Stenosemus simplicissimus ( Thiele, 1906 )

Sirenko, Boris, 2023, A small collection of rare and new chitons (Mollusca: Polyplacophora) from the Tasmanian Museum and Art Gallery, Zootaxa 5325 (3), pp. 359-392 : 381

Stenosemus simplicissimus ( Thiele, 1906)

( Figures 23C, D , 27–32 )

Ischnochiton (Chondropleura) simplicissimus Thiele, 1906: 335 , pl. 29, figs. 21–25; 1908: 21; 1909: 112; E.A. Smith, 1910: 211; Barnard, 1963: 6.

Ischnochiton simplicissimus ; Day, 1963: 6.

Ischnochiton (Stenosemus) simplicissimus ; Kaas & Van Belle 1990: 67, fig. 27, map 1.

Stenosemus simplicissimus ; Sirenko 1994: 164; Schwabe 2008: 71 (bibliography and description).

Type material. Holotype ( ZMB Moll 59908).

Type locality. South Africa, Cape of Good Hope , 318 m.

Material examined. Holotype, Southern Ocean , Heard Island, 52.3889°S, 72. 6272°E, Locality HIMI-Western Platean, 302 m, cruise SC 26, Haul N 161 B, 2 spms, BL 20.0 mm, ( TMAG, E 26179 ), 01.05.2003. 52.5969°S, 71. 9952°E, Locality HIMI-Aurora Bank, 310 m, cruise SC 26, Mol 37, benthic sled, 6 spms, BL 10.0– 14.5 mm, ( TMAG, E 26178 ), 30.04.2003.

Distribution. Stenosemus simplicissimus is wide distributed species in the South Ocean. It was found near South America (Shag Rocks near South Georgia Island), near Antarctica (eastern Weddell Sea), near South Africa (Cape off Good Hope) and Heard Island. The latest find has greatly expanded the range of this species. The species inhabits depth from 284 to 1064 m ( Schwabe 2008).

Remarks. Holotype (RMNH.MOLL.K.5155) has a somewhat glossy tegmentum, dorsal corpuscles with short longitudinal riblets in the upper part and with granules on the top. The studied specimens (BL 7.0 mm and 20.0 mm) from Heard Island have 9–10 longitudinal riblets on all dorsal surface of the corpuscles and have no granules on the top of the corpuscles. The larger specimen has 23 gills per side arranged from valve III to valve VII. The radula of the smaller specimen is 3.4 mm long and has 28 transverse rows of mature teeth.

Figures

FIGURE 23. Stenosemus exaratus, Southern Ocean, South Tasmania slope, (TMAG, E 45526), BL 11.5 mm: (A, B) and Stenosemus simplicissimus, Southern Ocean, Heart Island, (TMAG, E 26179) BL 20.0 mm (C, D).

FIGURE 27. Stenosemus simplicissimus, Southern Ocean, Heard Island, (TMAG, E 26179), BL 20.0 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 in central area; F. Valve V, rostral view; G. Valve VIII, lateral view.

FIGURE 28. Stenosemus simplicissimus, Southern Ocean, Heard Island, (TMAG, E 26179), BL 20.0 mm: A, B, D. Dorsal corpuscles, ventral scales and marginal spicules and bristles; C. Dorsal corpuscles and ventral spicules

FIGURE 29. Stenosemus simplicissimus, Southern Ocean, Heard Island, (TMAG, E 26179), BL 20.0 mm: A. Ventral scales and marginal spicules; B. Middle part of radula; C. Dorsal corpuscles; D. Central, first lateral teeth of radula and head of major lateral teeth of radula.

FIGURE 30. Stenosemus simplicissimus, Southern Ocean, Heard Island, (TMAG, E 26178), BL 7.0 mm: A. Part of 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 31. Stenosemus simplicissimus, Southern Ocean, Heard Island, (TMAG, E 26178), BL 7.0 mm: A. Dorsal corpuscles; B. Marginal spicules; C. Dorsal corpuscles, marginal spicules, needles and bristle; D. Ventral scales.

FIGURE 32. Stenosemus simplicissimus, Southern Ocean, Heard Island, (TMAG, E 26178), BL 7.0 mm: A. Middle part of radula; B. Central and first lateral teeth of radula.