Bull. Natl. Mus. Nat. Sci., Ser. A, 47(2), pp. 55–63, May 21, 2021 DOI: 10.50826/bnmnszool.47.2-55 A New Species of Leptochiton (Mollusca: Polyplacophora) from Shallow Water of the Andaman Sea Hiroshi Saito1* and Mu Mu Aung2 1Department of Zoology, National Museum of Nature and Science, 4–1–1 Amakubo, Tsukuba, Ibaraki 305–0005, Japan 2Forest Research Institute, Forest Department, Ministry of Natural Resources and Environmental Conservation, Yezin, Nay Phi Taw, Myanmar *E-mail: [email protected] (Received 2 March 2021; accepted 24 March 2021) http://zoobank.org/0244307D-B3D1-4B84-87FB-D4FB04AAA7F8 Abstract A new chiton species, Leptochiton myeikensis n. sp. is described from the southern Myeik (Mergui) Islands, Andaman Sea, Myanmar. This is the fifth species of the genus Leptochi- ton from tropical shallow waters. The new species is distinguishable from other congeners by the sculptures of tegmentum and girdle scales, and shape of the radula teeth. Key words: chiton, Myeik Islands, Myanmar, Indian Ocean, new species, taxonomy. Introduction in 75% ethanol. The valves, girdle scales and spic- ules, and the radula were observed by the scanning The genus Leptochiton has large species rich- electron microscope (SEM) JEOL-LV6380, Jeol ness, including 140 species (Molluscabase) Co., Tokyo. Preparation for the examination by which are distributed in the world seas. However, SEM followed Sirenko (2020) with modification: there is no record of Leptochiton species from girdle elements (hyponotum and perinotum) and the shallow water of the Andaman Sea (Kaas and radula were dried with a critical point dryer JCPD- Van Belle, 1985; 1998; Schwabe, 2006). This 5, Jeol Co. The girdle scales and spicules, and a absence is due not only to a lack of research in part of the radula were observed also with light this region, but also the scarceness of Leptochi- microscope, AxioImager® M1 light microscope ton species in the tropical shallow waters with AxioCam® HRC image capture equipment (Sirenko and Schwabe, 2011): so far only four controlled by AxioVision® ver. 4.8, Carl Zeiss, species were known from the vast area of the Jena. Those hard parts were mounted on glass global tropics (Leloup, 1981; Schwabe and slides with Entellan neu® Merck, Darmstadt. Lozouet, 2006; Sirenko and Schwabe, 2011; Drawings were made based on the images Sirenko and Saito, 2020). This paper describes a obtained with both SEM and light microscope. new species of the genus Leptochiton collected from the shallow water of the Myeik (Mergui) Taxonomy Islands, Myanmar coast in the Andaman Sea. Class Polyplacophora Gray, 1821 Order Lepidopleurida Thiele, 1909 Material and Methods Family Leptochitonidae Dall, 1889 The collected specimen was fixed and preserved Genus Leptochiton Gray, 1847 © 2021 National Museum of Nature and Science Type species. Chiton cinereus (sensu) Montagu, 56 H. Saito and M. M. Aung 1803 (non Linnaeus, 1767) ＝ Leptochiton gin; lateral areas not raised. Tail valve semicircu- asellus (Gmelin, 1791) fide Lovén (1846), sub- lar, narrower than head valve (width ratio: head sequent designation by Gray (1847). valve/tail valve ＝ 1.08); mucro central; antemu- cronal slope slightly convex; postmucronal slope almost straight. Leptochiton myeikensis n. sp. (Figures 1–5) Tegmentum of head valve, lateral areas of intermediate valves and postmucronal area of tail Type locality. Andaman Sea, Maung Yin valve sculptured with radial rows of small, oval, Island, southern Myeik Islands, Myanmar, raised granules, which are semi-fused with con- 10.7657°N, 98.0580°E, 15.6 m. tinuous granules by low, narrow ridges; number Type depository. Department of Zoology, of radial rows 55, 6–8 and 37, respectively. Cen- National Museum of Nature and Science, Tsu- tral area of intermediate valves and antemucronal kuba. area of tail valve sculptured with longitudinal Etymology. From the name of the locality, rows of oval, flat-topped granules, which are Myeik Islands. semi-fused with continuous granules as those on Material examined. Holotype: NSMT-Mo the head valve, and also connected to granules of 79255, ethanol preserved specimen, part of girdle both sides with weakly raised commarginal and radula mounted on glass slides and brass ridges; the number of rows 32 and 25, respec- stubs for SEM, body length 4.3 mm, collected tively. Aesthete pore group on each granule in from the Andaman Sea, Maung Yin Island, central and antemucronal areas comprising one southern Myeik Islands, Myanmar, 10.7657°N, megalaesthete and 4–5 micraesthetes on each 98.0580°E, 15.6 m, underside of dead coral piece side. In head valve, lateral areas and postmucro- on coarse shell sand, by H. Saito with scuba, 16 nal area, of one megalaesthete and 3-4 micraes- February 2020. thete pores on each side at basal portion of gran- Diagnosis. Animal very small. Shells moder- ules; some of frontal micraesthete pores may be ately elevated, round-backed. Central area with obscured below the granule in dorsal view. Artic- longitudinal rows of semi-fused granules with ulamentum thin, white. Apophyses small, trian- wide interspaces. Each granule also connected to gular in intermediate valves, rectangular in tail granules of both sides with weakly raised com- valve, widely apart. marginal ridges. Girdle dorsally densely covered Girdle narrow, dorsally densely covered with with small, bent, imbricating, round-topped small round-topped scales, up to 40 (length)×62 scales which are ornamented with 12–16 sharp (width) μm, bent, imbricating, ornamented with ribs. Radula with ca. 80 transverse rows of very 12–16 strong, occasionally bifurcated ribs, and small teeth; central tooth narrow, taller than first scattered long needles, up to 230×12 μm, with lateral teeth; head of major lateral teeth with two fine longitudinal grooves, although they may be denticles. caused by destruction during preparation for Description. Animal very small, 4.3 mm in observation. Marginal fringe with two kinds of body length, elongate-oval. Valves moderately elongate scales: wider type attaining to elevated (dorsal elevation 0.30 in valve IV), 80×21 μm, obtusely pointed at the top, truncated round-backed, not beaked. Color of tegmentum at the base, sculptured with 4–5 strong, longitu- and girdle light beige; soft parts reddish hue in dinal riblets extending from near the base to the living animal. tip; other type narrower, attaining to 70×12 μm, Head valve semicircular; anterior slope more pointed at the tip, sculptured with 2–3 lon- slightly concave. Intermediate valves rectangular, gitudinal riblets also extending to the tip. Ven- rounded in lateral margins, slightly convex in trally girdle covered with scales similar to wider anterior margin, nearly straight in posterior mar- marginal scales, but shorter, longitudinal riblets A New Species of Leptochiton from the Andaman Sea 57 Fig. 1. Leptochiton myeikensis, holotype (NSMT-Mo 79255), Andaman Sea, Maung Yin Island, Myeik Islands, Myanmar, BL 4.3 mm. A, B. specimen in life, dorsal and ventral views; C, D. specimen in preserved condi- tion, ventral and lateral views (s: Schwabe organ); E. specimen in situ, underwater photograph. Scale: 1 mm. not extending to the tip. Ventral scales becoming Gills seven per side, arranged from valve VI to shorter, more roundish, and having weaker sculp- anus. ture towards pallial groove; surface of those Schwabe organ (Sigwart et al., 2014) present on along pallial groove almost smooth. both side of oral lappets (Fig. 1C, s), brownish. 58 H. Saito and M. M. Aung Fig. 2. Leptochiton myeikensis, holotype (NSMT-Mo 79255), Andaman Sea, Maung Yin Island, Myeik Islands, Myanmar, BL 4.3 mm. A. valve I, dorsal view; B. valve III, dorsal view; C. valve IV, frontal view; D, E. valve VIII, dorsal and lateral views; F. valve V, sculpture of lateral area; G. valve V, granules of central area, show- ing arrangement of aesthete pores; H. valve I, granules and arrangement of aesthete pores. Scale: 500 μm for A–E; 100 μm for F; 50 μm for G, H. Radula 1.3 mm long with 80 transverse rows than outer main one; shaft plate-like with folded of very small mature teeth. Central tooth narrow, part at proximal portion and fin-like process in with small blade at top, keeled, widened at basal outer-basal portion; fin-like process with small half. First lateral teeth longer than central tooth, notch at distal portion. Major uncinal teeth rather bearing small cusp at top. Major lateral teeth wide, with folded blade in distal half. Inner mar- with bidentate cusp; inner denticle much smaller ginal teeth small, roughly u-shape, holding base A New Species of Leptochiton from the Andaman Sea 59 Fig. 3. Leptochiton myeikensis, holotype (NSMT-Mo 79255), Andaman Sea, Maung Yin Island, Myeik Islands, Myanmar, BL 4.3 mm. A. dorsal girdle scales; B. dorsal girdle scales, close up; C. dorsal needle; D. ventral girdle scales; E. ventral girdle scales near girdle margin. Scale: 20 μm for A, D, E; 10 μm for B; 50 μm for C. of major uncinal tooth. Outer marginal teeth Distribution. Only known from type locality. wide, obtusely pointed at both sides. Remarks. The present new species is charac- terized by longitudinal rows of granules on the 60 H. Saito and M. M. Aung Fig. 4. Leptochiton myeikensis, holotype (NSMT-Mo 79255), Andaman Sea, Maung Yin Island, Myeik Islands, Myanmar, BL 4.3 mm. A. radula, dorsal view; B. close up of left side of radula; C. close up of central part of radula; D. close up of right side of radula. Scale: 20 μm for A; 10 μm for B–D. central area of the intermediate valves and on the groups. The present species does not match any antemucronal area of the tail valve, 9–11 aes- of those five groups. Among approximately thetes in each granule, wide, round-topped dorsal ninety remaining species in this genus, the pres- scales with 12–16 riblets, a high number of trans- ent species most resembles Leptochiton las- verse rows of radula teeth (80 rows), narrow cen- crusesi Sirenko, 2015 described from intertidal tral tooth of the radula and bidentate cusps of the zone of Las Cruses, Chile, 33°30′S, 71°37′E, in major lateral teeth, with the inner denticle being having longitudinal rows of granules in central distinctly shorter than the outer one.