Stenosemus sharpii ( Pilsbry, 1896 )

Sirenko, Boris I. & Khabibulina, Valeriya, 2025, The widespread chiton Stenosemus albus (Mollusca: Polyplacophora) is absent from the Pacific Ocean, and a new chiton species, Zootaxa 5729 (2), pp. 201-232 : 222-228

Stenosemus sharpii ( Pilsbry, 1896)

( Figures 4C , 11P, Q , 19–25 )

Trachydermon sharpii Pilsbry, 1896: 50 ; Ferreira 1982: 98, fig. 2.

Stenosemus sharpii ; Clark 2002: 62, fig. 1–3, 6; Sirenko 2013: 148.

Type material. Lectotype ( ANSP 69140 ).

Type locality. Unalaska Bay , Unalaska Island, Aleutian Islands, U.S.A., 53°55′N, 166°35′W .

Material examined. A total of 27 spms.:

Kurile Islands, Simushir Island. Broughton Bight, stn 208, sample 485, 10 m, sand, stones and shells, 6 spms, BL 5–15 mm, 15.08.1970 ; stn 209, sample 486, 15 m, sand, stones and shells, 4 spms, BL 6–11 mm, 25.08.1970 ; stn 208, sample 488, 15 m, sand, stones and shells, 3 spms, BL 4–10 mm, 25.08.1970 ; stn 208, sample 490, 15 m, sand, stones and shells, 2 spms, BL 15–20 mm, 25.08.1970 ; stn 210, sample 492, 20 m, sand, stones, 1 spm, BL 8 mm, 25.08.1970 ; stn 210, sample 495, 20 m, sand, stones and shells, 2 spms, BL 11–18 mm, 25.08.1970 ; stn 214, sample 505, 20 m, sand, stones and shells, 6 spms, BL 7–16 mm, 25.08.1970 ; stn 214, sample 507, 20 m, sand and shells, 6 spms, BL 5–15 mm, 25.08.1970; 46°46′N, 151°41′E stn 255, sample 732, 45– 15 m, rocks, 1 spm, BL 9 mm, 14.08.1987 .

Urup Island. Khiva Cape, 46°02′N, 150°19′E, stn 181, sample 513, 50 m, pebbles and small boulders, 1 spms, BL 7 mm, 97.08.1987 .

Aleutian Islands, Dutch Harbor, Unalaska Island. 53°55′N, 166°35′W, 10–15 m, stones resting on sand, 1 spm, BL 15 mm, 07.05.1999, leg. R.N. Clark ; Amaknak Island , Captains Bay , near Small South America, 5–15 m, 4 spms, BL 13–15 mm, 05.06.2006, leg. B. Sirenko.

Distribution. Alaska, eastern and central Aleutian Islands (Unalaska, Amaknak and Amchitka Islands, Kurile Islands (Simushir and Urup Islands).

Diagnosis. Chitons more than 20 mm, with an expanded-oval body shape and a low carinated shell. General coloration white. Head valves 1.0–1.1 times wider than the tail valves. Intermediate valves carinated, little elevated (dorsal elevation 0.32), not beaked, lateral area slightly raised only on the valve II. Antemucronal area slightly convex, and postmucronal area concave. Tegmentum uniformly microgranulose, lateral areas with several concentric, low growth lines. Slit formula 13–17/1–2/12–14. Jugal plate well developed. Eaves with well-marked pores. Girdle narrow, about 5 times narrower than valve V. Dorsal spicules elongate finger-shaped with distinct ribbing (up to 16 ribs). Length of largest spiculae 230–240 µm. Marginal spicules of four forms like in S. magnidentis sp. nov. Specimen with BL 15.5 mm, has radula 5.8 mm long, with 77 rows of mature teeth. Central tooth pyriform with wide rounded base, head of major lateral tooth wide (ratio of length of head to width of head 2.50–2.73). Head has two denticles, largest of which wide, bluntly tipped in large specimens, smallest denticle very small, but clearly marked. Large denticle in juveniles of about 4.5 mm has a sharp tooth, which becomes blunt and rounded with age. Number of transverse rows of mature radula varies from 60 to 115. Specimens with BL 3.0 and 20 mm have 15 and 27 gills at each side, accordingly, arranged from valve III to valve VIII.

Remarks. The main differences of Stenosemus sharpii to closely related species of the genus having finegrained tegmentum devoid of ribs ( S. albus , S. magnidentis sp. nov. and S. golikovi ) is the wide head of the major lateral radular teeth with a blunt tip, the pyriform central tooth with a very wide rounded base, and distinctly ribbed dorsal spicules.

This species lives near Simushir Island at depths from 7 to 50 m (mostly 15–20 m) on stones and pebbles, shell and sand, at temperatures from -1.7 C to +7 C and salinity of 33.5 ‰. Clark (2002) noted that this species inhabits the Aleutian Islands at depths of 2–30 meters. The average settlement density and biomass of individuals of the species at Simushir Island are 34 spms/m 2 and a biomass of 4.1 g /m 2, respectively. The largest specimens, 21 mm long and 12.4 mm wide, were collected in Broughton Bay, Simushir Island at a depth of 15 meters. They apparently feed on detritus. In addition to detritus, foraminifera, spirorbids and sand were also found in the digestive tract.

Figures

FIGURE 4. Complete specimen of chitons of genus Stenosemus: S. albus, Barents Sea, Murman coast, Jarnishnaja Inlet, intertidal, BL 15.0 mm (A). S. magnidentis sp. nov., Bering Sea, 73 m, paratype, BL 12.0 mm (B). S. sharpii, Kurile Islands, Simushir Island, 15 m, BL 8.2 mm (C).

FIGURE 11. Central teeth and head of major lateral teeth of Stenosemus albus (A-I), S. magnidentis sp. nov. (J–O) and S. sharpii (P, Q). A. Norwegian Sea, Tromsø, BL 5.5 mm; B. Barents Sea, Murman coast, Jarnishnaja Inlet, BL 9.0 mm; C. Atlantic Ocean, Newfoundland Bank, 9.5 mm; D. Norwegian Sea, Tromsø, BL 9.5 mm; E. Arctic Ocean, Kara Sea, St. Anna Trough, 13.0 mm; F. Spitsbergen Islands, Kongsfjord, BL 12.0 mm; G. Arctic Ocean, Kara Sea, St. Anna Trough, 17.0 mm; H. Norwegian Sea, Storfjord, BL 16.0 mm; I. Franz Josef Land, Kheisa Island, BL 20.0 mm; J. East Siberian Sea, Chauna Inlet, BL 9.0 mm; K. Bering Sea, Gulf of Anadyr, BL 10.0 mm; L. Bering Sea, Navarin Cape, BL 10 mm; M. Sea of Japan, BL 11.0 mm; N. Commander Islands, Bering Island, BL 14.0 mm; O. Bering Sea, near Plover Bay, BL 14.5 mm; P. Kurile Islands, Simushir Island, BL 15.5 mm; Q. Aleutian Islands, Unalashka Island, near Small South America, BL 13.5 mm. Scale bar 100 µm.

FIGURE 19. Distribution of Stenosemus sharpii. Black circles – specimens examined, empty circles – by literature data.

FIGURE 20. Stenosemus sharpii, Aleutian Islands, Unalashka Island, Captains Bay, near Dutch Harbor, 5–15 m, BL 13.5 mm: 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 surface of central area; F. Valve V, rostral view; G. Valve VIII, lateral view; H. Valve VI, ventral view.

FIGURE 21. Stenosemus sharpii, Aleutian Islands, Unalashka Island, Captains Bay, near Dutch Harbor, 5–15 m, BL 13.5 mm: A, B, D. Dorsal and marginal spicules and scales, and ventral scales; C. Dorsal spicules.

FIGURE 22. Stenosemus sharpii, Aleutian Islands, Unalashka Island, Captains Bay, near Dutch Harbor, 5–15 m, BL 13.5 mm: A. Ventral scales and marginal spicules and scale; B, D. Radula; C. Central and first lateral teeth of radula.

FIGURE 23. Stenosemus sharpie, Kurile Islands, Simushir Island, 15 m, BL 15.5 mm: 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 surface of central area; F.Valve V, rostral view; G. Valve VIII, lateral view.

FIGURE 24. Stenosemus sharpie, Kurile Islands, Simushir Island, 15 m, BL 15.5 mm: A. Dorsal and marginal spicules and scales, and ventral scales; B, C. Dorsal spicules; D. dorsal and marginal spicules and scales.

FIGURE 25. Stenosemus sharpie, Kurile Islands, Simushir Island, 15 m, BL 15.5 mm: A. Radula; B. Central and first lateral teeth of radula. C. Central, first lateral and major lateral teeth of radula; D. Major uncinal teeth and head of major lateral teeth.