Onisimus barentsi Stebbing 1894

Onisimus barentsi (Stebbing, 1894)

Figure 1–4

Paronesimus barentsi Stebbing 1894: 14; Stephensen 1932: 354, 1935: 51; Gurjanova 1936: 34, 1951: 198, 1962: 143; Dunbar 1954: 715; Shoemaker 1955: 6; Barnard 1958: 98; Brunel 1970: 35; Watling 1979: 266; Palerud & Vader 1991: 42.

Onisimus barentsi Lowry & Stoddart 1993: 176

Onisimus krassini Gurjanova 1951: 175. New synonomy.

Type locality

West of Novaja Zemlya: 75° 49'N 53° 42'E, 125m.

Diagnosis

Antenna 1 peduncle article 1 produced anterodistally (nose). Uropod 1 peduncle length about 1.5 times rami. Cephalic lobe rounded. Coxa 1 expanded ventrally. Pereopod 1 propodus length 1.5 times carpus length, palm clearly shorter than dactylus. Telson cleft 40%.

Material examined

CRU, female, 12mm, Godthaab-Exp St 139, 71 ° 21’ N 54° 29’ W. Greenland, 1spm. (Det: O. plautus.); CMNC 1998-0010, female 5mm, Northumberland Canada, 35m; Type material of O. krassini at the ZMS.

Description (based upon female 12mm, adult)

Head and body: Head as long as pereon 1. Cephalic lobe little extended forward, rounded, dorsal margin straight and ventral margin convex. Eyes oval. Urosomite 1 with slight dorsal depression.

Antennae: Antenna 1 as long as cephalon and next 1.5 body segment; peduncle article 1 slightly longer than head, longer than deep, produced anterodorsally; accessory flagellum length 0.6 times primary flagellum, 5-articulate, article 1 clearly shorter than the rest of flagellum, as long as primary flagellum article 1; primary flagellum 0.9 times peduncle, 10-articulate, article 1 as long as next 3 articles combined; callynophore present; calceoli absent on flagellum.

Antenna 2 about 1.2 times as long as antenna 1; flagellum 12-articulate; peduncle article 4 clearly longer than article 5, article 5 as long as the three first articles in flagellum; calceoli absent on flagellum.

Mouthparts: Epistome and labrum weakly convex, labrum not in front of epistome.

Mandible incisor with convex margin; left lacinia mobilis present; 3 accessory setae present on left and right mandible; molar oval and not triturate; palp attached opposite molar, article 1 without setae, article 2 longer than 3, article 2 with 14 distoventral setae, article 3 with 23 distoventral setae.

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

Maxilliped inner plate distally with 3 nodular setae, inner and medial setae rows present and almost continuous, setae pappopectinate, distal setae row reduced, 1 simple seta; outer plate subovate, without subapical notch, distal setae group with 2 robust setae, inner setae row with 8 simple setae, outer setae row with 14 short robust setae; palp 4-articulate, unguis present on dactyl.

Pereopods: Pereopod 1 subchelate, coxa subrectangular but slightly expanded ventrally, upper anterior margin slightly concave, anteroventral corner subangular, ventral margin straight, posterior margin weakly convex with a little notch, medial setae present; basis 2 times as long as broad, strong setation along anterior margin and weak setation along posterior margin; ischium length posterior margin> breadth, with 4 medioventral setae-groups; merus shorter than ischium; carpus as long as merus, carpal lobe absent; propodus length 1.5 times carpus, slightly tapering distally, length> 2 times breadth; palm slightly oblique, nearly transverse, convex and serrate, corner clearly defined with 2 slender robust setae; dactylus longer than palm, accessory tooth present, cuticular teeth along posterior margin.

Pereopod 2: Missing.

Pereopod 3: Missing.

Pereopod 4 coxa anterior margin convex, moderate posteroventral lobe; merus without mediolateral setae row; propodus without setae row along anterior margin, posterior margin with 4 slender setae and 5 short robust setae, 2 apical robust setae.

Pereopod 5 coxa equilobate; basis with anterior margin weakly concave, robust setae present; basis expanded with posterior margin crenulate, convex, no submarginal spine-row; merus expanded posterodistally, length anterior margin shorter than breadth, 3 robust setae present on posterior margin; propodus anterior margin with 4 paired short robust setae and 2 apical robust setae.

Pereopod 6 coxa small, lobate posteriorly; basis expanded posteriorly with crenulate margin, convex, posteroventral corner rounded; merus expanded posterodistally, length anterior margin equal breadth, 4 robust setae present on posterior margin; propodus anterior margin with 4 paired short robust setae and 2 apical robust setae.

Pereopod 7 basis expanded posteriorly with crenulate posterior margin, convex; merus slightly expanded posterodistally, anterior margin longer than breadth, 4 robust setae on posterior margin; propodus anterior margin with 4 paired short robust setae, the rest of the limb missing.

Gills and oostegites: Gills on pereopods 2–6, fully developed with extra lobe on P5 and P6; Oostegites on pereopods 2–5.

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

Epimeral plate 2 anteroventral corner rounded, ventral margin straight, posteroventral corner subangular. Epimeral plate 3 anteroventral corner rounded, ventral margin straight; posteroventral corner produced into a horizontal tooth.

Uropods: Uropod 1 peduncle longer than rami (1.5 times), outer margin with 5 robust setae, inner margin with 6 robust setae; outer ramus slightly longer than inner, with 4 robust setae on outer margin; inner ramus with 4 robust setae on outer margin.

Uropod 2 peduncle equal to rami, outer margin with 6 robust setae, inner margin with 1 apical robust seta; rami subequal; outer ramus with 4 robust setae on outer margin; inner ramus without constriction, with 3 robust setae on outer margin.

Uropod 3 peduncle equal to rami, with 1 medioapical robust seta and 2 apical robust setae on inner margin; outer ramus partly broken; inner ramus without robust setae.

Telson: Longer than broad (1.4 times), rectangular, weakly convex and narrowing distally, cleft 40%, incision deeper than broad, with 2 dorsolateral setae and 1–2 apical setae on each lobe.

Distribution

Circum-arctic; the most recent Russian survey of the distribution of arctic amphipods (Tzvetkova & Golikov 2001) recorded Onisimus barentsi from the Barents, Kara and Chukchi Seas, and its synonym O. krassini additionally from the Laptev and East Siberian Seas. Gurjanova (1962) reported this species also from the Bering and Ochotsk Seas, as well as from off Sakhalin and from the Japan Sea; for O. krassini she gave records from the same area, but also including Korea. The species has never been recorded from Japanese waters (Ishimaru 1994). In the western Palaearctic, there are records from Ungava Bay, Canada (Dunbar 1954) and Point Barrow, Alaska (Shoemaker 1955), as well as from the St Laurens estuary (cf Brunel et al. 1998). Dunbar (1954) concluded ‘It is probably a species of Pacific origin which has spread both east and west of the Bering Sea along the Siberian and North American coasts and it is to be expected that it will be taken in due course in Eastern Siberia and North America.’ This expectation has come true and Dunbar’s hypothesis seems still valid.

Biology

Little is known, although some Russian data may have been overlooked. Bryazgin (1997, sub O. krassini) collected the species from 94m on a clay and silt bottom, Gurjanova’s (1962) records from the NW Pacific are from 43–110m, but in the East-Siberian Sea the species has been collected from as shallow as 10m (Golikov & Tzvetkova 1994). Dunbar recorded the species at three stations, from 12–120m depths, and Brunel (1970, sub Paronesimus, Brunel et al. 1998) collected material in the St Lawrence estuary between 20 and 180 m.

This species has not been reported from baited traps.

Remarks

The only deviation from Lowry and Stoddart’s (1993) redescription of the holotype is the presence of 3 accessory setae on both the right and left mandible. Examination of the type material of O. krassini in the Zoological Museum of St Petersburg revealed no observable differences between the two nominal taxa. Onisimus krassini is hence considered as a junior synonym of O. barentsi.