Onisimus plautus ( Krøyer, 1845 ), Kroyer, 1845

Berge, Jørgen, Vader, Wim & Johnsen, Jan Roger, 2007, Studies on the genus Onisimus Boeck, 1871 (Crustacea, Amphipoda, Uristidae) II. The barentsi and edwardsii groups, Zootaxa 1410, pp. 55-68 : 60-63

Onisimus plautus ( Krøyer, 1845)

Figure 5–8

Type locality

South East Greenland.

Diagnosis

Antenna 1 peduncle anterodorsally produced. Uropod 1 peduncle weakly longer than rami. Cephalic lobe triangular and acute. Coxa 1 rectangular (not expanded ventrally). Pereopod 1 propodus as long as carpus, dactylus as long as palm. Telson cleft 1/3.

Material examined

USNM 119980, Dolphin and Union Strait, Canada, 2 spm.

F 13184, Karmøy, Norway, 6 spm.

CRU Type, Greenland; CRU 3208, male, 8mm, South East Greenland; CRU 3208, female, 9mm, South East Greenland.

N 23433 female, 10mm, Laptev sea; N 23434 male, 12 mm, Laptev sea.

SMNH 3509, Møller bay. Novaja Semlya Exp, 1875, about 30 spm; SMNH 6669, 8 fvn, 70° 12’ 65° 45’, Kara sea, 1 spm; SMNH 6662, 1 spm; SMNH 6659, 1 spm.

SMNH 6667, Novaja Semlya exp. 1875, 10 fvn, 73° 40’ 69° 10’, 10 fvn, 3 spm.

SMNH 6664, 5 spm; SMNH 3502 female, 6.5mm, Møller bay.

Description (based upon N23433 adult female, 10mm, and N 23434 adult male, 12 mm; both from Laptev sea. Zool.Inst. St.Petersburg. Det: O. turgidus .)

Head and body: Head slightly longer than first pereonite. Cephalic lobe broadly triangular, distally acute; margins straight. Eyes yellow-brown in alcohol, oval, enlarged in adult male. Urosomite 1 with moderate dorsal depression.

Antennae: Antenna 1 as long as head and the next two body segments; peduncle slightly longer than head, article1 longer than broad, anterodorsal ‘nose’ not conspicuous; accessory flagellum exceeds half the length of primary flagellum, 5-articulate, article 1 as long as the rest of primary flagellum, longer than article 1 of primary flagellum; primary flagellum 11-articulate, article 1 length equals next 3 articles, article 1 and article 2 primary flagellum with 2 slender robust setae distally; no calceoli on flagellum.

Antenna 2 length about 1.3 times antenna 1; flagellum 19-articulate; peduncle article 4 longer than article 5; no calceoli on flagellum.

Both antennae longer, and with higher number of articles, in males than in females.

Mouthparts: Epistome weakly convex; labrum straight, not in front of epistome, with 1 apical robust seta on each lobe.

Mandibular incisors with convex margin; left lacinia mobilis present, right absent; accessory robust setae row present on left and right mandible, 3 long and slender robust setae; molar oval without visible triturating surface; palp article 1 longer than broad, article 2 longer than 3 with 11 to 13 distolateral setae (male with 21 setae), article 3 with 20–22 distolateral setae (male with 25).

Maxilla 1 with 2 plumose setae on inner plate; outer plate with ST in a 7/4-crown arrangement; palp large, 2- articulate, 8 terminal robust setae and one simple seta present on distolateral corner; distomedial margin serrate.

8 Maxilliped inner plate with 3 nodular setae; inner and medial setae rows present, almost continuous with 8 plumose setae; outer plate subovate, without subapical notch, distal setae group with two robust setae, outer setae row with 12 marginal short robust setae, inner setae row present with 5 slender setae; palp 4-articulate; article 2 slender; unguis present on dactylus.

Pereopods: Pereopod 1 subchelate; coxa with 1 seta, rectangular with rounded corners, upper anterior margin straight, ventral margin weakly convex, posterior margin straight with a little notch and a small seta distally; basis long, slender, length> 2 times breadth, moderate setation along anterior margin; ischium posterior margin longer than breadth; merus as long as ischium; carpus longer than merus, as long as propodus; propodus rectangular, length 2.2 times breadth, margins subparallel; palm not clearly defined, oblique, convex and serrate, palm corner with 1 robust seta; dactylus as long as palm, accessory tooth present, cuticular teeth along posterior margin.

Pereopod 2 subchelate; coxa rectangular with notch on posterodistal corner; ischium longer than merus; merus length 0.6 times carpus; propodus 0.6 to 0.7 times carpus, subrectangular, lateral margins slightly convex, distal margin slightly produced posterodistally; palm transverse; dactylus medial, shorter than palm, medium sized.

Pereopod 3 coxa with notch at posterodistal corner; merus slightly expanded, without mediolateral setae row; propodus without setae row along anterior margin; dactylus long and slender.

Pereopod 4 coxa anterior margin convex; weak posteroventral lobe.

Pereopod 5 coxa asymmetrically bilobate, posterior lobe clearly produced ventrally; basis with anterior margin straight and with robust setae, expanded with posterior margin crenulate, convex, no submarginal robust setae-row; merus broad; propodus anterior margin with 4 paired robust setae and 2 apical robust setae; dactylus long and slender.

Pereopod 6 coxa small, clearly lobate posteriorly; basis expanded posteriorly with crenulate margin, weakly convex, posteroventral corner rounded; merus broad; propodus slightly longer than propodus on pereopods 5 and 7, anterior margin with 4 paired robust setae and 2 apical robust setae; dactylus long and slender.

Pereopod 7 basis expanded posteriorly with crenulate posterior margin, convex, posteroventral corner rounded; merus slightly expanded posterodistally; propodus anterior margin with 4 paired robust setae and 2 apical robust setae; dactylus short and slender.

Gills on pereopods 2–6, with extra lobe on pereopods 5 and 6; oostegites on pereopods 2–5.

Epimeral plates: Epimeral plate 1 anteroventral corner subangular, not produced, ventral margin straight, posteroventral corner subrounded.

Epimeral plate 2 anteroventral corner rounded, ventral margin weakly convex, posteroventral corner angular with a small tooth, distal part of posterior margin straight.

Epimeral plate 3 anteroventral corner rounded, ventral margin straight, posteroventral corner produced into a strong, slightly upturned and distally subacute tooth, posterior margin straight.

Uropods: Uropod 1 peduncle longer than rami, outer margin with 9 robust setae, inner margin with 4 to 5 robust setae; rami subequal, outer ramus with 3 robust setae on outer margin; inner ramus with 3 robust setae on outer margin and 1 robust seta proximally on inner margin (absent in male).

Uropod 2 peduncle shorter than rami, outer margin with 5 to 7 robust setae, inner margin with 1 apical robust seta; outer ramus shorter than inner, outer margin with 4 robust setae; inner ramus without constriction, outer margin with 1 robust seta; male with 3 robust setae on outer margin of each rami.

Uropod 3 peduncle shorter than rami, with 2 medioapical robust setae and 1 apical robust seta on inner margin; outer ramus longer than inner ramus, 2-articulate; rami without setae (male and female).

Telson: Telson longer than broad, weakly convex and distally narrowing, cleft 30%, incision deeper than broad, 2 dorsolateral setae and 2 apical setae present on each lobe.

Distribution

This is one of the more well-known species of Onisimus , with a wide holarctic distribution. Stephensen (1923) presented a distribution map covering the Atlantic distribution of the species; however, the localities in SW Norway and W. Sweden, noted by Sars (1890) are doubtful. We have checked the material from Haugesund, SW Norway, in the Oslo Museum, and the sample really contains O. plautus , but we greatly suspect some mix-up in the labelling. O. plautus has never been found in Norway since. Stappers (1911) recorded this species from N. Norway, with Sparre Schneider as a source, but this was an error (see Stephensen 1938, p.37). The single old specimen from Bohuslän was checked by Oldevig (1959) and likewise confirmed as O. plautus ; again, the locality may well be incorrect. Onisimus plautus does occur, on the other hand, in W. Greenland (Stephensen 1923), in the Newfoundland-Nova Scotia area (Shoemaker 1930) as well as in the Gulf of St Lawrence (cf Brunel et al. 1998). O. plautus has been collected many places in the American Arctic (Shoemaker 1920, Stephensen 1944, Dunbar 1954, Oldevig 1959). Tzvetkova & Golikov (2001) report the species from the entire arctic coast line of Russia. In the Pacific the southernmost records seem to be in the Bering Strait ( Gurjanova 1962). Onisimus plautus may therefore be said to have a true holarctic distribution, while further south it is clearly confined to cold waters.

Biology

Onisimus plautus is a common species many places, but it has almost never been collected from baited traps (a single specimen was reported by Legezynska (2001)), so maybe it is not a scavenger. Bryazgin (1987) collected the species in the Barents Sea at depths of 14–230m on various bottoms (clay, silt and stones), and the West Atlantic material of Shoemaker (1920) was likewise collected from sandy and muddy bottoms at depths of 100– 250m. In fully Arctic waters the species may be found in more shallow water: Dunbar (1954) gave 10–70m from Ungava Bay, and most of the E. Greenland specimens were collected between 10–50m ( Stephensen 1944). The species was nevertheless apparently absent in the shallow Chaun Bay area in the East Siberian Sea ( Golikov & Tzvetkova 1994). We have been unable to check the very anomalous record by Sars (1886) of O. plautus collected from 1400m West of Bear Island.

This species is parasitized by a podasconid isopod, probably of the genus Parapodascon (Vader, unpubl. data).

The female is usually carrying from 4 to 7 eggs.

Remarks

Sars (1891) describes the eyes of living specimens as red. The description and figures given by Sars (1891) otherwise fit the examined specimens very well.

Figures

FIGURES 5 – 8: Onisimus plautus (Krøyer, 1845).