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, 8 (7), e66981

Antarctic Crabs: Invasion or Endurance?

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Antarctic Crabs: Invasion or Endurance?

Huw J Griffiths et al. PLoS One.

Abstract

Recent scientific interest following the "discovery" of lithodid crabs around Antarctica has centred on a hypothesis that these crabs might be poised to invade the Antarctic shelf if the recent warming trend continues, potentially decimating its native fauna. This "invasion hypothesis" suggests that decapod crabs were driven out of Antarctica 40-15 million years ago and are only now returning as "warm" enough habitats become available. The hypothesis is based on a geographically and spatially poor fossil record of a different group of crabs (Brachyura), and examination of relatively few Recent lithodid samples from the Antarctic slope. In this paper, we examine the existing lithodid fossil record and present the distribution and biogeographic patterns derived from over 16,000 records of Recent Southern Hemisphere crabs and lobsters. Globally, the lithodid fossil record consists of only two known specimens, neither of which comes from the Antarctic. Recent records show that 22 species of crabs and lobsters have been reported from the Southern Ocean, with 12 species found south of 60 °S. All are restricted to waters warmer than 0 °C, with their Antarctic distribution limited to the areas of seafloor dominated by Circumpolar Deep Water (CDW). Currently, CDW extends further and shallower onto the West Antarctic shelf than the known distribution ranges of most lithodid species examined. Geological evidence suggests that West Antarctic shelf could have been available for colonisation during the last 9,000 years. Distribution patterns, species richness, and levels of endemism all suggest that, rather than becoming extinct and recently re-invading from outside Antarctica, the lithodid crabs have likely persisted, and even radiated, on or near to Antarctic slope. We conclude there is no evidence for a modern-day "crab invasion". We recommend a repeated targeted lithodid sampling program along the West Antarctic shelf to fully test the validity of the "invasion hypothesis".

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Geographic distribution of recorded Lithodidae from the Southern Ocean with seafloor temperature.
Lithodid crab records = red circles, mean position of Polar Front (Moore et al., 1999) = white dotted line, Purple arrows = ABW production, Blue arrows = ABW direction (Orsi 2010 Nat Geo Sci), Red arrows = Circumpolar Deep Water on shelf. Temperature data from Clarke et al., 2009.
Figure 2
Figure 2. Ranges of fossil decapods in Antarctica.
Bars represent family ranges, colours show water depth of depositional unit in which the fossils were found. Dotted lines are inferred ranges between occurrences. ? = inferred position for end of a range. Red dashed line indicates the uncertainty of the fossil history of the Lithodidae due to the complete lack of a fossil record. All known marine macrofossil sites are shown on the right hand side (grey bars). Red letters indicate the sites in which crab fossils were found. References for ranges, unit water depths and fossil sites are given in Appendix S1. Infraorders labelled as; 1 = Astacidea, 2 = Glypheidea, 3 = Axiidea, 4 = Achelata, 5 = Polychelida, 6 = Anomura & 7 = Brachyura. Stratigraphic units labelled as; Nordenskjöld Formation = A, Fossil Bluff Formation = B, Santa Marta Formation = C, López de Bertodano Formation = D, Snow Hill Island Formation = E, Sobral Formation = F, La Meseta Formation = G, McMurdo Sound erratics = H, DSDP drill holes = I, Polonez Cove Formation = J, Destruction Bay Formation = K, Cape Melville Formation = L, Battye Glacier Formation = M, Hobbs glacier Formation = N, Prospect Mesa gravels = O, Bull Pass Formation = Q, Scallop Hill Formation = R, Cockburn Island Formation = S and Weddell Sea Formation = T, Sørsdal Formation = P.
Figure 3
Figure 3. Distributions of anomuran crab species found south 60°S.
Neolithodes yaldwyni (A), Paralomis stevensi (B), Paralomis birsteini (C), Neolithodes capensis (D), Neolithodes diomedeae (E), Lithodes murrayi (F – red circle) and Munidopsis albatrossae (F – blue square). Red line = Mean position of Polar Front (Moore et al., 1999).
Figure 4
Figure 4. Distribution of brachyuran crabs found south of 60°S.
Red circle = Halicarcinus planatus = , Blue square = Hyas araneus and Yellow triangle = Rochinia gracilipes. Red line = Mean position of Polar Front (Moore et al., 1999).
Figure 5
Figure 5. Southern Ocean crabs and lobsters with distributions south of the sub-Antarctic Front.
Paralomis anamerae (A – Red circle), Lithodes macquariae (A – Blue square), Kiwa sp. (B – Red circle), Paralomis elongata (B – Blue square), Thymopsis nilenta (C), Thymopides grobovi (D – Green triangle), Paralomis aculeata (D – Blue squares) and Neolithodes duhameli (D – Red circle). Red line = Mean position of Polar Front (Moore et al., 1999).
Figure 6
Figure 6. Crab and lobster species shared only between Southern South America and the northern Scotia arc.
Thymops takedai (A – Blue Square), Thymops birsteini (A – Red circle), Paralomis formosa (B) and Paralomis spinosissima (C). Red line = Mean position of Polar Front (Moore et al., 1999).
Figure 7
Figure 7. Depth ranges of Southern Ocean crabs and lobsters.
Thick black vertical bars = mean depth of records, horizontal bars indicate standard deviation from that mean. Grey circles = all records from the Southern Ocean and neighbouring regions, black dots = records from south of 60°S, red dots = records from the Antarctic shelf/slope.
Figure 8
Figure 8. Biogeographic patterns in the distributions of the Lithodidae.
Dendrogram of faunal similarity for Southern Ocean regions from Bray-Curtis similarities using complete linkage clustering. Coloured boxes represent biogeographic groupings at a 35% faunal similarity cut off value. Map of the geographic locations of the clusters. Black dotted line = Polar Front, purple solid line = Sub-Antarctic Front, orange solid line = southern Antarctic Circumpolar Current boundary (SACCB), grey = Seafloor temperature <0°C.

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Grant support

This work was funded by the Natural Environment Research Council (UK)as a contribution to the British Antarctic Survey core project, Environmental Change and Evolution. Funding for MB was provided by the Government of South Georgia and the South Sandwich Islands. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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