Pseudorchomene rossi Walker 1903

Pseudorchomene rossi (Walker, 1903)

(Figs. 28–30)

Orchomenopsis rossi Walker, 1903: 45, pl. 18–23.

Orchomenopsis chilensis f. rossi. — Schellenberg, 1926: 287 –290, fig. 26.

Orchomenella rossi. — Hurley, 1965b: 15, figs. 1–2.

Orchomene rossi. — Thurston, 1974b: 59 -60. — Andres, 1979: 96 -97. — Andres, 1983: 204 -205.

Abyssorchomene rossi. — De Broyer, 1983: 150 –152, fig. 12a, photos 4–9, 15–16. — Andres, 1990: 135, 137, fig. 266 — De Broyer et al., 2007: 162 (ubi syn.).

Type material. Not seen. "Cape Adare. Lat. 78°35'S; Feb. 18, 1900; near surface. Many specimens" (Walker 1903). According to Hurley (1965b), the SYNTYPES are deposited in the British Museum (current name: Natural History Museum, London).

Material examined. Expedition ARC 94, sta. NA62, King George Island, Admiralty Bay, 62°08'S 58°27'W, 470 m, 29.xii.1963 – 02.i.1964: 3 small (about 10 mm) specimens, RBINS, INV. 100983. — R/V “Polarstern” cruise ANT–XXIII/8, Larsen B, sta. 698–1, 65°59.99'S 60°24.90'W, 383–390 m, baited trap, 11–12.i.2007: 1 male dissected and 4 other specimens, RBINS, INV. 100981. — R/V “Polarstern” cruise ANT–XV/3 (EASIZ II), Atka Bay, sta. 280/284, Trap 13, 70°27.4'S 07°55.9'W, 550 m, baited trap, 29.ii.1998: 1 specimen, Specimen Id with corresponding GenBank accession number: AR–3110078 (HM054002), RBINS, INV. 100957. — R/V “Polarstern” cruise ANT XXI–2 (BENDEX), Eastern Weddell Sea, sta. 288, 72°47.58’S 19°29.86’W, 847 m, Fish Trap, 31.xii.2003 – 03.i.2004: 1 specimen, Specimen Id with corresponding GenBank Accession number: AR–1010076 (HM054001), RBINS, INV. 100973. — R/V “Polarstern” cruise ANT–XXIII/8, sta. 698–1, 65°59’S 60°24’W, 383 m, Larsen B, Amphipod Trap, 11–12.i.2007: 1 specimen, Specimen Id with corresponding GenBank accession number: AR– I19 (HM054003), RBINS, INV. 100982.

Type locality. Antarctica, Cape Adare, 78°35'S, no longitude given, near surface (Walker 1903).

Diagnosis. Eye black when alive. Somites of pereon and pleosome without posterior bumps. Mandibular palp inserted just proximal to molar process. Molar process narrow. Gnathopod 1: basis, anterior margin straight, palm transverse, basis 2.3 x, ischium 1.4 x, merus 1.4 x, carpus 0.9 x, propodus 1.8 x as long as wide. Gnathopod 2: carpus 3.8 x as long as wide. Pereopod 3: propodus with about 12 spines or pairs of small spines. Pereopod 3–7: on propodus, broadest spine of each pair or triplet with tip very blunt. Coxa 4 angular posteroventrally. Pereopod 5: coxa as long as broad; basis strongly expanded; merus posteriorly with short and stout spines only. Ratio length/ width of merus of pereopods 5–7: 1.2; 1.3; 1.3. Ratio length/width of carpus of pereopods 5–7: 1.3; 1.5; 1.7. Posterodistal angle of carpus of pereopods 5–7 with short and very stout spine(s). Pereopod 7: carpus, anterior margin normally spinose, posterior margin of carpus and propodus with postero-distal spines only. Epimeron 3 with large and very blunt tooth posterioventrally. Uropod 3: outer ramus medial margin without setae, inner ramus, medial margin with a few short setae, inner ramus reaching or overreaching tip of article 1 of outer ramus.

Maximal length. Up to 40 mm (Schellenberg 1926; Dauby et al. 2001).

Distribution. Circum-Antarctic, as far north as South Georgia (De Broyer et al. 2007), 7–1453 m (De Broyer et al. 2007) but mostly 200–600 m (De Broyer et al. 2004).

Biology. Occurs in water column as well as on benthic substrates (Dauby et al. 2001). Sometimes in large number under pack ice (Kaufmann et al. 1993, 1995). "Stomachs of benthic specimens were dominated by fluidish organic matter spotted with oily droplets, likely to be flesh at various stages of digestion; some other items are found, but in small quantity: sponge spicules, crustacean appendages and diatoms. Stomachs of pelagic individuals have a totally different content. While flesh was still present (about 25%), copepod remains formed the bulk (55%) of the diet; polychaete setae constitute a third, less common, item. The exact trophic position of A. rossi is unclear. Although an apparently selective copepod predator within the water column, it appears to be able to migrate down to the bottom to scavenge on different materials" (Dauby et al. 2001).

Remarks. In baited traps, P. rossi is sometimes found mixed with P. p l e b s, although this co-occurrence is rather infrequent (Thurston 1974b). The two species have the same general appearance, which can make their separation very time consuming in preserved samples. However, when live sorting, the species can be separated easily by eye colour: black in P. rossi, dark brown/reddish in P. p l e b s.