Stenosemus albus (Linnaeus, 1767)

(Figures 4A, 11A–I, 12–18)

Chiton albus Linnaeus 1767: 1107 .

Chiton oryza Spengler 1797: 94 .

Chiton aselloides Lowe 1825: 103, pl. 5: fig. 5.

Chiton sagrinatus Couthouy 1838: 217 .

Chiton (Stenosemus) albus; Middendorff 1847: 120

Lophyrus albus (Linnaeus 1767) var. infuscatus Sparre Schneider 1881: 57 .

Ischnochiton (Trachydermon) albus Pilsbry 1892:70 (in part) (synonymy).

Trachydermon albus Linnaeus 1767 var. infuscatus Pilsbry 1893: 64 .

Lophyrochiton albus; Jakovleva 1952: 102, figs 6/A, 11/3, 53, pl. 10: figs 3a–b (bibliography and synonymy (in part).

Lophyrochiton albus var. infuscatus Jakovleva 1952: 103 (in part).

Ischnochiton albus; Matthews 1953: 244, 246, fig. 5; Muus 1959: 51, fig. 29; Knudsen 1970: 5; 394; Langer 1978: 274, figs. 4, 6–7.

Ischnochiton (Stenosemus) albus; Kaas 1985: 318, 319; Sirenko 1986: 125.

Stenosemus albus; Sirenko 1987: 40, fig. 9; 2001: 103.

Type material. Syntype, LSL.4, In the Linnean Society of London,

Type locality. “In O. Islandico ”.

Material examined. A total of about 500 spms:

Severnaja Zemlja. Golomjanniy Island, 79°30′N, 90°40′E, sample 11, 8.5 m, pebbles, 8 spms, BL 8.5–14 mm .

Franz Josef Land. Alexander Land, 20 m, stones, pebbles, muddy sand, 1 spm, BL 6 mm, 08.10.1970 ; Kheisa Island, sample 556, 1.5 m, stones, 1 spm, BL 20 mm, 16.10.1982 .

Spitsbergen Islands. Kongsfjord, stn. 2, 78°59′N, 11°57′E, 15 m, stones, pebbles, 3 spms, BL 6–12 mm, August 1996 ; 78°03′N, 14°33′E, 28–30 m, stones, pebbles, 3 spms, BL 3–12 mm, 07.09.1899; R/ V Polarstern, stn. 91, 80°02′N, 12°58′E, 66 m, stones, shells, 1 spm, BL 14 mm, 14 07.1991 .

Kara Sea. Novaja Zemlja, Zelanija Cape, Klokova Cape, 73°06′N, 56°35′E, 25 m, pebbles, 7 spms, BL 5–12 mm, 21.09.1927 ; 76°49′N, 69°50′E, 404 m, 1 spm, BL 10 mm, 22.09.1931; 76°18′N, 68°49′E, 90 m, stones, mud, 3 spms, BL 10–12 mm, 23.09.1934; R/V Akademik Mstislav Keldish, stn 4988, St. Anna Trough, 76°36′82’’N, 71°15′08’’E, 220–225 m, 2 spms, BL 13–17 mm, 24.09.2007 .

Barents Sea. Jarnishnaja Inlet, intertidal, stones, 110 spms, BL 3–14 mm, 15.07.1987 ; Nemetskij Island, intertidal, stones, 14 spms, BL 7–16 mm, 12.07.1987 ; Cheshskaja Inlet, 67°30′N, 46°30′E, 60 m, stones, mud, 2 spms, BL 5–13 mm, 1925; 75°13′N, 35°08′E, 178 m, stones, mud, 4 spms, BL 10–13 mm, 15.08.1929; R/V Persey, stn 4008, 69°07′N, 36°29′E, 50 m, stones,mud, 3 spms, BL 9–13 mm, 25.08.1936; R/ V Persey, Motovskiy Bay, stn 1824, 69°37′N, 32°14′E, 215 m, stones, mud, 1 spm, BL 10 mm, 12.06.1931 .

White Sea. 65°12′N, 34°47′E, 10 m, stones, 4 spms, BL 10–13 mm, August. 1981; 65°10′N, 35°03′E, 60 m, stones, 8 spms, BL 10–11 mm, August. 1981; 65°08′N, 35°20′E, 20 m, stones, 5 spms, BL 3–10 mm, August. 1981; 64°58′N, 35°26′E, 38 m, stones, 32 spms, BL 3–12 mm, August. 1981; 65°12′N, 34°47′E, 10 m, stones, 4 spms, BL 10–13 mm, August. 1981; 65°42′N, 39°44′E, 40 m, stones, 8 spms, BL 4–11 mm, August. 1981; 66°10′N, 40°15′E, 80 m, stones, 4 spms, BL 10–12 mm, July. 1981; 66°26′N, 36°36′E, 7 m, stones, 3 spms, BL 6–9 mm, July. 1981 .

Norwegian Sea. Storfjord, 9 m, 1 spm, BL 16,0 mm, 13.08.1899; R/V Maslov, stn. 281, 77°28′N, 12°01′E, 150 m, stones, mud, 2 spms, BL 10–13 mm, 09.09.1969; stn 222, 74°24′N, 23°30′E, 195 m, stones, mud, 2 spms, BL 9– 10 mm, 18.08.1969; stn 237, 74°30′N, 25°30′E, 300 m, stones, mud, 1 spm, BL 9 mm, 30.08.1969; R/V Persey, stn 215, 76°15′N, 14°16′E, 140 m, stones, mud, 4 spms, BL 10–13 mm, 27.09.1924; R/V Tunets, 67°30′N, 06°45′E, stn 43, 1400 m, 3 spms, BL 5–11 mm, 08.07.1978 . Tromsø, intertidal pool, 6 spms, BL 5.5–9.5 mm; Tromsø, intertidal pool, 2 spm, BL 6–7 mm, 19.10.1995 .

Newfoundland Bank. R/V Sevastopol, stn. 3194, 43°05′N, 64°08′E, 87–93 m, 2 spms, BL 10–11 mm, 27.08.1960; R/V Nereida stn. 84, 42. 8408 N, 50.1062 E, 41 m, 1 spm, BL 9.5 mm, 2008 .

Distribution. Arctic Ocean (Kara, Barents, White, Norwegian, Baffin, Labrador seas and Canadian Arctic), North Atlantic Ocean up to the Gulf of Maine and Azores Islands.

Diagnosis. Chiton of small size, elongate oval, shell moderately elevated and carinated, intermediate valves not beaked, surface of tegmentum microgranulose. Slit formula 13–14/1–3/10–16. Girdle dorsally covered with juxtaposed, bent round-topped spicules, all surface of dorsal spicules smooth or weakly longitudinally grooved in upper half of spicule, with about 10 short ribs on top. Marginal spicules of four forms like in S. magnidentis sp. nov. Central tooth of radula elongate, more or less rectangular slightly widened in basal part and with narrow, somewhat convex blade in distal part, first lateral teeth oar-shaped, head of major lateral tooth relatively narrow (ratio of length of head to width of head 3.03–4.07), bicuspidate, inner denticle long, narrow and sharply pointed, outer denticle very small, pointed, sometimes not developed. Color of tegmentum white often with concentric brown streaks. 16–23 gills per side, arranged from valve IV to valve VIII in specimens with body length of 7.5 to 17 mm accordingly.

Remarks. In addition to the above-mentioned variability in the number of gills, age-related variability is also noted in the slight enlargement of the head of the major radular tooth. At the same time, the head itself remains sharply pointed. The largest specimen (20 mm long) was collected near Franz Josef Land. Kaas and Van Belle (1990) reported the largest specimen with a body length of 22 mm. However, individuals with a body lengths greater than 16 mm are rare.

S. albus belongs to the group of widespread and numerous chitons on the shelf. This species inhabits mainly the intertidal zone to 300 m, but Kaas & Van Belle (1990) reported this species down to about almost 1000 m. It forms especially high densities of settlement (up to 240 spm/m 2) and biomass (up to 11 g /m 2) at a depth of 6–90 m in the White Sea. Judging from the temperatures at the northern and southern edges of its range, this species breeds at low temperatures ranging from negative temperatures close to 0°C to +5°C. In the Arctic Ocean (particularly off Franz Josef Land), the species inhabits constant subzero temperatures at which it apparently breeds. Females with mature gonads were usually observed in late summer (White Sea, Barents Sea). Mature eggs are covered with a hull with numerous appendages in the form of tubular outgrowths, like in S. magnidentis sp. nov. and S. sharpii .