Griffiths, Barnes and Linse (2009) analysed the distributions of roughly 7,600 shelf and slope species of bivalve and gastropod molluscs and of cheilostome and cyclostome bryozoans across 29 Southern Hemisphere regions and concluded that the Southern Ocean benthos obeys a few general rules: the Antarctic forms a single circumpolar biogeographic province (no east-west split), Antarctic class-level species endemism is on the order of 50%, and the Antarctic Circumpolar Current imposes a directional, distance-decaying South American faunal signature. Gastropods were the exception, showing both much higher endemism (about 74%) and an east-west provincial split. The authors flagged four linked uncertainties: whether finer provincial structure would emerge with more complete sampling of under-sampled sectors and the deep sea; whether the deep sea acts as a reservoir and dispersal route that bypasses the Polar Front (the near-surface oceanographic boundary separating Antarctic from sub-Antarctic water); which mechanisms produce the taxon-specific exceptions; and whether the ~50% endemism rule generalizes across taxa, taxonomic levels and depths. Subsequent literature shows partial progress — the gastropod east-west split is robust (Pierrat et al. 2013), and Polar-Front isolation demonstrably weakens with depth in pycnogonids (sea spiders; Maxwell et al. 2024) — but no comprehensive multi-taxon, depth-integrated, circumpolar test has settled the provincial or endemism rules, and deep-sea exchange remains unquantified. The problem is open.

Background

The Southern Ocean is the ring of water encircling Antarctica, dominated by the Antarctic Circumpolar Current (ACC), the world's strongest current, which flows eastward around the continent unimpeded by land. Its northern biogeographic boundary is the Antarctic Polar Front (PF), a sharp near-surface temperature front where cold Antarctic surface water meets sub-Antarctic water. The benthos is the seafloor-dwelling fauna. A biogeographic province is a region whose species composition is more similar within it than to any neighbouring region; endemism is the proportion of a region's species found nowhere else. Because Antarctica has been isolated by the PF and by continental drift for tens of millions of years, and because repeated ice-sheet advances during glacial cycles eradicated much of the shelf fauna, Southern Ocean benthic species show unusually high endemism, broad depth tolerance (eurybathy, i.e. species spanning large depth ranges) and many cryptic species (morphologically similar but genetically distinct lineages, which distort both endemism counts and provincial classifications).

Twentieth-century classification schemes, built on qualitative synthesis, generally split Antarctic waters into separate east and west provinces and asserted very high (75-90%) endemism. Griffiths, Barnes and Linse (2009) tested this quantitatively. They compiled species inventories (0-3000 m depth) for four benthic classes — Bivalvia and Gastropoda (two classes of molluscs, clams and snails) and Cheilostomata and Cyclostomata (two classes of bryozoans, colonial 'moss animals') — across 29 regions of the Southern Hemisphere, and analysed faunal similarity (Bray-Curtis similarity, a standard index comparing species lists between regions; visualized with non-metric multidimensional scaling and tested with analysis of similarities, ANOSIM, a permutation test of group differences). Three of the four classes treated the Southern Ocean as a single 'functional unit' with no east-west division, supporting a single Antarctic province; only gastropods split into eastern and western components, with the apparent split resting partly on almost unexplored Bellingshausen Sea records. Antarctic class-level endemism was 42.6% (bivalves), 55.6% (cheilostomes), 47.0% (cyclostomes) and 73.7% (gastropods), suggesting a practical rule of roughly 50% class-level endemism, with gastropods the marked exception. The prevalence of South American species declined with distance from South America in the direction of ACC flow in three classes (Spearman rank correlations rsr_s=0.68-0.90) but not in bivalves (rsr_s=0.19, not significant), evidence for a long-term ACC transport signature with bivalves as an exception. The deep sea was largely absent from the analysis: sampling then concentrated on the shelf, and deep basins were (and remain) the least sampled habitat, so the possible role of the deep sea in moving fauna past the PF could not be assessed. Earlier deep-sea sampling (the ANDEEP expeditions, an international series of deep-water survey cruises of the Antarctic margin and abyss, summarized by Brandt et al. 2007 in Nature) had shown that some slope assemblages contain species that invaded from the shelf, that abyssal faunas connect to other oceans mainly in good dispersers, and that poor dispersers include many Southern-Ocean endemics — but coverage was geographically tiny relative to the vast deep Southern Ocean.

Problem Statement

Determine whether the generalized biogeography proposed for the Southern Ocean benthos - a single circumpolar Antarctic province and Antarctic class-level species endemism of roughly 50% - is robust once sampling gaps are filled, and identify the connectivity mechanisms that sustain it. Concretely, once the historically under-sampled sectors (Weddell, Bellingshausen and Amundsen seas, sub-ice-shelf shelves, and depths below the shelf) are included: (1) establish whether the Antarctic benthos forms a single biogeographic province or whether finer east-west or additional provincial subdivisions (as already seen in gastropods) recur across classes; (2) establish whether the ~50% Antarctic class-level endemism rule holds across additional taxa, at finer taxonomic levels (species endemism computed within orders and families) and when deep records are incorporated; (3) establish whether and how the deep sea acts as a reservoir and dispersal route for shelf taxa connecting Antarctic waters across/beyond the Antarctic Polar Front, including via northward Antarctic Bottom Water pathways, and whether such routes are restricted to species able to survive at abyssal depths; and (4) identify the mechanisms (larval biology, rafting, life-history traits such as brooding, i.e. incubating embryos on or in the parent instead of releasing planktonic larvae, or polyembryony, i.e. many embryos budding from a single fertilized egg, and eurybathy) that produce the taxon-specific exceptions to the general rules, notably the high endemism and east-west structure of gastropods, the absence of a distance-decay trend in bivalves, and the broad ranges of cyclostomes. These four questions jointly ask whether the single-province and ~50%-endemism generalizations are real, mechanistically coherent features of the Southern Ocean benthos or artifacts of incomplete, shelf-biased sampling; a resolution requires all four.

References

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