Stenosemus exaratus (G.O. Sars, 1878 )

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 : 378-381

Stenosemus exaratus (G.O. Sars, 1878)

( Figures 23A, B 24–26 )

Lophyrus exaratus G.O. Sars, 1878: 113 , pl. 8, fig. 1, pl. II, fig. 1.

Lepidopleurus dorsuosus Haddon, 1886: 18 , pl. 1, fig. 5, pl. 3, figs. 5a–i.

Lepidopleurus dallii Haddon, 1886: 19 , pl. 1, fig. 6, pl. 3, figs. 6a–i.

Ischnochiton (Chondropleura) affinis Thiele, 1906 , 334, pl. 29, figs. 17–20.

Lepidopleurus (Leptochiton) nicomedes Dall, 1919: 501 .

Ischnochiton johnstoni Cotton, 1937: 11 , figs. 10–18.

Lepidopleurus nicomedes ; Leloup, 1956: 15 (bibliography).

Ischnochiton exaratus ; Leloup, 1956: 41, fig. 21; Kaas, 1972: 92, figs. 195–199, pl. 6, fig. 9.

Ischnochiton dorsuosus ; Dell, 1964: 114 (bibliography); Kaas, 1979: 29; Ferreira, 1980: 59.

Ischnochiton (Stenosemus) exaratus ; Kaas, 1979: 29; Kaas & Van Belle 1990: 62, fig. 25. maps 1, 23 (synonymy and bibliography).

Stenosemus exaratus ; Ferreira, 1981: 327.

Type material. Two syntypes ( NHMO D33573 ). Type locality. Norway, Bodö (67°17’N, 14°25’E) and Florö (61°36’N, 5°0’E), 180–360 m.

Material examined. Southern Ocean, Macquarie Islands , R/V Dmitry Mendeleev, cruise 16, stn 1294, 54°56.5’S, 158°49.7’E, 320 m, Sigsby trawl, 4 spms, BL 11.0–14.0 mm, 25.01.1976 ; Macquarie Islands , offshore to W, 54.7119°S, 158.7679°E, 200–468 m, stn SS1999_01_060Lot_60, Benthic dredge, CSIRO, 1 spm (TMAG, E 45537 ) BL 16.0 mm, 22.01.1999 . Macquarie Islands , offshore to E, 54.5959°S, 158.9433°E, 250–500 m, stn SS1999_093Lot_093, Benthic dredge, CSIRO, 28 spm (TMAG, E 45535 ) BL 10.5–17.0 mm, 26.01.1999 ; Southern Tasmania Slope: Southern Sirveyor , 44.0306°S, 147.5796°E, 830–1030 m, stn SS0207_08_Lot_8, Sherman sled, CSIRO, 2 spm (TMAG, E 45526 ) BL 11.0–12.0 mm, 31.03.2007 .

Distribution. Wide-spread species. It inhabits the Atlantic Ocean from Greenland, Iceland, Norway up to Terra del Fuego, South Shetlands and South Orkney Islands, Pacific Ocean near Southern Chile, Indian Ocean near Prince Edward, Kerguelen and Amsterdam Islands. The four above mentioned samples of this species are the first find for South Australia and Macquarie Islands. The species lives at depths from 23 up to 2580 m ( Kaas & Van Belle 1990; Schwabe & Sellanes 2010).

Remarks. The specimen (TMAG, E 45526, BL 11.5 mm) studied under SEM has 21 gills on each side arranged from valve IV to the anus; radula 4.1 mm long with 31 transverse rows of mature teeth. Gut content contains detritus (50%), foraminifera (40%, about 100% calcareous shells) and sand (10%).

The studied specimen collected south of Tasmania, as well as individuals collected from Macquarie, differs from Norwegian individuals in the almost complete absence of radial grooves on the postmucronal area of the tail valve and the presence of granules on the top of the dorsal calcareous corpuscles (not metioned in the description by Kaas & Van Belle 1990).

Stenosemus exaratus and S. beui ( O’Neill, 1987) have a very similar sculptured tegmentum but S. exaratus differs from S. beui by having slightly striated dorsal calcareous corpuscles (vs. coarse lattice-like sculptured, longitudinally ribbed corpuscles in S. beui ), the head of major lateral teeth lacks a small appendix at its outer side (vs. head of major lateral teeth with small appendix at outer side in S. beui ).

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 24. Stenosemus exaratus, Southern Ocean, South Tasmania slope, (TMAG, E 45526), BL 11.5 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 in central area; F. Valve V, rostral view; G. Valve VIII, lateral view.

FIGURE 25. Stenosemus exaratus, Southern Ocean, South Tasmania slope, (TMAG, E 45526), BL 11.5 mm: A, C. Dorsal corpuscles; B, D. Ventral scales

FIGURE 26. Stenosemus exaratus, Southern Ocean, South Tasmania slope, (TMAG, E 45526), BL 11.5 mm: A. Middle part of radula; B. Central and first lateral teeth of radula.