Leptochiton subantarcticus (Iredale & Hull, 1930)

Sirenko, Boris, 2020, New Zealand and Australian species of the genus Leptochiton (Mollusca Polyplacophora), Zootaxa 4851 (3), pp. 401-449 : 433-436

Leptochiton subantarcticus (Iredale & Hull, 1930)

( Figures 36–43 , 44E, F , 46F, G )

Lepidopleurus columnarius ; Ashby 1929b: 372, pl. 32, fig. 9. (non Hedley & May 1908).

Parachiton subantarcticus Iredale & Hull, 1930: 157 , pl. 16, figs. 6–7; Kaas 1977: 85; Powell 1979: 22.

Lepidopleurus subantarcticus ; Beu 1977: 39.

Leptochiton (Leptochiton) subantarcticus ; Kaas & Van Belle 1985: 160, fig. 73, map 29.

Leptochiton subantarcticus ; Kaas & Van Belle 1994: 24, fig. 8.

Syn. nov. Leptochiton (Leptochiton) deecresswellae Anseeuw & Terryn, 2002: 77 , figs. 1–3.

Type material. Holotype ( AM C149662 ), a single eroded intermediate valve.

Type locality. New Zealand, off Auckland Island , 171 m.

Material examined. Photographs of the type material ( AM C 149662 ); New Zealand, Papanui Canyon, off Otago Heads, South Island, 45°50’S, 171°1’E, stn. Mu 70–45, 540– 490 m, 9 spms ( ZISP 2384 ), BL 5.0– 7.5 mm, 22.10.1970, leg. A.G. Beu; off Dunedin, 540– 490 m, 3 spms ( ZISP 2385 ), BL 5–7 mm, 22.10.1970, leg. A.G. Beu; New Zealand, South Island , shelf NE of Tiaroa Head, 45°41’S, 170°53’E, R / V Munida, stn 1975008, 50 m, 30 spms, BL 4.0– 5.7 mm ( NMNZ M 319795 ), 08.10.1975; shelf NE of Taiaroa Head , 45°42’S, 170°51’E, R / V Munida, stn 1975009, 44 m, 1 spm, BL 8.0 mm ( NMNZ M 319796 ), 08.10.1975; Crooked Arm Sill, Doubtful Sound, Fiordland , 45°21.42’S, 167°1.78’E, R / V Munida, stn 1997104, 54– 60 m, 2 spms, BL 2 mm, ( NMNZ M 319788 ), 25.05.1997; Crooked Arm Sill, Doubtful Sound, Fiordland , 45°21.42’S, 167°1.78’E, R / V Munida, stn 1997104, 54– 60 m, 6 spms, BL 4.0–5.0 mm, ( NMNZ M 319792 ), 25.05.1997; North Island, stn 81833, 143– 153 m, 11 spms, BL 2.5–7.0 mm, ( NMNZ M 149934 ), 22.01.1981; Head of Pegasus Canyon, Pegasus Bay , R / V Acheron, stn 1976558, 446 m, mud, 5 spms, BL 3.5–6.0 mm, ( NMNZ M 152576 ), 27.09.1976; Campbell Island , 52°43.9’S, 169°32.5’E, R / V Dmitriy Mendeleev, 16 cruise, stn 1279, Sigsby trawl, 186 m, dead bryozoans and brachiopods, 9 spms, BL 3–6 mm, 16.01.1976; wall of Pegasus Canyon, stn 76559, 512– 1006 m, 5 spms, BL 5.0–9.0 mm, ( NMNZ M 052841 ), 27.09.1976; North Island, stn 56081, 796 m, 1 spms, BL 8.0 mm, ( NMNZ M 100811 ), 23.12.1981; Pegasus Canyon , NE of Banks Peninsula, 43°26.90’S, 173°29.50’E, R / V Acheron, stn 1985940, 505 m, 4 spms, BL 5.0–7.0 mm, ( NMNZ M 319763 ), 13.04.1985. All samples without registered numbers of the NMNZ are kept in the ZISP.

Distribution. All around New Zealand, Auckland and Campbell Islands, 44– 796 m.

The type material of this species comprises a single intermediate valve. The present study attempts to improve the re-description by Kaas and Van Belle (1994). The new observations are based mainly on a specimen from South Island, stn. Mu 70–45, 540– 490 m, BL 7.5 mm (figs. 36–38A, B, 39), and additionally on three specimens from: South Island, Pegasus Canyon, NE of Banks Peninsula, 505 m, BL 7.0 mm (NMNZ M 319763) (figs. 42C, D); South Island, shelf NE of Tiaroa Head, 50 m, BL 5.5 mm (NMNZ M 319795) (figs. 40–42A, B, 43); Head of Pegasus Canyon, Pegasus Bay, 446 m, mud, BL 6.0 mm, (NMNZ M 152576) (figs. 38C, D). These specimens and two more were disarticulated and studied by means of SEM and light microscopes.

Emended description. Small size, length usually 6–8 mm, sometimes up to 9 mm. Valves rather thick, from subcarinated to arched, low or strongly elevated (elevation ratio varies from 0.33 up to 0.78 in valve V) not beaked. Color of tegmentum white.

Head valve semicircular, posterior margin widely V-shaped. Intermediate valves rhomboidal, broadly rectangular or roughly triangular, anterior margin concave in the jugal part, convex to obliquely truncated in the pleural parts, posterior margin rather convex or straight, lateral area not raised, lateral margins rounded. Tail valve narrower than head valve, bluntly triangular; mucro anterior or subcentral, not prominent, antemucronal slope straight, postmucronal slope little convex or concave. Head valve, lateral areas and postmucronal area with distinct growth line.

Tegmentum sculptured with flattened, oval granules, arranged in longitudinal rows on central area of intermediate valves (50–60 rows) and antemucronal area of tail valve, in vague radial rows on head valve, and in a random manner on lateral areas of intermediate valves and on postmucronal area of tail valve. Granules in lateral areas are flattened and obscure. Each granule with one megalaesthete and four or rare six micraesthetes.

Articulamentum thick, well developed, apophyses small, widely separated, narrower than jugal sinus in old specimens.

Girdle narrow (about 0.3 mm near valve V), dorsally covered with bluntly pointed scales (75 x 27–40 μm) with four to eight distinct narrow ribs. Sutural and dorsal needles long and smooth (120 x 11 µm). Margin with two kinds of spicules: elongate pointed needles (80 x 11 µm) and sharply pointed spicules with eight riblets. Ventrally girdle covered with elongate, smooth, pointed scales (55 x 20–25 µm), next to margin with two short riblets in their distal ventral part.

Radula of specimen with BL 7.5 mm 2.9 mm long with 32 transverse rows of mature teeth. Central tooth narrow, elongate with distinct blade, major lateral teeth with bidentate cusps, inner denticle rather shorter than outer one. Radula of specimen with BL 7.0 mm 3.0 mm long with 38 transverse rows of mature teeth.

Specimen with BL 7.5 mm has 6 gills and specimen with BL 7.0 mm – 7 gills per side arranged from valve VI to anus.

Remarks. Despite the fact that the intermediate valve (length 2.4 mm, width 3.4 mm) of the holotype of L. subantarcticus is strongly damaged, the majority of the studied larger specimens are very similar in shape and sculpture to the holotype. The examination of more than 40 available specimens show a very strong variability in the shape of valves, especially in the range of elevation from low to high (figs. 36C, G, 38C, D, 40B, F, 42C, D). Smaller specimens often have a lower elevation and more distinct radial rows of granules on the head valve and on the lateral areas of intermediate valves than older specimens. The features of the girdle and radula are very similar in all studied specimens. It is noteworthy that Leptochiton kerguelensis ( Haddon, 1886) from Antarctic and Subantarctic waters shows a similar variability of the valve profile from subcarinated to roundly arched ( Sirenko 2015).

In their re-description of L. subantarcticus, Kaas & Van Belle (1994) used a specimen from the same sample as the present author (off Otago Heads, South Island, 45°50’S, 171°1’E, stn. Mu 70–45, 540– 490 m), but their description and drawings of the central and major lateral teeth of the radula differ considerable from the present observations. While the examined radula in this study shows the central tooth narrow and the cusp of the major lateral tooth bidentated, Kaas & Van Belle (1994) detected an unidentate cusp of the major lateral tooth and a wider shaft in the central tooth. All other features of shell and girdle of both specimens from off Otago Heads are similar.

Leptochiton subantarcticus is most similar to L. columnarius from South Australia and Tasmania but differs from the latter by having an anterior or subcentral mucro (vs. posterior mucro in L. columnarius ), bluntly pointed dorsal scales (vs. sharply pointed scales in L. columnarius ), subcarinated or arched valves (vs. carinated valves in L. columnarius ). The differences are not strong. It is possible that both taxa are synonymous but it is necessary to study L. columnarius from Australia to substantiate this.

Leptochiton subantarcticus is also very similar to L. deecresswellae . There are only two small differences between these two species: L. subantarcticus has strongly elevated valves (vs. weak elevated in L. deecresswellae ) and granules are arranged in vague radial rows on the head valve and on the lateral areas of the intermediate valves in L. subantarcticus (vs. in a random manner in L. deecresswellae ). The first character obviously depends on the age variability, whereas the second difference is probably a result of intraspecific variability. Moreover, L. deecresswellae was described from Papanui Canyon, off Otago Heads ( Anseeuw & Terryn 2002), where specimens very similar to the holotype of L. subantarcticus were collected (see present study, figs. 34–37 and Kaas & Van Belle 1994, page 24). Consequently, L. deecresswellae regarded to represent a synonym of L. subantarcticus .

Figures

FIGURE 36. Leptochiton subantarcticus, New Zealand, South Island, Papanui Canyon, off Otago Heads, stn. Mu 70–45, BL 7.5 mm, (ZISP 2384). A. Valve I, dorsal view; B. Valve II, dorsal view; C. Valve V, dorsal view; D. Valve VIII, dorsal view; E. Valve VI, ventral view; F. Valve V, tegmentum sculpture in central area; G. Valve V, rostral view; H. Valve VIII, lateral view.

FIGURE 37. Leptochiton subantarcticus, New Zealand, South Island, Papanui Canyon, off Otago Heads, stn. Mu 70–45, BL 7.5 mm, (ZISP 2384). A. Dorsal and ventral scales and marginal needles; B. Dorsal scales near suture; C. Dorsal scales near suture and sutural needle; D. Marginal needles and ventral scales.

FIGURE 38. Leptochiton subantarcticus, New Zealand, South Island, Papanui Canyon, off Otago Heads, stn. Mu 70–45, BL 7.5 mm, (ZISP 2384). (A, B), Head of Pegasus Canyon, Pegasus Bay, 446 m, BL 6.0 mm, (NMNZ M 152576) (C, D); A. Central portion of radula from anterior part; B. Central and first lateral tooth of radula; C. Valve V, dorsal view; D. Valve V, rostral view.

FIGURE 39. Leptochiton subantarcticus, New Zealand, South Island, Papanui Canyon, off Otago Heads, stn. Mu 70–45, BL 7.5 mm, (ZISP 2384). A. Sutural needle; B. Dorsal scales near suture; C. Dorsal scale in middle; D. Marginal needle; E. Ventral scale near margin; F. Ventral scale from mid-girdle; G. Central and first lateral teeth; H. Heads of major lateral teeth; I. Aesthete group in central area. Scale bar 50 μm (A–H).

FIGURE 40. Leptochiton subantarcticus, New Zealand, South Island, shelf NE of Tiaroa Head, BL 5.5 mm (NMNZ M 319795). A. Valve I, dorsal view; B. Valve V, dorsal view; C. Valve VIII, dorsal view; D. Valve VI, ventral view; E. Valve V, tegmentum sculpture in central and lateral areas; F. Valve V, rostral view; G. Valve VIII, lateral view.

FIGURE 41. Leptochiton subantarcticus, New Zealand, South Island, shelf NE of Tiaroa Head, BL 5.5 mm (NMNZ M 319795). A-C. Dorsal and ventral scales; D. Marginal needles and spicules, and dorsal scales.

FIGURE 42. Leptochiton subantarcticus, New Zealand, South Island, shelf NE of Tiaroa Head, BL 5.5 mm (NMNZ M 319795) (A, B) South Island, Pegasus Canyon, NE of Banks Peninsula, 505 m, BL 7.0 mm (NMNZ M 319763) C, D). A. Radula from anterior part; B. Central part of radula; C. Valve V, dorsal view; D. valve V, rostral view.

FIGURE 43. Leptochiton subantarcticus, New Zealand, South Island, shelf NE of Tiaroa Head, BL 5.5 mm (NMNZ M 319795). A. Sutural needle; B. Dorsal scales; C. Marginal needle; D. Marginal spicule; E. Ventral scale near margin; F. Ventral scale from mid-girdle; G. Central and first lateral teeth; H, I. Heads of major lateral teeth; J. Aesthete group in central area. Scale bar 50 μm (A–I).

FIGURE 44. Images of species of genus Leptochiton from New Zealand and Australia: A, B. L. cf. opiparus, New Zealand, Kermadec Islands, R/V Acheron, stn 1973312, 1188-1225 m, BL 16 mm, (NMNZ M 222132); C, D. L. marshalli n. sp., holotype, New Zealand, Three Kings Islands, Tasman Bay, Great Island, 6 m, BL 10.0 mm, (NMNZ M 117376); E, F. L. subantarcticus, New Zealand, Head of Pegasus Canyon, Pegasus Bay, 446 m, mud, BL 5.5 mm, (NMNZ M 152576); G, H. L. cf. laurae, New Zealand, South Island, SSE of Kaikoura, R/V James Cook, stn 1986020, 927–995 m, BL 10.5 mm, (NMNZ M 319764).

FIGURE 46. Images of type specimens: A. Leptochiton otagoensis, probably holotype (AM C.159221); B. L. sperandus, holotype (AM C.126815); C. L. liratellus, paratype AM C.149661); D. L. liratus, neotype (AM C.10410); E. L. opiparus, holotype (AM C.32026); F, G. L. subantarcticus, holotype (AM C.149662); H. L. columnarius, holotype (AM C.29060).