Genome-wide SNPs showed that populations of the brittle star Astrotoma agassizii on either side of the Antarctic Polar Front (APF) are deeply divergent (FST about 0.72) yet connected by rare, recent admixture, making the APF a strong but leaky barrier and this species one of the clearest documented cases of trans-frontal genetic exchange in the Southern Ocean. A 2019 follow-up resolved the taxonomic thread: A. agassizii is a complex of seven cryptic clades, including a giant Antarctic broadcaster and a dwarf brooder sympatric at South Georgia. What remains open is the dynamic core of the 2017 problem: the true frequency, geographic origins, and direction of trans-frontal migrants (the 2017 sample was spatially coarse and left the western Chilean coast unsampled); the dispersal mechanism producing the admixture (planktonic larvae versus rafting in the Antarctic Circumpolar Current); and the quantitative sensitivity of this connectivity to climate-driven shifts in APF position, temperature, and salinity.

Background

The Antarctic Polar Front (APF) — the sharp latitudinal jump in sea-surface temperature where cold Antarctic Surface Water meets sub-Antarctic water, roughly encircling Antarctica near 50–60°S — is one of the strongest and most persistent oceanographic boundaries on Earth and is widely held responsible for the high endemism (species found nowhere else) of Southern Ocean benthic (seafloor-dwelling) invertebrates. The Antarctic Circumpolar Current (ACC), the world's strongest current, flows eastward along and through this front; it can either reinforce isolation by sweeping propagules past suitable habitat or promote long-distance dispersal. Whether the APF is an absolute barrier to benthic gene flow or a leaky one, and by what vector rare exchanges occur, is a central question of Southern Ocean phylogeography.

The gorgonocephalid brittle star Astrotoma agassizii (family Gorgonocephalidae, the basket stars) is a textbook case. It lives on the continental shelves of both southern South America (the Patagonian Shelf) and Antarctica and was long treated as one morphologically uniform species. Mitochondrial DNA studies (Hunter and Halanych 2008; Heimeier et al. 2010) found three deeply divergent but morphologically indistinguishable lineages — one Antarctic and two South American. Heimeier et al. matched planktonic embryos from the Ross Sea to the Antarctic Peninsula 16S haplotype over more than 5000 km, implying a planktonic dispersive stage in the Antarctic lineage, whereas the South American lineages were described as brooders (retaining embryos on the parent); the co-occurrence of multiple developmental modes within one nominal species, called poecilogony, is rarely confirmed and usually resolves into cryptic species.

Galaska et al. (2017) revisited this system with 2b-RAD sequencing — a reduced-representation genome method that scores thousands of restriction-site-associated single nucleotide polymorphisms (SNPs) — obtaining 955 SNPs for 94 individuals. They confirmed extreme nuclear divergence across the Drake Passage (FST = 0.721, meaning about 72% of the genetic variance partitions between regions, with roughly one-third of loci fixed for alternate alleles), yet found five individuals with mixed-ancestry genomes — four on the Patagonian Shelf and one in the Southern Ocean, the latter also carrying a South American mitochondrial haplotype — plus six admixed individuals between two distinct Patagonian Shelf genetic clusters. The APF is therefore a strong but imperfect barrier, with demonstrably recent or ongoing, apparently bidirectional genetic contact.

Jossart et al. (2019) combined mitochondrial and reproductive data across the wider range and settled the taxonomy: A. agassizii comprises seven deeply divergent clades, including a giant Antarctic broadcaster (clade I) and a dwarf brooder (clade V) living in sympatry (co-occurring in the same locality) at South Georgia — cryptic species rather than poecilogony — although the clades have not been formally described or named.

The dynamic picture nonetheless remains unresolved. The true frequency, geographic origins, and direction of trans-frontal migrants are unknown (the 2017 sample was spatially coarse: the western Chilean coast where the Humboldt Current meets the ACC was unsampled, and the Chilean Patagonian Shelf and additional Southern Ocean shelf localities were coarsely sampled or unsampled). The mechanism generating the admixture — long-lived planktonic (teleplanic) larvae of the broadcasting Antarctic clade, versus rafting of adults or juveniles on macroalgae or floating substrata carried by the ACC, as proposed for other Southern Ocean invertebrates — is untested. Finally, how ongoing climate-driven shifts in APF position, temperature, and salinity will quantitatively alter exchange rates, the survivorship of migrants, and the balance between Southern Ocean endemism and gene flow is an open empirical question.

Problem Statement

For the Astrotoma agassizii species complex across the Drake Passage and the wider Antarctic Polar Front, determine quantitatively and mechanistically the trans-frontal genetic connectivity left open by Galaska et al. (2017) and Jossart et al. (2019). The components jointly support one overarching objective — to quantify and mechanistically explain the permeability of the APF for this species complex and its climatic sensitivity — because identifying migrants is prerequisite to attributing mechanisms, which is prerequisite to projecting climate responses. Specifically: (1) Migrant frequency, origins, and direction: identify first-generation migrants and admixed individuals, estimate contemporary trans-APF migration rates and their directionality with uncertainty, and test the asymmetric incidence of admixture observed in 2017. (2) Mechanism: establish which dispersal vector(s) — planktonic larvae of the broadcasting Antarctic clade, rafting on macroalgae or floating substrata carried by the ACC, or other vectors — actually produce the observed trans-frontal admixture, and in what relative proportion. (3) Climatic sensitivity: determine how ongoing and projected climate-driven shifts in APF position, temperature, and salinity quantitatively alter trans-frontal exchange rates, the survivorship of migrants, and the balance between Southern Ocean endemism and gene flow.

References

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