биота российских вод японского моря - Materials of Alexey Shipunov

1. Thysanoessa inermis (Kroyer, 1846)(Pl. I, fig. 1; II, fig. 1)Kroyer, 1846: pl. 7, figs. 2,3 (Thysanopoda inermis, T. neglecta); G.O. Sars, 1883: 51–52(Euphausia inermis, Thysanoessa borealis); Hansen, 1911: 8; Lomakina, 1978: 183–185, figs.112, 113; Baker et al., 1990: p. 74, fig. 9b (Thysanoessa inermis).Description. Frontal plate of rostrum long, reaching end of first segment of antennularpeduncle, lanceolate, with pointed tip. Eyes without (forma inermis) or withconstriction; upper lobe narrower (f. neglecta). Carapace without lateral spine. Thoracopodsidentical in arrangement (f. inermis), or thoracopods 2 modified into graspingorgans (f. neglecta) due to elongated segments 4 and 5 and inflated basal segments.Distal 6th abdominal segment shorter than 2 penultimate segments (4 and 5) takentogether; its posterior end with small mid-dorsal spine pointed backward. Fifth abdominalsegment often with similar spine.Length of body in euphausiids from northern part of Sea of Japan up to 34 mm.Remarks. The studies of the North Atlantic euphausiid fauna showed that formaneglecta is a stage of T. inermis, represented by late larval stages and juveniles. Adultspecimens of f. neglecta are very rare and have not been recorded in the Northern Pacific,particularly in the Sea of Japan (Lomakina, 1978).Nemoto (1966) discovered a phenomenon of wedgewise change of the rate ofspecimens with spines at the end of fifth and sixth abdominal segments. The percentageof specimens with two spines decreases westward, from 75% in Alaska Bay to16% in the Sea of Okhotsk. He also reports single finds of specimens with spines onabdominal segments 6, 5, and 4.Distribution. T. inermis is a widespread boreal-arctic species. In the northwesternpart of the Pacific Ocean it has not been recorded south of 37˚ N in the Sea of Japanand the Pacific side of the South Kuril Islands. In the northeastern part of the PacificOcean the species has not been recorded south of 52˚ N. In the Atlantic Ocean itis found north of 39˚ N near the American coasts, in the La Manche and Skagerrakstraits. In the Arctic T. inermis is possibly circumpolar, found in all seas of Russia, inthe Beaufort Sea, in the east of the Canadian part of the Arctic, and around Greenland.In the Sea of Japan the southern boundary of the distributional range of this specieslies north of 37˚ N near the continent and 39˚ N near the Japan coast. In the TatarStrait it occurs to 51˚ N. The densest swarms of this species were observed in the easternhalf of the sea near the northern Hokkaido and the South Sakhalin in spring.Habitat and breeding. T. inermis is an epi- to mesopelagic species, in the Sea ofJapan found to depths around 1000 m (Vinogradov, 1968). Spring pre-breeding andbreeding assemblages begin to accumulate near the northern Hokkaido in the secondhalf of March, with maximum concentration in the near-surface layer. Then assemblagesmove northward, to the coastal zone of the southwestern Sakhalin up toIliynsky shoal (about 48˚ N). Phenological wave passes over the eastern part of the seanorthward from Hokkaido to 50˚ N from March to late May-early June. In Peter theGreat Bay mass breeding of T. inermis begins in the second to third decade of April.Diameter of egg capsules 600–925 µm, perivitelline space well developed (80–86% ofthe whole egg volume).Euphausiids of this species feed on meso- and microzooplankton, phytoplankton,and if food is scarce, on detritus and other euphausiids.84