Pseudorchomene plebs (Hurley, 1965)

D’Acoz, Cedric D’Udekem & Havermans, Charlotte, 2012, Two new Pseudorchomene species from the Southern Ocean, with phylogenetic remarks on the genus and related species (Crustacea: Amphipoda: Lysianassoidea: Lysianassidae: Tryphosinae), Zootaxa 3310, pp. 1-50 : 36-40

Pseudorchomene plebs (Hurley, 1965)

( Figs. 23–26 )

Orchomenella plebs Hurley, 1965a: 109 , figs. 1–2.

Orchomene plebs . — Thurston, 1974b: 59. — Andres, 1979: 96. — Andres, 1983: 203 -204. Abyssorchomene plebs . — De Broyer, 1983: 146 –149, fig. 12a. — Andres, 1990: 135, 137, fig. 267 — De Broyer et al., 2007: 161 (ubi syn.).

Type material. Not seen. " HOLOTYPE No. 112416 deposited in the U.S. National Museum, length 20 mm; also PARATYPES. PARATYPES also deposited in N.Z. Oceanographic Institute, Dominion Museum, and British Museum (Nat. Hist.). The material described is from Station A.32, "White Island, 15 Nov. 1961." ( Hurley 1965a).

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: about 35 specimens, RBINS, INV. 100980 (2 tubes) — 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, 28–29.ii.1998: 5 specimens, Specimen Id with corresponding GenBank accession number: AP–31100710 ( HM054000 ), RBINS, INV. 100956. — R/V “Polarstern” cruise ANT–XIX–5 (LAMPOS), sta. 191–1, Saunders Island, 57°41'S 26°24’W, 270 m, baited trap, 15.iv.2002: 29 specimens, Specimen Ids with corresponding GenBank accession numbers: AP– SS270 ( GU109258 ), AP–08100719 ( HM053987 ), AP–08100722 ( HM053988 ), AP–0506081 ( HM053989 ), RBINS, INV. 100958 (5 tubes). —R/V “Ivan Papanin” cruise BELARE, Crown Bay, 70°S 23°E, 230 m, baited trap, December 2008, 10 specimens, Specimen Id with corresponding GenBank accession number: AP–23110992 ( JQ423245 ), RBINS, INV. 100960. — R/V “Polarstern” cruise ANT–XXIII/8, Larsen B, sta. 698–1, 65°59’S 60°24’W, 383 m, Amphipod Trap, 11–12.i.2007: 1 specimen, Specimen Id with corresponding GenBank accession number: AP–LB383 ( GU109233 ), RBINS, INV. 100959. — R/V “Polarstern” cruise ANT–XXIII/8, Larsen B, sta. 713–1, 65°06'S 60°46'W, 299 m, amphipod trap, 18–19.i.2007. 1 female, dissected and mounted on 12 slides in Euparal, RBINS, INV. 100989 /1-12.

Diagnosis. Eye dark brownish/reddish when alive. Somites of pereon and pleosome without posterior humps. Mandibular palp inserted just proximal to molar process. Molar process narrow. Gnathopod 1: basis, anterior margin straight, palm transverse, with basis 2.2 x, ischium 1.9 x, merus 1.5 x, carpus 0.8 x, propodus 1.8 x as long as wide. Gnathopod 2: carpus 2.6 x as long as wide. Pereopod 3: propodus with about 14 spines or pairs of spines (which are small). Pereopod 3–7: on propodus, broadest spine of each pair or triplet with tip acute to subacute. Coxa 4 angular posteroventrally. Pereopod 5: coxa as long as broad; posterior half of basis extremely expanded; merus with setae posteriorly and a few posterodistal fairly slender spines. Ratio length/width of merus of pereopods 5–7: 1.4; 1.7; 1.8. Ratio length/width of carpus of pereopods 5–7: 1.8; 2.3; 2.1. Posterodistal angle of carpus of pereopods 5–7 with spines of normal length and stoutness. Pereopod 7: carpus anterior margin normally spinose, posterior border of carpus and propodus with posterodistal spines only. Epimeron 3 posterior margin regularly rounded. Uropod 3 with medial margin of both rami with many long setae, inner ramus not reaching tip of article 1 of outer ramus.

Maximal length. Up to 25 mm ( Dauby et al. 2001).

Distribution. Circum-Antarctic, as far north as South Orkneys and Macquarie, 0–2889 m ( De Broyer et al. 2007; Havermans et al. 2011), mostly between 400–800 m ( De Broyer et al. 2004).

Biology. "... collected by midwater trawls, indicating a bentho-pelagic way of life. Stomach and gut contents varied from one individual to another. Crustacean parts (eyes or ommatidia, appendages and chitinous plates) were frequent. Some individuals contained fragments of carrion (muscles), while others had ingested diatoms. It is worth noting that A. plebs is more commonly found in baited traps (from 1 to 98% of attracted amphipods) than A. rossi (only few specimens), which could indicate a preference for scavenging" ( Dauby et al. 2001). While it is not an obligatory scavenger, P. p l e b s often enters baited traps in considerable swarms (hence the name of the species) ( Fig. 26 ), sometimes of thousands of specimens ( Rakusa-Suszczewski 1982; De Broyer & Klages 1990; d'Udekem d'Acoz & Robert 2008, Havermans & Robert in press). Such swarms are able to devour fish carcasses in three days, leaving perfectly clean skeletons ( Fig. 27 ).

Remarks. In Figure 2 of Hurley (1965a), the illustration labelled ‘epimeron 3’ is actually (left) epimeron 2 and that labelled ‘epimera 1–2’ shows (left) epimera 1 and 3. Coxa 4 of the same Hurley’s illustration does not look as angular posteriorly as in specimens studied herein.

Figures

FIGURE 2. Pseudorchomene coatsi (Chilton, 1912), female, 19 mm, habitus, ANT – XIX / 3 – 4 (ANDEEP I & II), sta. 0 83, West of Elephant Island.

FIGURE 23. Pseudorchomene plebs (Hurley, 1965), female, habitus, size unrecorded. ANT – XXIII / 8 sta. 625 – 1 / 625 – 2, Elephant, 60 ° 59 ' S 055 ° 57 ' W, 280 – 322 m, 23 – 25. xii. 2006, (specimen not preserved). Note the reddish / brownish colour of the eye.

FIGURE 24. Pseudorchomene plebs (Hurley, 1965), female. 24 mm, ANT – XXIII / 8 sta. 713 – 1, Larsen B. A, left Md; B, tip of lacinia mobilis; C, left Gn 1; D, left coxa 4; E, right P 5; F, left Ep 1; G, left Ep 2; H, left Ep 3; I, left U 3.

FIGURE 25. Pseudorchomene plebs (Hurley, 1965), female. 24 mm, ANT – XXIII / 8 sta. 713 – 1, Larsen B. A, propodus and dactylus of left P 3; B, distal spines of propodus of right P 3; C, distal half of right P 5; D, merus and carpus of right P 6; E, merus and carpus of right P 7.

FIGURE 26. Swarms of Pseudorchomene plebs (Hurley, 1965) and one Natatolana sp., scavenging on dead fish in baited trap, ANT – XXIII / 8 sta. 625 – 1 / 625 – 2, Elephant Island, trap deployed on 23. xii. 2006 and hauled up on 25. xii. 2006.

FIGURE 27. Fish skeleton from a baited trap, cleaned up by swarms of Pseudorchomene plebs (Hurley, 1965) (and by a few Natatolana spp.); ANT – XXIII / 8 sta. 625 – 1 / 625 – 2, Elephant Island, trap deployed on 23. xii. 2006 and hauled up on 25. xii. 2006.