Belknapchiton belknapi (Dall, 1878) Sirenko & Saito & Schwabe, 2022

Sirenko, Boris, Saito, Hiroshi & Schwabe, Enrico, 2022, A redescription of Leptochiton belknapi Dall, 1878 (Mollusca: Polyplacophora Leptochitonidae), the type species of the new genus Belknapchiton, Zootaxa 5205 (2), pp. 101-124 : 104-118

Belknapchiton belknapi (Dall, 1878) n. comb.

( Figs 1–15 )

Leptochiton belknapi Dall 1878a: 1 ; 1878b: 99 (in part); 1879: 317 (in part); 1882: 411 (in part); Smith 1947a: 4; 1947b: 17; Ferreira 1979: 152, 153, 156, fig. 10; Wu & Okutani 1984: 8, pl. 1: figs 5–6, pl. 4: figs 2–10, pl. 5: figs 5–9, pl. 6: figs 1–3 (in part, see below); Kaas 1985a: 305; 1985b: 335; Kaas & Van Belle 1987: 23, fig. 10; 1998: 29; Skoglund 1989: 79 (in part); 1990: 28 (in part); 2001: 3 (in part); Bank 1994: 15 (in part); Sirenko 1994: 162; 2013: 148; 2015: 154–155 ; Sirenko & Agapova 1997: 213, fig. 2B; Kaiser 1997: 47; Schwabe 2008: 213 (in part); Sigwart 2008a: 45; Schwabe & Sellanes 2010: 33, figs 1L–M; Sigwart et al. 2014: 7, fig. 5B; Bertolaso et al. 2015: 10; Sirenko & Sellanes 2016: 262 , figs 1A, B, 2–4; Drumm et al. 2016: 88 (in part); Reyes-Gómez 2016: 5 (in part), fig. 4.

Lepidopleurus mesogonus Dall 1902: 555 ; 1908: 354; 1921: 187; Oldroyd 1927: 250; Clarke 1962: 5; Abbott 1974: 393; Kaas & Van Belle 1998: 123.

Lepidopleurus farallonis Dall 1902: 557 ; 1921: 187; Oldroyd 1927: 250; Clarke 1962: 5; Thorpe 1971: 882; Abbott 1974: 393; Scott et al. 1990: 4; Kaas & Van Belle 1998: 71.

Lepidopleurus opacus Dall 1908: 354 ; Clarke 1962: 5; Thorpe 1971: 882; Scott et al. 1990: 4; Kaas & Van Belle 1998: 135.

Lepidopleurus alveolus ; Jakovleva 1952: 57, fig. 18, pl. 1, fig. 6 (in part, non Chiton alveolus M. Sars MS, Lovén, 1846 ).

Lepidopleurus japonicus ; Jakovleva 1952: 56, fig. 17, pl. 1, fig. 5 (non Lepidopleurus japonicus Thiele, 1909 )

Lepidopleurus belknapi ; Abbott 1974: 393

Leptochiton alveolus ; Ferreira 1979:152, fig. 10 (in part, non Chiton alveolus M. Sars MS, Lovén, 1846 ); Kaas & Van Belle 1985: 36, fig. 14, map 6 (in part, non Chiton alveolus M. Sars MS, Lovén, 1846 ); Squires & Goedert 1995: 49, figs 3–6 (in part, non Chiton alveolus M. Sars MS, Lovén, 1846 ).

Leptochiton farallonis ; Smith 1947a: 5; 1947b: 17; Burghardt & Burghardt 1969: 24; Kaas & Van Belle 1980: 45.

Leptochiton mesogonus ; Smith 1947a: 5; 1947b: 17; Burghardt & Burghardt 1969: 25; Kaas & Van Belle 1980: 84.

Leptochiton opacus ; Smith & Ferreira 1977: 84, figs 3–4; Kaas & Van Belle 1980: 93.

Leptochiton sp. Sellanes et al. 2004: 1066.

Leptochiton cf. belknapi ; Avdeev & Sirenko 2005: 527, 537, 540; Stebbins & Eernisse 2009: 55; Schwabe et al. 2014: 172, 176, 180.

non: Leptochiton belknapi ; Kaas 1991: 10 (in part) (= Connexochiton kaasi Saito, 1997 fide Sirenko 2021)

Excluded here is the Challenger Expedition material (see Discussion) from Luzon, referred to as:

Leptochiton belknapi ; Haddon 1886: 10, pl. 1, fig. 2, pl. 2, figs 2a–d; Burghardt & Burghardt 1969: 23; 1972: 79

Lepidopleurus belknapi ; Pilsbry 1892: 7, pl. 1, figs 18–22; Nierstrasz 1905: 4, 5, 10, 110; Maluquer 1915: 208; Dall 1921: 187; Oldroyd 1927: 8, 249; Taki 1940: 489; Clarke 1962: 5; Kaas & Van Belle 1980: 15

Type material. Holotype ( USNM 30972 ).

Type locality. Off western Aleutian Islands, 53°08’N, 171°19’W, 1840 m depth .

Material examined. Total 344 spms. Holotype; Japan, northeastern Honshu , RV Vitjaz , st. 667, 38°40.1’N, 143°29.3’E, 3012 m, Sigsbee trawl, 2 spms, BL 32.0–33.0 mm, 10.05.1957; st. 6671, 40°12’N, 143°35.8’E, 2500 m, Sigsbee trawl, 60 spms, BL 9.0–29.0 mm, 23.06.1972; RV Soyo-Maru, st. R2 , 37°29.0’N, 142°37.4’E, 1730– 1740 m, Agassiz trawl, 2 spms (NSMT-Mo 60003), BL c. 25–40 mm, 22.06.1977; Izu-Ogasawara Islands, RV Soyo-Maru, st. B2, 34°04.0’N, 139°54.9’E, 1230–1340 m, Agassiz trawl, 13 spms (NSMT-Mo 59993), BL 4.7– 12.7. mm, 09.03.1959; st. B4, 31°58.1’N, 140°21.0’E, 2230–2245 m, Agassiz trawl, 1 spm (NSMT-Mo 59994), BL c. 20 mm, 12.07.1964; st. B2, 34°04.0’N, 140°01.5’E, 1270–1280 m, Agassiz trawl, 3 spms (NSMT-Mo 59995), BL 11.3–21.7. mm, 09.03.1965; st. B5, 30°37.5’N, 140°31.0’E, 1660–1830 m, Agassiz trawl, 1 spm (NSMT-Mo 59997), BL 15.8. mm, 05.12.1967; st. B2, 33°10.0’N, 140°06.1’E, 1410–1450 m, Agassiz trawl, 3 spms (NSMT-Mo 59998), BL 10.7–19.2 mm, 05.07.1968; st. B5, 30°37.0’N, 140°40.5’E, 2140 m, Agassiz trawl, 2 spms (NSMT-Mo 59999), BL 12.2–15.3 mm, 03.07.1969; st. B2, 34°08.0’N, 139°58.0’E, 1250 m, Agassiz trawl, 2 spms (NSMT-Mo 60000), BL 7.9–17.0 mm, 10.11.1971; st. B2, 34°06.5’N, 139°59.0’E, 1350 m, Agassiz trawl, 5 spms (NSMT-Mo 60001), BL 14.2–20.3 mm, 08.06.1973; st. B2, 33°58.5’N, 140°24.3’E, 1560 m, Agassiz trawl, 2 spms (NSMT-Mo 60002), BL 13.0–c. 25 mm, 28.07.1976; Kurile Islands and southeastern Kamchatka, RV Toporok, st. 17, 43°32.5’N, 147°20.5’E, 1450–1530 m, 4 spms, BL 11.0–42.0 mm, 28.08.1948 ; RV Vitjaz, st. 5638, 44°36’N, 149°07’E, 1710– 1880 m, Galathea trawl, 2 spms, BL 25.0–32.0 mm, 10.09.1966 ; RV Tikhookeansky, st. 7, sample 19, 43°25’11’’N, 146°04’11’’E, 490 m, Ocean grab, 1 spm, BL 3.2 mm, 08.07.1987 ; st. 107, sample 301, 44°44.3’N, 146°59.1’E, 700 m, dredge, 4 spms, BL 23.0–30.0 mm, 26.07.1987; RV Vitjaz, st. 3303, 53°54.2’N, 160°42.7’E, 684 m, Sigsbee trawl, 1 spm, BL 10.0 mm, 24.05.1955 ; st. 3304, 53°54.2’N, 160°42.7’E, 1000 m, Ocean grab, 1 spm, BL 28.0 mm, 24.05.1955; RV Akademik Mstislav Keldish, st. 2325, 53°27.7’N, 160°59.3’E, 3106– 2992 m, Sigsbee trawl, 1 spm, BL 25.0 mm, 12.08.1990 ; st. 2328, 53°26.59’N, 160°21.0’E, 1814–1920 m, dredge, 5 spms, BL 20.0–22.0 mm, 12.08.1990; Sea of Okhotsk , RV Vitjaz, st. 1853, 55°38’N, 143°0’E, 420 m, Sigsbee trawl, 1 spm, BL 24.0 mm, 18.10.1952 ; st. 1854, 55°49’N, 143°4’E, 590 m, Ocean grab, 1 spm, BL 15.0 mm, 18.10.1952; st. 861, 49°46.5’N, 155°04’E, 390 m, 1 spm, BL 10.0 mm, 24.06.1951; RV Partizansk, st. 43, 49°20’N, 146°18’E, 370 m, dredge, 1 spm, BL 29.0 mm, 06.08.1978 ; Commander Islands , RV Vitjaz, st. 626, 55°37’N, 164°36.7’E, 2440 m, Sigsbee trawl, 4 spms, BL 8.0–14.0 mm, 28.09.1950 ; st. 3353, 53°53.5’N, 169°15.15’E, 1600 m, Sigsbee trawl, 1 spm, BL 7.0 mm, 05.06.1955; RV Rakitnoe, st. 174, sample 409, 55°17’6’’N, 166°37’8’’E, 150– 130 m, 3 spms, BL 22.0–31.0 mm, 21.09.1973 ; st. 183, sample 418, 55°08’0’’N, 165°08’3’’E, 250– 130 m, 3 spm, BL 19.0 mm, 22.09.1973; st. 192, sample 454, 55°05’5’’N, 165°48’0’’E, 150–200 m, 3 spms, BL 19.0–20.0 mm, 23.09.1973; st. 224, sample 582, 54°36’8’’N, 167°21’5’’E, 130–250 m, 22 spms, BL 6.0–21.0 mm, 02.10.1973; st. 229, sample 587, 54°29’3’’N, 167°31’1’’E, 150–200 m, 31 spms, BL 4.5–23.0 mm, 03.10.1973; st. 240, sample 598, 55°11’7’’N, 165°34’0’’E, 130–250 m, 7 spms, BL 13.0–25.0 mm, 05.10.1973; st. 241, sample 599, 55°12’5’’N, 165°34’5’’E, 100 m, 1 spm, BL 10.0 mm, 05.10.1973; st. 245, sample 603, 55°25’3’’N, 165°58’0’’E, 150–300 m, 10 spms, BL 7.0–9.0 mm, 06.10.1973; RV Akademik Mstislav Keldish, st. 2306, 54°57.1’N, 165°49.9’E, 4805– 3724 m, Sigsbee trawl, 2 spms, BL 6.0– 8.5 mm, 29.07.1990 ; st. 2310, 55°23.38’N, 167°15.7’E, 580–750 m, dredge, 1 spm, BL 2.0 mm, 01.08.1990; st. 2312, 55°25.16’N, 167°15.15’E, 470– 375 m, dredge, 1 spm, BL 6.0 mm, 02.08.1990; st. 2321, 55°23.3’N, 167°18.8’E, 1490–1554 m, dredge, 2 spms, BL 13.0–14.0 mm, 08.08.1990; RV Akademik Oparin st. 4, 54°11’8’’N, 168°36’5’’E, 508 m, 3 spms, BL 23.0–31.0 mm, 02.08.1991 ; st. 5, 54°12’0’’N, 168°37’3’’E, 569 m, 2 spms, BL 22.0– 22.5 mm, 02.08.1991; st. 7, 54°13’5’’N, 168°43’3’’E, 386 m, 1 spm, BL 13.0 mm, 02.08.1991; st. 43, 55°36’9’’N, 164°54’0’’E, 350–523 m, 3 spms, BL 10.0–15.0 mm, 24.08.1991; st. 44, 55°35’4’’N, 165°00’4’’E, 205 m, 2 spms, BL 15.0–22.0 mm, 24.08.1991; st. 51, 55°25’3’’N, 167°47’3’’E, 372 m, 1 spm, BL 10.0, 25.08.1991; Bering Sea, RV Vitjaz st. 542, 60°02’N, 179°48’E, 1440 m, Sigsbee trawl, 1 spm, BL 19.0 mm, 27.08.1950 ; st. 608, 59°14’5’’N, 170°30’E, 510 m, Sigsbee trawl, 1 spm, BL 6.0 mm, 20.09.1950; st. 626, 55°37’N, 164°36’7’’E, 2440 m, Sigsbee trawl, 1 spm, BL 20.0 mm, 27.08.1950; RV Pervenets, st. 125, 61°43’3’’N, 177°32’0’’E, 450 m, Ocean grab, 1 spm, BL 20.0 mm, 15.08.1959 ; st. 239, 56°36’0’’N, 172°18’1’’E, 203 m, 1 spm, BL 15.0 mm, 15.09.1959; st. 286, 55°36’5’’N, 165°30’0’’E, 435 m, 2 spms, BL 4.0–16.0 mm, 20.09.1959; RV Ametist st. 127, 53°43’N, 164°50’W, 191 m, Ocean grab, 1 spm, BL 8.0 mm, 28.08.1960 ; st. 162, 55°45’N, 165°34’W, 227 m, Ocean grab, 1 spm, BL 10.0 mm, 07.09.1960; st. 173, 56°41’3’’N, 163°57’5’’W, 294 m, Ocean grab, 1 spm, BL 10.0 mm, 09.09.1960; st. 175, 56°55’0’’N, 163°45’0’’W, 294 m, Ocean grab, 1 spm, BL 10.0 mm, 09.09.1960; st. 181, 56°38’0’’N, 165°09’W, 215 m, Ocean grab, 1 spm, BL 15.0 mm, 10.09.1960; st. 213, 57°01’8’’N, 168°20’0’’W, 408 m, Ocean grab, 4 spm, BL 7.0–9.0 mm, 15.09.1960; RV Zemtchug st. 105, 58°26’0’’N, 174°41’0’’E, 2083 m, Ocean grab, 1 spm, BL 17.0 mm, 03.10.1963 ; st. 221, 61°35’0’’N, 177°22’0’’E, 1240 m, Ocean grab, 1 spm, BL 10.0 mm, 02.10.1963; st. 226, 61°47’0’’N, 177°28’0’’E, 346 m, Ocean grab, 1 spm, BL 21.0 mm, 03.08.1963; RV Akademik Shirshov st. 22, 57°31’4’’N, 174°02’0’’E, 1320 m, Sigsbee trawl, 1 spm, BL 13.0 mm, 21.06.1981 ; to S from Navarin Cape , 60°41’N, 179°11’E, 550–560 m, 1 spm, BL 26.0 mm, 29.09.1996 ; RV Professor Khromov dredge N4, 55°23’7’’N, 167°18’0’’E, 480–660 m, 15 spms, BL 7.0–13.0 mm, 01.08.2004 ; RV Akademik Lavrentyev st. LV82–21, 60°8343’N, 174°3722’E, 660 m, 1 spm, BL 17.0 mm, 03.07.2018 ; East Aleutian Islands, RV Vityaz st. 6130, 53°47’5’’N, 163°50’0’’W, 250 m, Ocean grab, 1 spm, BL 12.0 mm, 05.06.1969 ; st. 6131, 53°47’5’’N, 163°51’0’’W, 510–700 m, Sigsbee trawl, 4 spms, BL 15.0–20.0 mm, 05.06.1969; st. 6135, 53°32’N, 163°22’W, 2880–2930 m, Sigsbee trawl, 3 spms, BL 8.0–13.0 mm, 09.06.1969; Gulf of Alaska, RV Ametist st. 115, 57°26’5’’N, 150°18’0’’W, 182 m, Ocean grab, 1 spm, BL 7.0 mm, 24.08.1960 ; st. 117, 56°55’0’’N, 150°45’W, 181 m, Ocean grab, 3 spms, BL 7.0–10.0 mm, 25.08.1960; st. 137, 54°51’1’’N, 157°32’W, 392 m, Ocean grab, 2 spms, BL 10.0– 11.0 mm, 31.08.1960; RV Vitjaz, st. 6089, 58°02’0’’N, 149°02’W, 180 m, Ocean grab, 3 spms, BL 5.0–8.0 mm, 1969; st. 6090, 58°01’7’’N, 149°01’8’’W, 455 m, Ocean grab, 4 spms, BL 7.0–11.0 mm, 1969; st. 6122, 56°45’5’’N, 136°02’0’’W, 1000–1180 m, Sigsbee trawl, 7 spms, BL 10.0–13.0 mm, 21.05.1969 ; st. 6123, 56°45’0’’N, 136°03’0’’W, 650 m, Ocean grab, 2 spms, BL 2.0–10.0 mm, 21.05.1969; st. 6125, 55°23’7’’N, 134°46’0’’W, 550 m, Ocean grab, 1 spm, BL 7.0 mm, 22.05.1969; British Columbia, RV Vitjaz, st. 4139, 54°24’N, 133°49’7’’W, 290 m, Ocean grab, 1 spm, BL 5.5 mm, 1958; Cowan collection, N1270, 52°49’N, 132°52’W, 1601 m, mud & boulders, 1 spm, BL 12.0 mm, August, 1964, col. F. Bernard; Oregon, RV Vitjaz, st. 4177, 44°51’7’’N, 125°04’0’’W, 650 m, Ocean grab, 1 spm, BL 10.0 mm, 1958; st. 4179, 44°40’N, 124°59’6’’W, 1233–1258 m, Sigsbee trawl, 30 spms, BL 11.0–17.0 mm, 04.12.1958 ; California, RV Albatross, st. 4423, between Santa Barbara and San Nicolas, 389–610 m, 1 spm, BL 20.0 mm, USNM 218730 , 1913 ; South America , RV Akademik Kurchatov, st. 217, 3°49’0’’N, 82°47’W, 2800 m, Sigsbee trawl, 1 spm, BL 20.0 mm, 27.08.1968 ; st. 241, 23°50’S, 70°56’6’’W, 2900 m, Sigsbee trawl, 2 spm,, BL 20.0 mm, 15.09.1968; RV AGOR Vidal Gormáz, st. VG07-7, AGT-8, 36.0023°– 36.0058°S, 73.3841°– 73.3706°W, 922–1013 m, 2 spms ( ZISP 2274 ), BL 18.0–20.0 mm, 07.10.2007 ; RV Melville st. INSPIRE, AGT-3_533, 36.22523°S, 73.43115°W, 724 m, 4 spms ( SCBUCN 4136 ) BL 12.0–30.0 mm, 09.03.2010 .

Distribution. Widespread species, inhabits the Pacific Ocean from off Pacific side of Hokkaido (38°40.1’N, 143°29.3’E), the Sea of Okhotsk and Bering Sea and North and South America’s Pacific waters southwards to 36.22523°S, 73.43115°W, at depths of 100 to 3724 m, based on more than 340 investigated specimens.

Wu & Okutani (1984) reported B. belknapi based on 50 specimens collected by R/V Soyo-Maru from 14 stations along the Japan Trench and the Izu-Ogasawara Trench (30˚–38˚N), 1230–3020 m. Among them we re-examined 44 specimens deposited in NSMT. Most of the specimens were correctly identified as B. belknapi (NSMT-Mo 59993, 59994, 59997–60003, and parts of 59995), but some specimens were found to be one or two distinct species (NSMT-Mo 59996, 60004, and part of 59995). Further study is needed to identify those specimens of the distinct species.

Kaas (1989, 1990) reported B. belknapi from off Luzon Island, the Philippines and Sulu Sea. Unfortunately, he did not provide any description or drawings by which it would be possible to verify his identification. Two of the specimens from the Sulu Sea, (st. CP128) identified by Kaas (1989) as Leptochiton belknapi turned out to be Nierstraszella lineata ( Nierstrasz, 1905) . The other samples of B. belknapi from the Philippines ( Kaas 1990) could not be located (Virginie Heros, pers. com.) and thus not restudied and verified ( Sirenko 2020).

Revised diagnosis. Animal of large size, elongate oval. Intermediate valves subcarinated. Tail valve narrower than head valve and with posterior mucro. Antemucronal area 1.4 times longer than postmucronal area. Tegmentum sculptured with small granules evenly arranged quincuncially. Each granule with one megalaesthete and two, exceptionally up to four micraesthetes positioned in front of megalaesthete. Perinotum densely covered with pointed scales with 2–4 ribs. Central tooth of radula short and wide, major lateral tooth with big, unicuspid head and very small denticle-like appendage at inner edge.

Amended description. Of large size, BL up to 42 mm (ZIN 2444, from near Shikotan Island, Kurile Islands). Valves thick, subcarinated, highly elevated (elevation ratio from 0.36 to 0.54 in valve V), not beaked. Valve slopes slightly convex. Color of tegmentum white often with black or brown deposits on old parts of valves.

Head valve semicircular, slightly wider than tail valve. Intermediate valves not beaked, rectangular, anterior margin convex in valve II and slightly concave in the jugal part of remaining valves, lateral areas slightly raised, lateral margins rounded, posterior margin straight or slightly concave at both sides of the hardly perceptible apex. Tail valve semicircular to roughly triangular, slightly narrower as head valve, anterior margin slightly concave in jugal part, mucro posterior, slightly protruding, postmucronal slope concave or straight in most specimens.

Tegmentum uniformly sculptured with raised, quincuncially arranged granules. Each granule with one megalaesthete and two, rarely up to four, micraesthetes in front, pores of megalaesthetes slightly larger than micraesthetes.

Articulamentum white, strongly developed, apophyses small, widely separated, subtriangular in intermediate valves, more or less trapezoidal in tail valve.

Girdle narrow (it is about five times narrower than valve V), dorsally covered with sharply pointed, slightly curved scales (75 x 24 μm), usually showing two (rarely 3–4) longitudinal ribs, and scattered long, smooth, needles (200 x 20 μm). Intersegmental needles similar but longer (292 x 24 μm). Marginal needles shorter than sutural needles (210 x 22 μm). Ventrally girdle covered with elongate smooth, sharply pointed scales (100 x 28 μm).

The length of the radula depends on the length of the body, in individuals with a body length of 8.2 and 21.0 mm, the length of the radula is 1.8 mm and 8.7 mm, respectively, and the number of mature transverse rows varies in the studied individuals from 21 to 33 without any dependence on age. Central tooth nearly rectangular with rounded base. Head of major lateral tooth unidentate, strong, with small denticle-like appendage at inner edge.

Number of gills depends on the body length. Individuals with body lengths of 4.5, 6.5, 8.2, 12.0 and 25.0 mm, have 8, 9, 12, 14 and 17 gills per side, respectively.

Sigwart (2008a) observed, for a specimen with 14 gills per side, the location of the nephridiopore between gills 5–6 and the gonopore between gills 7–8, counting from behind.

Remarks. In addition to the age-related variability in the number of gills and the length of the radula mentioned above, cases of variability in the shape of the postmucronal slope of the tail valve were detected. This slope can be concave in smaller specimens, becoming straight to convex with increasing size ( Fig. 14 ), but the vast majority of examined specimens, like the holotype, have a concave postmucronal slope. Also, the number of tegmental micraesthetes on the granules may slightly vary; besides the usual two micraesthetes we sometimes detected three or even four micraesthetes. Given the absence of other differences from the holotype, we attribute these slight variations to intraspecific variability.

The following species may be easily separate from B. belknapi by having the tail valve wider than the head valve: B. sigwartae , B. mutschkeae , B. similis , B. halistreptus halistreptus and B. halistreptus abbreviatus. In addition, the last two mentioned subspecies have twice as many gills and they are located from the valve V to the anus, which turns out to be very rare in the entire order Lepidopleurida (vs. there are fewer gills and they are located from the valve VII to the anus of B. belknapi , like most in the order).

B. belknapi differs from B. japonicus in having two (rarely 3–4) ribs in dorsal scale (vs. 6–8 ribs in B. japonicus ), a unidentate head of major lateral tooth with a small denticle-like appendage at inner edge (vs. without appendage in B. japonicus ), two (rarely 3–4) micraesthetes in granule of tegmentum (vs. 4–6 micraesthetes in B. japonicus ).

B.belknapi differs from B. aequispinus in having subcarinated intermediate valve (vs. rounded in B. aequispinus ), unidentate head of major lateral tooth with appendage at inner edge (vs. without appendage in B. aequispinus ), two (rarely 3–4) micraesthetes in granule of tegmentum (vs. 4 micraesthetes in B. aequispinus ).

B. belknapi differs from B. alveolus in having subcarinated intermediate valves (vs. rounded in B. alveolus ), two (rarely 3–4) ribs in dorsal scales (vs. 4–5 ribs in B. alveolus ), two (rarely 3–4) micraesthetes in granule (vs. 4–6 micraesthetes in B. alveolus ).

B. belknapi differs from B. taiwanensis in having two (rarely 3–4) micraesthetes in granule (vs. 6 micraesthetes in B. taiwanensis ), two (rarely 3–4) ribs in dorsal scales (vs. 14–16 ribs in B. taiwanensis ), smooth sutural and marginal needles (vs. ribbed sutural and marginal needles in B. taiwanensis ).

B. belknapi differs from B. giganteus in having two (rarely 3–4) micraesthetes in granule (vs. 8 micraesthetes in B. giganteus ), two (rarely 3–4) ribs in dorsal scales (vs. no ribs in B. giganteus ), ratio of width of jugal sinus to width of apophyse in valve V is 2.1 (vs. 1.2–1.5 in mature B. giganteus ).

B. belknapi differs from B. simplex in having two (rarely 3–4) ribs in dorsal scales (vs. 6 ribs in B. simplex ), subcarinated intermediate valves (vs. carinated in B. simplex ).

B. belknapi differs from B. macleani in having two (rarely 3–4) ribs in dorsal scales (vs. 1 rib in B. macleani ), head of major lateral tooth with small appendage at inner edge (vs. no appendage in B. macleani ). Moreover B. belknapi does not have depressions between the antemucronal and postmucronal areas, as found in B. macleani . Owing to these depressions the mucro of B. macleani is not expressed at all (vs. mucro protruding in B. belknapi ).

B. belknapi differs from B. ater and B. okamurai in having two (rarely 3–4) micraesthetes in granule (vs. 4–6 micraesthetes in B. ater and six micraesthetes in B. okamurai ), head of major lateral tooth with small appendage at inner edge (vs. no appendage in both species).

B. belknapi differs from B. seishinmaruae in having subcarinated intermediate valves (vs. rounded in B. seishinmaruae ), elongate oval granules in central and antemucronal areas (vs. oval in B. seishinmaruae ), lateral areas of intermediate valves and both areas of tail valve with well visible growth line (growth line are absent or hardly visible in B. seishinmaruae ).

B. belknapi differs from B. kaasi in having perinotum with wide scales (vs. perinotum with narrow spicules in B. kaasi ), lateral margins with one row of small pores (vs. lateral margins with numerous large pores in B. kaasi ).

B. belknapi differs from B. bergenhayni in having two (rarely 3–4) ribs in dorsal scales (vs. 10–12 ribs in B. bergenhayni ), subcarinated intermediate valve (vs. rounded in B. bergenhayni ), two (rarely 3–4) micraesthetes in granule (vs. 8–9 micraesthetes in B. bergenhayni ).

B. belknapi differs from B. fijiensis in having two (rarely 3–4) ribs in dorsal scales (vs. seven ribs in B. fijiensis ), two (rarely 3–4) micraesthetes in granule (vs. 12 micraesthetes in B. fijiensis ), mucro posterior (vs. mucro anterior in B. fijiensis ).

B. belknapi differs from B. sp. ( Leptochiton cf. opiparus in Sirenko 2020) by having dorsal scales with two (rarely 3–4) distinct ribs (vs. lots of small ribs in B. sp.), narrower jugal sinus, ratio of width of jugal sinus to width of apophysis is 2.1 (vs. 2.6 in B. sp.).

In 2019, Sirenko was sent several samples of chitons from south Australia and Tasmania by TMAG. Among the chitons one specimen was collected in the Great Australian Bight, 200 km W of S Eyre Peninsula, 35.1406˚S, 134.2747˚E, 450 m. The locality is not so far from the type locality of B. opiparus . All the features of the newly found specimen coincide with the description and photograph of the holotype of B. opiparus . The latter species differs from B. belknapi in that its valve II turns out to be the widest of all intermediate valves, as well as the presence of 6–7 well-developed ribs on dorsal scales (two [rarely 3–4] ribs in B. belknapi ). In addition, B. opiparus has long, ribbed marginal scales (smooth marginal needles in B. belknapi ).

B. belknapi differs from B. benthus by having a posterior mucro (vs. central mucro in B. benthus ) and fewer gills, holotype of B. benthus with BL 8 mm has 7 gills whereas specimens of B. belknapi with BL 4.5mm, 6.5 mm, 8.2 mm and 12 mm have 8, 9, 12, 14 gills per side accordingly. Perhaps B. benthus is closest to B. macleani ( Sirenko, 2015) , known from Peru-Chile Trench to Clarion-Clipperton zone, Central Pacific ( Wiklund et al. 2017) at depth 4600–4850 m. B. macleani also has relatively few gills (eight gills at a BL 11.0 mm).

Finally, B. belknapi differs from B. costatoacus by having two (rarely 3–4) ribs in dorsal scales (vs. 11–12 ribs in B. costatoacus), smooth sutural needles (vs. ribbed sutural needles in B. costatoacus), an all-over quincuncially arranged granule pattern (vs. more longitudinally arranged in pleural areas of B. costatoacus), two (rarely 3–4) micraesthetes in granule (vs. 10–12 micraesthetes in B. costatoacus).

The specimen collected at station 205 off Luzon Island, Philippines during the voyage of H.M.S. Challenger and identified by Haddon (1886) as B. belknapi is most likely not such. The Haddon’s specimen (BL 12 mm) differs from the specimens of B. belknapi studied by us by a smaller number of gills (7–8 gills) (vs. 14 gills in our B. belknapi with BL 12 mm). In addition, the intermediate valves of Haddon’s specimen have a shallow depression on each side of the median ridge, which gives the latter a pinched appearance; this depression is most apparent at the hinder border of each valve ( Haddon 1886). Such depressions near the apex were noted in L. vanbellei Sirenko, 2001 from another group of deep-sea chiton, namely the group L. vitjazae . Perhaps Haddon’s chiton belongs to this group, of which the main characteristics are a small number of gills and a tridentate head of the major lateral tooth of the radula.

Figures

FIGURE 1. Images of Belknapchiton belknapi. A, B. Specimen from Commander Islands, RV Rakitnoe, st. 224, BL 19.0 mm (ZIN 2455); C. Specimen from East of Northern Honshu, RV Vitjaz, st. 6671, BL 29.0 mm (ZIN 2456); A. Dorsal view; B, C. Lateral view.

FIGURE 2. Belknapchiton belknapi, to East of Northern Honshu, RV Vitjaz, st. 6671, BL 11.0 mm (ZIN 2456): A. Valve I, dorsal view; B. Valve II, dorsal view; C. Valve V, dorsal view; D. Valve VIII, dorsal view; E. Valve V, detail of tegmentum in central area; F. Valve V, rostral view; G. Valve VIII, lateral view.

FIGURE 3. Belknapchiton belknapi, to East of Northern Honshu, RV Vitjaz, st. 6671, BL 11.0 mm (ZIN 2456): A. Dorsal scales and needles; B. Radula; C. Dorsal scales and needles, marginal needles and ventral scales; D. Central and first lateral teeth of radula.

FIGURE 4. Belknapchiton belknapi, Commander Islands, RV Rakitnoe, st. 229, BL 12.0 mm (ZIN 2457): A. Valve I, dorsal view; B. Valve V, dorsal view; C. Valve VIII, dorsal view; D. Valve IV, ventral view; E. Valve V, detail of tegmentum in central area; F. Valve V, rostral view; G. Valve VIII, lateral view.

FIGURE 5. Belknapchiton belknapi, Commander Islands, RV Rakitnoe, st. 229, BL 12.0 mm (ZIN 2457): A. Dorsal scales and needles; B. valve VIII, detail of tegmentum in jugal area; C. Dorsal scales, Marginal needles and ventral scales; D. Radula.

FIGURE 6. Belknapchiton belknapi, East Kamchatka, Pacific Ocean, RV Akademik Keldish, st. 2328, BL 21.0 mm (ZIN 2458): A. Valve V, dorsal view; B. Valve VIII, dorsal view; C. Valve VIII, posterior view; D. Valve V, detail of tegmentum in central area; E. Valve V, rostral view; F. Valve VIII, lateral view.

FIGURE 7. Belknapchiton belknapi, East Kamchatka, Pacific Ocean, RV Akademik Keldish, st. 2328, BL 21.0 mm (ZIN 2458): A. Dorsal scales and needles; B, D. Radula; C. Dorsal scales.

FIGURE 8. Belknapchiton belknapi, Bering Sea, south of the Pribilov Islands, RV Ametist, st. 173, BL 10.0 mm (ZIN 2459): A. Valve I, dorsal view; B. Valve V, dorsal view; C. Valve VIII, dorsal view; D. Valve IV, ventral view; E. Valve V, detail of tegmentum in central area; F. Valve V, rostral view; G. Valve VIII, lateral view.

FIGURE 9. Belknapchiton belknapi, Bering Sea, south of the Pribilov Islands, RV Ametist, st. 173, BL 10.0 mm (ZIN 2459): A. Ventral scales; B. Dorsal scales; C. Radula.

FIGURE 10. Belknapchiton belknapi, Bering Sea, south of the Pribilov Islands, RV Ametist, st. 175, BL 15.0 mm (ZIN 2460): A. Valve I, dorsal view; B. Valve V, dorsal view; C. Valve VIII, dorsal view; D. Valve V, detail of tegmentum in central area; E. Valve V, rostral view; F. Valve VIII, lateral view.

FIGURE 11. Belknapchiton belknapi, Bering Sea, south of the Pribilov Islands, RV Ametist, st. 175, BL 15.0 mm (ZIN 2460): A, B. Dorsal scales and needles, marginal needles and ventral scales; C. Dorsal scales and needles; D. Radula.

FIGURE 12. Belknapchiton belknapi, Oregon, RV Vitjaz, st. 4179, BL 12.0 mm (ZIN 2461): A. Valve I, dorsal view; B. Valve V, dorsal view; C. Valve VIII, dorsal view; D. Valve VIII, lateral view; E. Valve V, detail of tegmentum in central area; F. Valve V, rostral view; G. Valve VI, ventral view.

FIGURE 13. Belknapchiton belknapi, Oregon, RV Vitjaz, st. 4179, BL 12.0 mm (ZIN 2461): A. Dorsal and ventral scales; B, D. Radula; C. Dorsal scales and needle.

FIGURE 14. Age variability of Belknapchiton belknapi, to East of Northern Honshu, RV Vitjaz, st. 6671 (ZIN 2456): A, C, E, G chiton with BL 32.0 mm; B, D, F, H chiton with BL 9.0 mm; A, B. Valve V, dorsal views; C, D. Valve V, Rostral views; E, F. Tail valve, lateral views; G, H. Tail valve, dorsal views.

FIGURE 15. Confirmed distribution of Belknapchiton belknapi as characterized in this study.