Leptochiton beui, Sirenko, 2020

Sirenko, Boris, 2020, New Zealand and Australian species of the genus Leptochiton (Mollusca Polyplacophora), Zootaxa 4851 (3), pp. 401-449 : 406-409

Leptochiton beui n. sp.

( Figures 5–8 , 45C )

Type material. Holotype ( NMNZ M 149935 ) and a paratype ( NMNZ M 152577 ).

Type locality. New Zealand, South Island , wall of Hokitika Trench, 42°36.10’S, 170°40.00’E, 300 m, on Alcyonarias and rock slab .

Etymology. Named after my colleague Alan G. Beu (GNS Science) for his large contribution to the knowledge of New Zealand Recent chitons and gastropods as well as fossil molluscs.

Material examined. New Zealand, South Island , wall of Hokitika Trench, 42°36.10’S, 170°40.00’E, R / V James Cook , stn 1978630, 300 m, on Alcyonarias and rock slab, holotype, BL 3.5 mm, ( NMNZ M 149935 ), 13.04.1978 ; Conway Rise , Kaikoura, 42°40.00’S, 173°39.00’E, 400 m, paratype, BL 4.5 mm, ( NMNZ M 152577 ), 17.08.1983, leg. E. Forbes ; Conway Rise , Kaikoura, 42°40.00’S, 173°39.00’E, 400 m, 2 spms, BL 2.5–3.5 mm, ( NMNZ M 076103 ), 17.08.1983, leg. E. Forbes ; Conway Rise , off Haumuri Bluff, 42°40.00’S, 173°39.00’E, 400 m, 4 spms, BL 2-4 mm, ( NMNZ M 095165 ), 02.09.1984, leg. E. Forbes ; Chatham Rise , 43°22.10’S, 179°54.98’E, R / V Tangaroa, stn 1995018, 430 m, 2 spms, BL 3.5 mm, ( NMNZ M 138100 ), 29.12.1995 ; Thompson Basin floor, Thompson Sound, Fiordland, 45°13.07’S, 166°58.02’E, R / V Munida, stn 1997114, 350 m, 1 spm, BL 3 mm, ( NMNZ M 138351 ), 28.05.1997 .

Distribution. New Zealand, around South Island and Chatham Rise, 300– 430 m.

Diagnosis. Small chiton. Valves moderately elevated, rounded, not beaked. Lateral areas not raised. Tail valve with anterior mucro, postmucronal slope slightly concave. Round granules arranged in more or less longitudinal rows on pleural areas becoming quincuncially arranged towards the jugum. Each granule with seven aesthete pores. Dorsal scales wide, obtusely pointed, with 21–24 distinct, narrow ribs. Radula with numerous transverse rows of very small teeth. Central teeth narrow, first lateral teeth narrow and long, major lateral teeth with long tridentate cusp.

Description. Holotype 3.5 mm long, elongate-oval, moderately elevated (elevation ratio in valve V 0.42). Valves thin, rounded, side slope convex. Valves not beaked, lateral areas not raised. Color of tegmentum yellowish white.

Head valve semicircular, wider than tail valve, tegmentum sculptured with rounded, flattened granules arranged quincuncially. Intermediate valves broadly rectangular, short and wide, 2.5 times broader than long, anterior and posterior margins nearly straight, not beaked, lateral margins rounded. Lateral areas not raised, sculptured like head valve, central area sculptured with more or less longitudinal rows of rounded granules in the pleural parts and becoming quincuncially arranged towards the jugum. Each granule has one megalaesthete and six micraesthetes. Tail valve with anterior mucro, antemucronal area with convex slope, sculptured like central areas, postmucronal area with slightly concave slope, sculptured like head valve.

Articulamentum weakly developed, apophyses small and widely separated.

Girdle very narrow, covered with wide, obtusely pointed dorsal scales (44 x 56–65 μm) with 21–24 distinct, narrow ribs, those near valves longer (75 x 57 μm). Intersegmental needles (90 x 13 μm), long and smooth. Margin with two kind of elements: elongate needles (84 x 12 µm) and elongate scales with seven riblets arranged on dorsal part of scale. Ventral scales at central part of girdle pointed, smooth (60 x 20 μm), near the girdle margin one row of ventral scales (68 x 24 μm), with 5–7 short riblets at distal portion.

Radula of holotype 1.7 mm long with slightly more than 130 transverse rows of mature teeth. Central tooth narrow, first lateral tooth elongated, major lateral teeth with long tridentate cusps, central denticle largest, side denticles arranged on different heights, inner more distal, while the outer is more proximal.

Holotype has six gills per side.

Remarks. The paratype of the new species (NMNZ M 152577) (BL 4.5 mm) has more pronounced longitudinal rows of granules in pleural areas and longer cusps of major radular lateral teeth than the holotype. The radula of the paratype is 1.6 mm long, with ca. 150 transverse rows of mature teeth and 6 gills per side arranged from valve VI to anus.

Leptochiton beui n. sp. belongs to the group of L. compostellanum in which currently eight species are grouped that have mainly a southern hemisphere distribution: L. badius , L. compostellanum , L. sanmatiensis Güller, Liuzzi & Zelaya, 2015 , L. linseae Sirenko, 2015 , L. peruvianus Sirenko, 2015 , L. ferreirai Sirenko & Sellanes, 2016 , L. smirnovi Sirenko, 2016 and L. beui n. sp. The new species differs from other members of this group by having 21–24 narrow ribs in dorsal scales (vs. 12–20 ribs in other members of the group) and shorter side denticles on the cusps of the major lateral teeth. Leptochiton beui n. sp. differs from the geographically closer L. badius by having flattened granules (vs. granules with round top in L. badius ) and dorsal scales with 21–24 narrow ribs (vs. 12–17 wide ribs in L. badius ).

The new species, differs from L. linseae from the Magellan Strait and L. compostellanum from Spain by having seven aesthete pores in each aesthete group (always three in L. linseae and L. compostellanum ). The new species differs from L. sanmatiensis from Argentina by having quincuncially arranged granules on the jugal area (longitudinal rows of granules in L. sanmatiensis ) and by the shape of the cusps of the major lateral radular teeth with side denticles arranged at a different level (side denticles arranged at equal level in L. sanmatiensis ). L e p t o c h iton beui n. sp. differs from L. peruvianus from Peru by having 6 gills arranged from valve VI to anus (vs. 11 gills arranged from valve V to anus in L. peruvianus ). The new species differs from L. smirnovi from South Africa by having smooth ventral scales (vs. ribbed ventral scales in L. smirnovi ). L. ferreirai differs from the new species by having five aesthete pores (vs. seven in L. beui n. sp.).

Figures

FIGURE 5. Leptochiton beui n. sp., holotype, BL 3.5 mm, (NMNZ M 149935). A. Valve I, dorsal view; B. Valve V, dorsal view; C. Valve VIII, dorsal view; D. Valve VI, ventral view; E. Valve V, rostral view; F. Valve V, tegmentum sculpture in central and lateral areas; G. Valve VIII tegmentum sculpture near mucro; H. Valve VIII, lateral view.

FIGURE 6. Leptochiton beui n. sp., holotype, BL 3.5 mm, (NMNZ M 149935). A, B, C. Dorsal, marginal and ventral scales; D. Dorsal scale.

FIGURE 7. Leptochiton beui n. sp., holotype, BL 3.5 mm, (NMNZ M 149935). A, B. Central portion of radula from anterior part.

FIGURE 8. Leptochiton beui n. sp., holotype, BL 3.5 mm, (NMNZ M 149935). 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 tooth and first lateral teeth; H. Heads of major lateral teeth; I. Aesthete group in central area. Scale bar 50 μm.

FIGURE 45. Images of species of genus Leptochiton from New Zealand and Australia: A. L. badius, Australia, NSW, Sydney, Long Reef, BL 4.2 mm, (ZISP 2381) 04.02.1928; B. L. finlayi, New Zealand, N of Cavalli Islands, R/V Tangaroa, stn TAN0906/81, 115 m, BL 8.0 mm, (NMNZ M 314504); C. L. beui n. sp., New Zealand, Chatham Rise, 430 m, BL 3.5 mm, (NMNZ M 138100); D. L. inquinatus, New Zealand, Lyall Bay, Cook Strait, low tide, under stones, BL 10.0 mm, (ZISP; E. L. subantarcticus, New Zealand, South Island, shelf NE of Tiaroa Head, 50 m, BL 5.7 mm, (NMNZ M 319795).