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 : 65-74

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. 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

Ty p e l o c a l i t y. Papua New Guinea, Bismarck Archipelago, Manus Island, Silver Sound , 02°01′ S, 146°53′ E, 1 m ( R / V Dmitriy Mendeleev, 18 cruise, sample 4, 27.01.1977) .

E t y m o l o g y. Named in honour of my wife Alena Nemirko who help me to live and to work.

D i s t r i b u t i o n. Only known from the type locality.

D i a g n o s i s. Animal of small size, shell rounded. Intermediate valves sinuate in anterior margin, concave in the jugal area, convex at side margins, evenly convex, not beaked in posterior margin. Tail valve slightly narrower than head valve. Tegmentum of head valve, lateral area of intermediate valves, and postmucronal area of tail valve smooth, only with growth line. Pleural areas with 8–9 very short and deep longitudinal, sometimes interrupted grooves along diagonal line. Insertion plates very short and smooth. Dorsal spicules small, blunt-topped, with 8–10 ribs around. Ventral scales more or less rectangular, ornamented around 8–10 distally converging riblets. Major lateral tooth bearing a cusp with 4 denticles.

D e s c r i p t i o n. Holotype of small size, BL 7.0 mm, elongate oval. Valves low (dorsal elevation 0.37), back rounded, side slopes convex, not beaked. Tegmentum light yellow with white spots.

Head valve semicircular, front slope convex, posterior margin widely V-shaped, shallowly notched in the middle. Intermediate valves more than twice as wide as long, anterior margin sinuate, concave in the jugal area, convex at the pleurae, posterior margin evenly convex not beaked, lateral areas slightly raised. Tail valve slightly narrower than head valve, anterior margin slightly concave, hind margin evenly convex, mucro posterior, hind slope almost steep, convex, antemucronal slope straight.

Tegmentum of head valve, lateral area of intermediate valves, and postmucronal area of tail valve smooth, only with well noticeable growth line. Pleural areas with 8–9 very short and deep longitudinal, sometimes interrupted grooves along diagonal line on each side.

Articulamentum white, apophyses triangular with rounded top in intermediate valves, and trapezoidal in tail valve, separated by a wide, deep, slightly concave, delicately dentate sinus (10–11 denticles in valves IV– VI). Slit formula 9/1/11, slit rays hardly or not indicated, teeth rather long, not pectinated, those of the tail valve much deformed, eaves narrow, porous.

Girdle dorsally covered with close-set small (45x21 µm), blunt-topped spicules, with 10–11 ribs around, and scattered curved needles (39–42x5 µm). Marginal spicules slightly ribbed, round-topped up to 79x17 µm. Ventral side of girdle clothed with more or less rectangular scales (25x21 µm), ornamented around 12–13 distally converging riblets.

Radula of the holotype was damaged, only the head of major lateral tooth is preserved, It has four denticles.

Due to the fact that the holotype was damaged, it was not possible to calculate the number of gills.

R e m a r k s. Despite the fact that this new species has a small size it looks like an adult chiton. Suffice it to recall a related species Tonicia disalvoi (Dell Angelo, Raines et Bonfitto, 2004) whose body length is two millimeters less. The present new species is most similar to Tonicia disalvoi that also has similar sculpture of tegmentum and not pectinated insertion plates, but differs from the latter in having ribbed ventral scales (vs. smooth scales in T. disalvoi ), posterior mucro (vs. anterior mucro in T. disalvoi ), jugal and most part of central area smooth (vs. with small pits in T. disalvoi ), and curved shape of intermediate valves (vs. broadly rectangular in T. disalvoi ).

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.