remain limited by the absence of fossils closely related to Primnoidae, and the geographic origin of the group is unresolved by ancestral-area reconstruction.

References

  1. Dueñas LF, Tracey DM, Crawford AJ, Wilke T, Alderslade P, Sánchez JA (2016) The Antarctic Circumpolar Current as a diversification trigger for deep-sea octocorals. BMC Evolutionary Biology 16:2. doi:10.1186/s12862-015-0574-z, https://doi.org/10.1186/s12862-015-0574-z
  2. Cairns SD, Wirshing HH (2018) A phylogenetic analysis of the Primnoidae (Anthozoa: Octocorallia: Calcaxonia) with analyses of character evolution and a key to the genera and subgenera. BMC Evolutionary Biology 18:66. doi:10.1186/s12862-018-1182-5, https://doi.org/10.1186/s12862-018-1182-5
  3. Núñez-Flores M, Gomez-Uchida D, López-González PJ (2020) Molecular and morphological data reveal three new species of Thouarella Gray, 1870 (Anthozoa: Octocorallia: Primnoidae) from the Southern Ocean. Marine Biodiversity 50(3):30. doi:10.1007/s12526-020-01053-z, https://doi.org/10.1007/s12526-020-01053-z
  4. Núñez-Flores M, Gomez-Uchida D, López-González PJ (2021) Molecular systematics of Thouarella (Octocorallia: Primnoidae) with the description of three new species from the Southern Ocean based on combined molecular and morphological evidence. Invertebrate Systematics 35(6):655–674. doi:10.1071/IS20078, https://doi.org/10.1071/IS20078
  5. Núñez-Flores M, Solórzano A, Avaria-Llautureo J, Gomez-Uchida D, López-González PJ (2024) Diversification dynamics of a common deep-sea octocoral family linked to the Paleocene-Eocene thermal maximum. Molecular Phylogenetics and Evolution 190:107945. doi:10.1016/j.ympev.2023.107945, https://doi.org/10.1016/j.ympev.2023.107945
  6. Yesson C, Wright E, Braga-Henriques A (2018) Population genetics of Narella versluysi (Octocorallia: Alcyonacea, Primnoidae) in the Bay of Biscay (NE Atlantic). Marine Biology 165(8):135. doi:10.1007/s00227-018-3394-z, https://doi.org/10.1007/s00227-018-3394-z
  7. Galaska MP, Sands CJ, Santos SR, Mahon AR, Halanych KM (2017) Crossing the Divide: Admixture Across the Antarctic Polar Front Revealed by the Brittle Star Astrotoma agassizii. The Biological Bulletin 232(3):198–211. doi:10.1086/693460, https://doi.org/10.1086/693460
  8. Zehnpfennig JR, Galaska MP, Halanych KM, Mahon AR (2024) Single nucleotide polymorphism data reveals distinct geographic structuring in the Antarctic circumpolar sea spider Nymphon australe. Marine Biology 171(9):179. doi:10.1007/s00227-024-04492-5, https://doi.org/10.1007/s00227-024-04492-5
  9. Moore JM, Carvajal JI, Rouse GW, Wilson NG (2018) The Antarctic Circumpolar Current isolates and connects: Structured circumpolarity in the sea star Glabraster antarctica. Ecology and Evolution 8(21):10621–10633. doi:10.1002/ece3.4551, https://doi.org/10.1002/ece3.4551
  10. González-Wevar CA, Segovia NI, Rosenfeld S, Maturana CS, Jeldres V, Pinochet R, Saucède T, Morley SA, Brickle P, Wilson NG, Spencer HG, Poulin E (2022) Seven snail species hidden in one: Biogeographic diversity in an apparently widespread periwinkle in the Southern Ocean. Journal of Biogeography 49(8):1521–1534. doi:10.1111/jbi.14453, https://doi.org/10.1111/jbi.14453
  11. Bruning P, Archaumbault P, Garrido I, de Lecea AM, Morley SA, Brante A, Ortiz P, Cárdenas L (2024) Phylogeography of cold water soft coral Alcyonium spp. (Anthozoa, Octocorallia: Alcyonacea) between South America and the West Antarctic Peninsula. Ecology and Evolution 14(12):e70522. doi:10.1002/ece3.70522, https://doi.org/10.1002/ece3.70522
  12. Zeng C, Rowden AA, Clark MR, Gardner JPA (2017) Population genetic structure and connectivity of deep-sea stony corals (Order Scleractinia) in the New Zealand region: Implications for the conservation and management of vulnerable marine ecosystems. Evolutionary Applications 10(10):1040–1054. doi:10.1111/eva.12509, https://doi.org/10.1111/eva.12509