Stenosemus magnidentis sp. nov.

(Figures 1, 4B, 6–10, 11 JO)

Trachydermon albus; Dall 1879: 295, 322; Krause 1885:256. (non Chiton albus Linnaeus, 1767).

Lepidochitona alba; Dall 1921: 188; Oldroyd 1927: 258. (non Chiton albus Linnaeus, 1767).

Ischnochiton albus; Pilsbry 1892: 70; G. & L. Burghardt 1969: 16; Abbott 1974: 394; Putman 1980: 68; Sirenko 1985: 349. (non Chiton albus Linnaeus, 1767, in part).

Lophyrochiton albus; Jakovleva 1952: 102, fig. 6A, 11, 3, pl. X, fig. 3. (non Chiton albus Linnaeus, 1767, in part).

Ischnochiton (Lepidopleuroides) albus; Sirenko 1979: 201. (non Chiton albus Linnaeus, 1767).

Ischnochiton (Stenosemus) albus; Kaas, Van Belle 1990: 60, fig. 24, map 2. (non Chiton albus Linnaeus, 1767, in part).

Stenosemus albus, Ferreira 1981: 327; Hanselman 1983: 30; Sirenko & Agapova 1997: 215, fig. 2G; Sirenko 2013: 148; Clark 2020: 297 (non Chiton albus Linnaeus, 1767).

Type material. Holotype BL 10.0 mm (ZIN 2592) and paratype add BL (ZIN 2593), now disarticulated consisting of SEM stub of 3 valves, part of perinotum and radula, mount of parts of perinotum and radula and vial with other valves. 20 paratypes, BL 3–10 mm, (ZIN 2594) and paratype, BL 10.0 mm (ZIN 2593).

Type locality. Bering Sea, east of the Navarin Cape, 62°05’N, 179°18’E, depth 72 m, (R/V Vitjaz, stn 583).

Material examined. A total of more than 400 specimens:

East Siberian Sea. Chauna Inlet, Pevek Cape, SCUBA, 12.0 m, stones, pebbles, muddy sand, 2 spms, BL 15–16 mm, 15.08.1986; 8.0 m, stones, pebbles, muddy sand, 2 spms, BL 15–16 mm, 15.08.1986;

Chukchi Sea. R/V Krasin stn. 53, 69°54′N, 171°09′W, 45 m, pebbles, sand, 2 spms, BL 12.0 mm, 15.09.1935; R/V Krasin stn. 26, 69°54′N, 171°09′W, 45 m, pebbles, sand, 2 spms, BL 12.0 mm, 15.09.1935; R/V Krasin stn. 59, 71°17′N, 175°54′W, 48 m, pebbles, sand, 3 spms, BL 9.0– 9.5 mm, 18.09.1935; R/V Professor Khromov, stn.SDI, 70°00′N, 169°59′W, 45 m, stones, sand, 2 spms, BL 14.0–15.0 mm, 11.09.2012;

Beaufort Sea. the Boulder Patch, 6 m, rocks, 1 spms, BL 11 mm, 24.07.1978, leg. Ken Dunton.

Bering Strait. R/V Professor Khromov, stn 8, 65°52.69′N, 169°05′W, 47 m, stones, pebbles, 21 spms, BL 7–10 mm, 10.08.2004 ; stn C517, 68°20.813′N, 167°10.123′W, 31 m, stones, pebbles, 1 spms, BL 7 mm, 01.09.2012 .

Bering Sea. R/V Dalnevostochnik; stn. 22, 65°52′N, 169°00′W, 47 m, stones, pebbles, 14 spms, BL 10–14 mm, 09.08.1932 R/V Dalnevostochnik; stn. 45, Gulf of Anadyr, 64°21′N, 178°46′W, 66 m, stones, pebbles, paratype, (ZIN 2593) BL 10 mm; 28.08.1932 ; R/V Dalnevostochnik; stn. 46, 64°22′N, 179°45′W, 46 m, stones, pebbles, 11 spms, BL 10 mm, 29.08.1932; R/V Dalnevostochnik; stn. 41, 64°15′N, 172°27′W, 25 m, stones, pebbles, 2 spms, BL 9–13 mm, 30.04.1932 R/V Dalnevostochnik; stn. 14, 64°16′N, 173°50′W, 55 m, stones, pebbles, 6 spms, BL 9–13 mm, 26.08.1932; near Plover Bay, SCUBA, 28–30.0 m, stones, pebbles, muddy sand, 3 spms, BL 13–17 mm, 06.09.1976 ; near Plover Bay, SCUBA, 8.0 m, pebbles, shells, muddy sand, 5 spms, BL 0.8–15.0 mm, 06.09.1976 ; near Plover Bay, SCUBA, 4.0 m, stones, 4 spms, BL 9.0–13.0 mm, 8.09.1976 ; near Plover Bay, SCUBA, 22–23 m, stones, pebbles, 2 spms, BL 6–13 mm, 10.09.1976 ; R/V Vitjaz, stn. 528, 54°46′N, 164°08′E, 120 m, stones, pebbles, 5 spms, BL 8–11 mm, 19.08.1950; R/V Vitjaz, stn. 550, 64°02′N, 172°57′W, 53 m, stones, pebbles, 1 spm, BL 9 mm, 29.08.1950; R/V Vitjaz, stn. 554, 64°14′N, 173°39′W, 38 m, stones, pebbles, 9 spms, BL 6–14 mm, 30.08.1950; R/V Vitjaz, stn. 583, 62°05′N, 179°18′E, 72 m, stones, pebbles, holotype (ZIN 2592, BL 10 mm, and 20 paratypes (ZIN 2594), BL 3–10 mm, 11.09.1950; R/V Vitjaz, stn. 605, 59°53′N, 179°18′E, 195 m, stones, pebbles, 5 spms, BL 3–10 mm, 11.09.1950; R/V Vitjaz, stn. 609, 59°50′N, 170°18′E, 75 m, stones, pebbles, sand, 4 spms, BL 6–11 mm, 18.09.1950; R/V Vitjaz, stn. 1459, 60°55′N, 172°26′E, 60 m, stones, pebbles, 16 spms, BL 6–11 mm, 07.06.1982; R/V Vitjaz, stn. 6130, 53°49′N, 163°40′W, 176–210 m, stones, pebbles, 18 spms, BL 6–13 mm, 05.06.1969; R/V Akademik Shirshov, cruise 27, stn. 11, 63°59′N, 173°29′E, 59 m, stones, pebbles, mud, 14 spms, BL 2–14 mm, 16.06.1987;

Commander Islands. Bering Island, dredge, 80 m, stones, 31 spms, BL 4–9 mm, 15.09.1973 ; Bering Island, dredge, 110–140 m, stones, 9 spms, BL 3–9 mm, 17.09.1973 ;, Bering Island, dredge, 100 m, stones, 13 spms, BL 4–10 mm, 15.09.1973 ; Bering Island, dredge, 100 m, stones, 22 spms, BL 3–9 mm, 03.09.1973 ; R/V Akademik Keldish, stn 2293, 55°05′N, 165°54′E, 92– 90 m, pebbles, sand,9 spms, BL 5.0–8.0 mm, 25.07.1990 ;

Kurile Islands. Paramushir Islands, R/V Lebed, stn 136, 50°24′N, 156°46′W, 118 m, pebbles, sand,1 spm, BL 5.0 mm, 03.08.1954 ; Shumshu Island, R/V Lebed, stn 41, 50°24′N, 157°13′W, 190 m, pebbles, muddy sand, 1 spms, BL 7.0 mm, 03.07.1954 ;

Sea of Okhotsk. R/V Vitjaz, stn. 1715, 48°45′N, 145°05′E, 152 m, stones, pebbles, 1 spm, BL 7 mm, 21.09.1952 ; R/V Vitjaz, stn. 1799, 58°02′N, 156°24′E, 104 m, stones, pebbles, 2 spms, BL 15 mm, 06.10.1952 ; R/V Vitjaz, stn. 11864, 59°01′N, 142°35′E, 62 m, stones, pebbles, 7 spms, BL 3–10 mm, 20.10.1952 ; R/V Professor Probatov, stn 62, 57°15′N, 155°37′E, 118 m, pebbles, sand, 2 spms, BL 3.0–5.0 mm, 05.07.2013 ; R/V Professor Probatov, stn 73, 58°08′N, 156°02′E, 145 m, pebbles, sand, 1 spm, BL 3.5.0 mm, 03.07.2015 ; Shantar Islands, Large Shantar Island, Jakshin Inlet, sample 225, 14 m, pebbles, muddy sand, 5 spms, BL 3–5 mm, 27.08.1978 ; Sakhalin Bay, sample 217, 52 m, muddy sand, pebbles, 11 spms, BL 8–12 mm, 27.08.1978 ; West Kamchatka, 57°15′N, 155°37′E, 118 m, stones, pebbles, 2 spms, BL 3–5 mm, 05.07.2014 .

Sea of Japan. R/V Atna, Egorov Cape, stn. 105, 44°40′N, 136°28′E, 100 m, stones, pebbles, mud, 2 spms, BL 5–13 mm, 20.08.1975 ; Bolshova Cape, stn. 110, 45°31′N, 137°23′E, 60 m, stones, pebbles, mud, 2 spms, BL 10–11 mm, 21.08.1975; Giljak Cape, stn. 156, 46°49′N, 138°40′E, 100 m, large pebbles, mud, 5 spms, BL 13–14 mm, 31.08.1975 Sosunova Cape, stn. 160, 46°30′N, 138°25′E, 70 m, pebbles, mud, 3 spms, BL 9–13 mm, 29.09.1975; Sosunova Cape, stn. 162, 46°27′N, 138°30′E, 120 m, pebbles, mud, 3 spms, BL 11–12 mm, 29.09.1975; Mosolova Cape, stn. 172, 45°03′N, 136°46′E, 70 m, stones, sand, pebbles, mud, 2 spms, BL 13–14 mm, 02.09.1975; Kudrina Cape, stn. 178, 43°25′N, 135°03′E, 70 m, pebbles, mud, 4 spms, BL 7–15 mm, 25.09.1975; stn. 110, 45°31′N, 137°23′E, 60 m, stones, pebbles, mud, 2 spms, BL 10–11 mm, 21.08.1975; Sajon Cape, stn. 116, 46°54′N, 138°02′E, 110 m, pebbles, mud, 6 spms, BL 10–14 mm, 30.09.1975; Sajon Cape, stn. 117, 45°54′N, 138°05′E, 130 m, pebbles, mud, 4 spms, BL 10–12 mm, 30.09.1975; Shternberg Cape, stn. 352, 50 m, pebbles, muddy sand, 2 spms, BL 10 mm, 03.08.1977; R/V Akademik Oparin, cruise 64, stn. 65, 44°33′N, 136°14′E, 65–72 m, 1 spms, BL 12 mm, 06.07.2021; R/V Akademik Oparin, cruise 64, stn. 86, 43°10′N, 134°41′E, 71–75 m, 3 spms, BL 8–12 mm, 11.07.2021 .

Pacific coast of north America. near Prince of Wales Island R/V Vitjaz, stn. 6126, 55°24′N, 134°40′E, 530 m, stones, pebbles, 1 spms, BL 7 mm, 22.05.1969 .

Etymology. Latin magnus =large and dentis =tooth), referring to large major lateral tooth of the radula of this species.

Distribution. North Pacific Ocean (Japan, Okhotsk and Bering seas, coast of North America from Chukchi Sea up to Queen Charlotte Islands), Arctic Ocean (Beaufort, Chukchi and East-Siberian seas), at depths from 6 to 385 m, mainly on pebble with sand, sometimes with shelly, silty substrates, and less frequently on rocky substrates.

Diagnosis. Chitons of small size up to 16.0, moderately elevated, subcarinated not beaked, ratio of width of head valve to width of tail valve 1.11, mucro antemedian, ratio of length of postmucronal area to length of antemucronal area 1.16, posterior slope nearly straight. apophyses broadly triangular with rounded top, slit formula 13/1/12. Girdle narrow, dorsally covered with elongate, curved, smooth, juxtaposed dactylic spicules with 6–8 short ribs on top. Radula has about 60 transverse rows of mature teeth. Central tooth pyriform wide near the base, first lateral tooth long oarshaped, head of major lateral tooth wide (ratio of length of head to width of head 2.60–2.70), bicuspidate, outer denticle very small but sharply pointed,

Description. Holotype, BL10.0 mm, body valves thin, moderately elevated (dorsal elevation 0.36), subcarinated, not beaked, side slopes slightly convex. Color of valves white.

Head valve semicircular, posterior margin widely V-shaped, notched in the middle. Intermediate valves broadly rectangular, front margin of valves especially of valve II concave at both sides of the strongly forwardly produced jugal part, side margins somewhat rounded, hind margin straight, lateral areas scarcely raised. Tail valve noticeably smaller than head valve (ratio of width of head valve to width of tail valve 1.11), mucro decidedly antemedian (ratio of length of postmucronal area to length of antemucronal area 1.16), posterior slope nearly straight.

Tegmentum smooth to the naked eye and microgranulose under magnification.

Articulamentum thin, translucent, white, apophyses broadly triangular with rounded top, connected across the wide sinus by a short jugal plate, insertion plates short, slit formula 13/1/12, teeth short, sharp, slit rays distinct, eaves narrow and solid.

Girdle relatively narrow, about 0.4 mm in width near valve V, covered dorsally with elongate, curved, more or less conical, smooth, juxtaposed, dactylic spicules with 6–8 short ribs on top, 180–230 x 80 μm on midgirdle, 70 x 40 μm near margin. Marginal spicules are of several types: narrow spicules 80 x 10 μm on top of short bristles, small spicules 38 x 8 μm on short chitinous bristles, long, bluntly pointed spicules 100 –150 x 20–24 μm with 5–6 obsolete ribs, flattened triangular scales 95 x 30 μm with 5–6 thin, oblique ribs, one row of pointed scales 60 x 30 µm with 3–4 short ribs on the inner side distally. Ventrally the girdle is covered with rows of elongate, rectangular, narrow, straight scales up to 80 x 25 μm.

Radula of the holotype is 3.3 mm long and has 61 transverse rows of mature teeth. Central tooth pyriform wide near the base, first lateral tooth long oarshaped, head of major lateral tooth wide (ratio of length of head to width of head 2.60–2.70), bicuspidate, outer denticle very small but sharply pointed, inner denticle wide, with an inwardly sloping process at the distal end.

Holotype with 16 gills on each side extending from valve III to valve VIII. Studied specimen with BL 12 mm has 20 gills, nephridiopore situated between 6 and 7 gills, gonopore situated between7 and 8 gills counted from the last one.

Gut contains mainly detritus and sand.

Remarks. Studies of specimens of the new species, ranging in size from 9.0 to 16.0 mm and collected from various regions of the Pacific and Polar Oceans, including the East Siberian Sea to the Sea of Japan, revealed some variability. In Anadyr Bay, individuals were found with dorsal spicules having poorly developed longitudinal ribs in the upper half. Additionally, the width and length of the central tooth and the head of the large lateral tooth of the radula showed small variations.

Some specimens of new species with more or less intense dark brown deposits along hind margin of the valves.

As noted in the introduction, the new species belongs to a group within this genus characterized by the absence of macrosculpture on the shell valves. Of the six species in this group, three ( S. albus, S. sharpii, and S. magnidentis sp. nov.) have a similar structure of radulae, while the other three species ( S. simplicissimus, S. golikovi and S. kolesnikovi) exhibit a markedly different morphology. The latter group is distinguished by a large, rectangular central radula tooth, short wing-shaped first lateral teeth, and a short, unidentate head on the major lateral tooth. In contrast, the former group is characterized by a small, pyriform central tooth, oar-shaped first lateral teeth, and relatively long, bidentate heads on the major lateral teeth.The new species is most similar to S. sharpii and the wide spread Stenosemus albus and differs from the second in having a wider head of major lateral tooth of radula (ratio of length of head of major lateral tooth to its width 2.60–2.70) (vs. 3.17–4.06 in S. albus); central tooth pyriform (vs. more or less rectangular in S. albus); inner denticle of head of major lateral tooth wide with an inwardly sloping process at distal end: (vs. inner denticle long, narrow and sharply pointed in S. albus); outer denticle always clearly marked (vs. weakly developed, sometimes is nearly absent in S. albus); the body shape is broader; the proximal half of the central tooth of the radula is noticeably broader than in S. albus .

S. magnidentis sp. nov. differs from S. sharpii by having a sharper top of the head of major lateral tooth of radula with an inwardly sloping process at the distal end (vs. blunt tipped in S. sharpii); narrower proximal part of the central teeth (vs. very wide proximal part in S. sharpii); smooth surface of dorsal spicules or rare poorly developed longitudinal ribs in the upper half of dorsal spicule, (vs. distinct longitudinal ribs in the upper half of dorsal spicules in S. sharpii).

According to our data, the vertical distribution of this widespread species depends on the location of cold water masses; therefore, the range of depths where chitons settle differs in different parts of their range. In the Sea of Okhotsk at depths of 6–385 m and in the Sea of Japan at depths of 40– 200 m. According to the samples taken VanVeen grab (0.25 m 2) the average settlement density in most areas is close to 7–8 spms/m 2.

Judging by temperatures at the northern and southern edges of its range, this species reproduces at low temperatures, ranging from below freezing (close to 0°C) to +5°C. Females with mature gonads were typically observed in late summer (in the Sea of Okhotsk and Sea of Japan) and in autumn (in northwestern Kamchatka and the northern Kuril Islands). Mature eggs are covered with a hull equipped with numerous appendages in the form of tubular outgrowths up to 200 µm long, tapering toward the distal end. The distal ends are often bent into a hook shape, which apparently allows the eggs to clump together and attach to underwater objects.

In some areas of its range, such as the Tatar Strait and the northeastern coast of Kamchatka, nearly 30% of the population is infected by the parasitic copepod Tesonesma reniformis Avdeev & Sirenko, 1994 . Mention here also as a consequence, that the basiobiont of this copepod is no longer S. albus . Accordingly Schwabe, et al. (2018) they observed this parasite in the N Atlantic also. This parasite resides in the body cavity of the chiton and occupies the space of the host’s gonads, producing a massive number of its own eggs there. The largest specimen of these chitons we collected at a depth of 105 m in the Sea of Okhotsk off northern Sakhalin is 22 mm long and 11 mm wide. Most chitons, judging by growth rings, live no more than 11–12 years, reaching 13–16 mm in length by this age.

The new species feeds mainly on detritus. The digestive tract often contains foraminifera, sand, and small amounts of diatom algae (own observations).