Onithochiton maklayi Sirenko, 2019

Sirenko, Boris I., 2019, New chitons (Mollusca: Polyplacophora) of the genera Rhyssoplax, Lucilina and Onithochiton from shallow waters of Papua New Guinea, The Bulletin of the Russian Far East Malacological Society 23 (1 / 2), pp. 63-81 : 74-79

Onithochiton maklayi Sirenko sp. nov.

Figs. 9–11 , 12A

urn:lsid:zoobank.org:act:6376B502-81F9-4981-98DA-E83BBFEC8DE0

Ty p e m a t e r i a l. The holotype ( ZISP 2305 ) now disarticulated, consisting of mount of part of girdle and radula, vial with valves, part of radula and part of girdle .

Ty p e l o c a l i t y. Papua New Guinea, Astrolabe Bay, Bilibili Island, intertidal, rocks ( R / V Kallisto, cruise 1, 03.01.1975, leg. B.I. Sirenko) .

E t y m o l o g y. Named in honour of the Russian ethnographer Nikolay Miklukho- Maklay, who studied the populations of South-East Asia, Australia and Oceania, including the indigenous tribes of North-Eastern Papua New Guinea in Astrolabe Bay.

D i s t r i b u t i o n. Known only from the type locality.

M a t e r i a l e x a m i n e d. Papua New Guinea, Astrolabe Bay , Bilibili Island, R / V Kallisto, cruise 1, intertidal, rocks, holotype ( ZISP 2305 ), BL 36.0 mm, 1 paratype ( ZISP 2306 ), BL 35 mm, 03.01.1975, leg. B.I. Sirenko.

D i a g n o s i s. Animal of medium size, shell rounded. Tail valve narrower than head valve. Tegmentum of head valve, lateral area of intermediate valves, and postmucronal area of tail valve with round or oval, convex pustules (up to 300–400 µm) more or less arranged in radial rows at least in head valve. Central area with flattened riblets along diagonal line arranged longitudinally at the sides, anteriorly converging towards the jugum; interstices in two times narrower than riblets. There are distinct growth lines in all areas near anterior and side margins. Dorsal side of girdle beset with sharp-topped spicules, with longitudinal ribs around and scattered smooth needles. Major lateral tooth bearing a cusp with two denticles the outer denticle large, blunt and much rounded, the innermost small and sharp.

D e s c r i p t i o n. The holotype 36.0x19.0 mm, elongated, oval, shell rather flat (dorsal elevation 0.25), rounded, slightly beaked. Color of tegmentum blackish-green in anterior margin of head valve, lateral and pleural areas of intermediate valves and hind margin of tail valve, other portion of central area and middle of head and tail valves yellow except red-brown in jugal area. Girdle yellow with reddish-brown spots.

Head valve semicircular, hind margin straight, Intermediate valves rectangular, front margin convex in valve II and concave medially, convex at the pleurae in other valves, hind margin beaked, straight to concave at both sides of the strongly, bluntly protruding apex, lateral areas not raised, Tail valve considerably narrower than head valve, triangular, the length almost half the width, front margin straight, hind margin angularly rounded, mucro terminal, antemucronal slope convex, postmucronal area restricted to a narrow strip on both sides of the terminal mucro.

Tegmentum of head valve, lateral area of intermediate valves and postmucronal area of tail valve with round or oval, convex pustules (up to 300–400 µm) more or less arranged in radial rows. Central area of intermediate valves and antemucronal area of tail valve with numerous flattish, longitudinal, forwardly converging ribs, interstices half as wide, ribs anteriorly converging, getting more fine and close set towards the very narrow, smooth jugum.

Articulamentum white with light brown in the middle, apophyses well rounded, separated by wide finely pectinated sinus (23 small, short denticles in valve V), insertion plates of head and intermediate plates pectinated. Slit formula 8/1/0 (callus), slit rays slightly indicated.

Girdle about 5.1 mm wide near valve V, dorsally covered with short, thick, sharp-topped spicules with 12–16 longitudinal riblets, 190–200x40 μm, among them randomly dispersed smooth needle 190x26 μm. Marginal spicules the same size. Ventral side of girdle covered with more or less rectangular scales with 18-20 distinct, distally converging riblets 60x28 µm.

Radula of the holotype 11.0 mm long with 42 transverse rows of mature teeth, central tooth somewhat pinched in the middle with rectangular blade, first lateral tooth twice longer than central tooth, cusp of major lateral tooth with two denticles, the outer denticle large, blunt and much rounded, the innermost small and sharp.

The holotype with 45 gills on both sides, extending from valve II to valve VIII.

R e m a r k s. The present new species much resembles Australian Onithochiton quercinus (Gould, 1846) , South African O. literatus (Krauss, 1848) and Vietnamese O. stracki Sirenko, 2012 , but differs from them by having bidentate cusp of major lateral tooth of radula (vs. tetracuspid in all three), tegmentum of head valve, lateral area of intermediate valves, and postmucronal area of tail valve with round or oval, convex pustules (vs. irregular, flattish, concentric rugosites or transversely elongate, flattish nodules in all three). O. maclayi sp. nov. differs also from O. quercinus by having less number of longitudinal riblets and wider interstices between the riblets in central areas of intermediate valves.

The new species differs from O. lyellii (Sowerby, 1832) and O. neglectus neglectus de Rochebrune, 1881 by having bidentate cusp of major lateral tooth of radula (vs. unicuspid cap in O. neglectus neglectus and tetracuspid cap in O. lyellii )

Figures

Fig. 9. Onithochiton maklayi, holotype (ZISP 2305), Papua New Guinea, Astrolabe Bay, Bilibili Island, intertidal, rocks, BL 36.0 mm. A – head valve, dorsal view; B – valve II, dorsal view; C – valve V, dorsal view; D – tail valve, dorsal view; E – tail valve, lateral view; F – valve IV, surface of tegmentum in central and lateral areas; G – valve VII, ventral view; H – valve VIII, ventral view; I – valve IV, rostral view.

Fig. 10. Onithochiton maklayi, holotype (ZISP 2305), Papua New Guinea, Astrolabe Bay, Bilibili Island, intertidal, rocks, BL 36.0 mm. A – valve IV, surface of tegmentum in central and lateral areas; B – dorsal spicules and needles; C – dorsal spicules and needles, marginal needles and ventral scales; D – dorsal spicules.

Fig. 11. Onithochiton maklayi, holotype (ZISP 2305), Papua New Guinea, Astrolabe Bay, Bilibili Island, intertidal, rocks, BL 36.0 mm. A – valve VIII, sculpture of insertion plates; B – radula; C – central and first lateral teeth of radula; D – head of major lateral teeth of radula.

Fig. 12. Color images of species of genera Onithochiton, Rhyssoplax and Lucilina. A – Onithochiton maklayi, holotype (ZISP 2305), Papua New Guinea, Astrolabe Bay, Bilibili Island, intertidal, rocks, BL 36.0 mm; B – Rhyssoplax venusta (ZISP 2302), Papua New Guinea, Bismarck Archipelago, Manus Island, 1 m, BL 14 mm; C – Lucilina insueta, holotype (ZISP 2303), Papua New Guinea, Astrolabe Bay, Bilibili Island, intertidal, rocks, BL 20 mm.