Onisimus turgidus

Onisimus turgidus (Sars, 1879)

Figs 4–10

Anonyx turgidus Sars, 1879: no 13.

Onisimus turgidus – Sars, 1885: 147, pl. xii, fig. 5, comb. nov. (described as sp. nov.). — Lowry & Stoddart 1993: 169. — Johnsen 2001: 60, fig. 21. — Zimina et al. 2019: 871.

Onisimus normani – Stephensen 1923: 48. — Shoemaker 1930: 11. — Vader 1975: 38. — Moore et al. 1994: 206. — Vader & Krapp-Schickel 1996: 65. — Bryazgin 1997: 99. — Lyubina et al. 2014: 246. (These are all not O. normani, but O. turgidus.)

NORWAY • 1 ♀, syntype (15 mm); Barents Sea; 72°53′ N, 21°51′ E; 408 m depth; 30 Jul. 1878; G.O. Sars leg.; stn 323 (trawl), Norwegian North sea Expedition; 1.5°C; silt.; F1764 (Figs 4–6) • 1 ♀ (10 mm); Kuberget Ullsfjorden; 100 m depth; Sep. 1974; Wim Vader leg.; from Actinostola callosa; TSZCr8573. (Figs 7–10) • 1 spec.; Ullsfjorden; from Actinostola callosa; TM IX-95 • 2 specs; Ullsfjorden; 69°39′48″ N, 19°45′18″ E; 106 m depth; 28 Apr. 1995; Wim Vader leg.; from Actinostola callosa; stn JohanRuud 255-95; TSZCr14488 • 21 specs; Ullsfjorden; Jun. 1995; Wim Vader leg.; from Actinostola callosa; stn Hyas 6-95; Arctic University Museum of Tromsø • 1 spec., Finnkroken; 50– 60 fathoms depth; 1900; J. Sparre Schneider leg.; TSZCr1171.

Johnsen (2001) studied samples of presumed Onisimus normani from different parts of the world, and concluded that they all (except the ones from S Norway) belonged in the turgidus species complex; unfortunately there was not sufficient material available to be able to revise this entire complex. The type material was collected from the Barents Sea and it has been collected from this area several times since (Bryazgin 1997; Lyubina et al. 2014, sub nomine O. normani; Zimina et al. 2019). Material from N Norway (Ullsfjorden) is quite similar to the type material, but the animals are smaller, 10 mm against 14 mm for the type. No fully adult or ovigerous specimens of O. turgidus have been found as yet, similar to the situation in O. normani. Onisimus turgidus also spends most of its life inside a sea anemone and like O. normani, likely leaves the host when almost adult.

Onisimus turgidus appears to be an obligate associate of sea anemones for most of its life-cycle. The preferred host of Onisimus turgidus in N Norway is Actinostola callosa. The numbers vary quite a lot from year to year (Moore et al. 1994; Vader & Krapp-Schickel 1996: table 2, sub nom. O. normani) but the species is never numerous even there: in 714 specimens of Actinostola Verrill, 1883 from Ullsfjorden 116 specimens of O. turgidus were found, an infection percentage of 0.16 amphipods per host. There is a significant positive correlation between the size of the Actinostola hosts and the number of Onisimus found inside: in the large sample of January 1974 sea anemones of 10–50 g wet weight contained 0.03 amphipods per host, those of 60–100 g 0.54 and the few of 100–150 g 3.00. Actinostola callosa in Ullsfjorden also is the host of another amphipod species, Stenothoe brevicornis Sars, 1883, that likewise feeds on host tissue, but this amphipod lives on the outside of the host, among the tentacles (Moore et al. 1994; Vader & Krapp-Schickel 1996).

Onisimus turgidus in Ullsfjorden is also sometimes found inside Bolocera tuediae, the almost exclusive host of O. normani in S Norway (Vader 1973), but here it is even less common than in A. callosa: in 297 specimens of Bolocera Gosse, 1860 from Ullsfjorden only 14 specimens of Onisimus were found (0.05 amphipods per host). The parasitic isopod Parapodascon sp. that is quite common on O. normani in S Norway, has also been found a few times on O. turgidus from the Ullsfjord.