Onisimus edwardsii Kroyer 1846

Onisimus edwardsii (Krøyer, 1846)

Anonyx edwardsii Krøyer 1846: 41

Onesimus Edwardsii Sars 1891: 105; Stephensen 1913: 121 Onisimus edwardsii Shoemaker 1930: 228; Lowry & Stoddart 1993:173. Onisimus edwardsi Gurjanova 1951: 162

Boeckosimus edwardsii Barnard 1969: 330

Type locality

Greenland.

Diagnosis

Epimeral plate 3 with slightly upturned tooth. Pereopod 1 propodus clearly longer than carpus, pereopods 3 and 4 propodus with posterior margin with paired robust setae and setae, P4 coxa with moderate posteroventral lobe. Telson cleft 30%.

Material examined

USNM Acc.no 182783, Indian Harbor, Labrador, 1 spm.

SMNH 6672 det: O. plautus, male, 9mm, Matoschkin scharr, Belusha viken, 6 spm. TMU xxx, Female, 10mm, Ekkerøy 2–3m; TMU 1168, male, 8 mm, Vardø; TMU 10025, Herwigshavna.

Distribution

Stephensen (1923) provided a distribution map of the Atlantic records of O. edwardsii; as in the case of O. plautus, there are also here a few old records from S. Norway and W. Sweden, where the species never has been refound, in spite of extensive collecting; there is even an old, vague record from the Kattegat, dating from Meinert (cf Stephensen 1923b). The most recent Norwegian catalogue (Vader et al. 1997) only has records from Tromsø (69*50’N) and north, and also here there are only a few reports. Otherwise this species is common and often numerous in the Atlantic Arctic (cf Stephensen 1923, 1935, 1944), and also along the entire arctic coast of Russia East to the Chukchi Sea (Gurjanova 1985, Tzvetkova & Golikov 2001) and in the cold eastern waters of Canada (Shoemaker 1930, 1930b, Dunbar 1954, Brunel et al. 1998) south to Nova Scotia. The species was not present in the extensive collections from Point Barrow, Alaska, however (Shoemaker 1955), nor does Gurjanova (1962) report on any Pacific material.

Biology

Onisimus edwardsii is a well-known general scavenger; it is easily and in large numbers collected in baited traps, and has also been found clustering on dead invertebrates in the field (Berge, pers. obs., Vader, pers. obs.).The observations and experiments of Legezynska (2001) have shown that O. edwardsii is an opportunistic predator and scavenger, with a wide range of prey animals. She also showed that this species avoids the very turbid waters of the inner fjord basins (where the much larger O. caricus is absolutely dominant), and only is found in the less turbid outer parts of the fjords. Life cycle data have been collected by Weslawski & Legezynska (2002). The animals appear to be K-strategists and winter breeders, and do not reproduce until they are one year old.

The species occurs mostly in very shallow waters in the high arctic, but somewhat lower down in low arctic areas: Stephensen (1944) noted occurrences from 6–36m in E. Greenland, Dunbar (1954) from 5 to 20 m in Ungava Bay, and Weslawski (1990) from 2–20m in the cold Hornsund area of Spitsbergen, with a clear maximum around 15 m (Legezynska et al. 2000). On the other hand, Bryazgin’s data from the Barents Sea noted depths of 30–70m, and also in the Gulf of St Lawrence the animals also occur in the infralittoral (Brunel et al. 1998, see also Shoemaker 1930a, b).