Lucilina insueta 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 : 65-69

Lucilina insueta Sirenko sp. nov.

Figs. 3–5 , 12C

urn:lsid:zoobank.org:act:9A18C9A0-1FCB-4BA3-9754-0549511DA62E

Ty p e m a t e r i a l. Holotype ( ZISP 2303 ), now disarticulated, consisting of the 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. From the Latin insuetus, uncommon, referring to the uncommon number of slits and rudiment of callus in the tail valve.

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

D i a g n o s i s. Animal of medium size, shell rather low, back rounded. Tegmentum smooth in all areas except for a few weak concentric growth lines. Slit formula 9/1/2 + rudiment of callus. Girdle looking smooth, leathery, dorsal scales very small and short with one longitudinal rib, ventral scales subquadrangular, ornamented around with 14–15 distally converging riblets. Major lateral tooth of radula with tetracuspid cap, the outer three denticles blunt and much rounded, the innermost sharpest.

D e s c r i p t i o n. Chiton of medium size, holotype, BL 20.0 mm, elongate oval. Shell low (dorsal elevation 0.27), back rounded, intermediate valves side slopes straight, valves slightly beaked. Tegmentum mainly yellowish with brown and olive spots. Girdle of fixed holotype colored in predominant tegmental color yellow.

Head valve semicircular. Intermediate valves with anterior margin straight in a wide central part, splayed laterally, side margins rounded, hind margin almost straight at both sides of rounded apex, lateral areas not raised. Tail valve slightly narrower than head valve, the length somewhat less than half the width, front margin weakly convex in the middle, hind margin evenly convex, mucro posterior, hind slope steep, convex, antemucronal area convex.

Tegmentum smooth in all areas, with a few weak concentric growth lines. Head valve with minute, black ocelli sparsely distributed in 8 radiating series in between, lateral and postmucronal areas with one row of ocelli in anterior portion.

Articulamentum white with light brown in middle portion, apophyses triangular with rounded top in intermediate valves, and trapezoidal in tail valve, jugal sinus straight, about 1/3 of the valve’s width, provided with a very short, finely denticulated plate. Slit formula 9/1/2+rudiment of callus, pectination and slit rays well indicated in head and intermediate valves, insertion teeth short, finely grooved on the upper side, sharply pectinated at outer edge, eaves short, solid. Tail valve with pectination on the hind parts of the apophyses, with two obsolete slits and a smooth rudiment of callus between the slits (Fig. 4B).

Girdle about 3.3 mm wide near valve V, looking smooth, leathery dorsally covered with not numerous, small scales (20x10 μm), protruded from cuticula, and ornamented with one longitudinal rib, and more rare small spicules (8x4 μm). Ventral scales (27–29x25 µm) subquadrangular, ornamented around with 14–15 distally converging riblets.

Radula of the holotype 6.5 mm long, with 42 transverse rows of mature teeth, central tooth of radula knobby in the lower part, slightly narrow in the central part, distally widening to an even wider quadrangular blade, major lateral tooth with tetracuspid cap, the outer three denticles blunt and much rounded, the innermost sharpest.

The holotype with 39 gills on both sides, extending from valve II to first half of valve VIII.

R e m a r k s. The rudiment of callus bring this species closer to species of the genus Onithochiton but pectination and two obsolete slits on the sides of insertion plate of the tail valve require to place this species in the genus Lucilina . L. insueta sp. n. is superficially similar to south Australian L. dupuisi (Leloup, 1973) that has also practically smooth tegmentum, but differs from the latter in having unique rudiment of callus and only two slits in insertion plate of the tail valve (vs. 9 equidistant slits in L. dupuisi).

Figures

Fig. 3. Lucilina insueta, holotype (ZISP 2303), Papua New Guinea, Astrolabe Bay, Bilibili Island, inter- tidal, rocks, BL 20 mm. A – head valve, dorsal view; B – valve V, dorsal view; C – tail valve, dorsal view; D – valve IV, ventral view; E – surface of tegmentum in central and lateral areas; F – tail valve, lateral view; G – valve V, rostral view.

Fig. 4. Lucilina insueta, holotype (ZISP 2303), Papua New Guinea,Astrolabe Bay, Bilibili Island, intertidal, rocks, BL 20 mm. A – valve IV, lateral view; B – tail valve, ventral view. Arrows show slits of insertion plates; C – dorsal and ventral scales; D – dorsal scals.

Fig. 5. Lucilina insueta, holotype (ZISP 2303), Papua New Guinea,Astrolabe Bay, Bilibili Island, intertidal, rocks, BL 20 mm. A, C – ventral scales; B – radula; D – head of major lateral tooth 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.