Terenochiton nomurai Sirenko et Saito, 2020

Sirenko, Boris I. & Saito, Hiroshi, 2020, Two new species of the family Leptochitonidae (Mollusca: Polyplacophora) from the tropical and subtropical shallow waters of the West Pacific, The Bulletin of the Russian Far East Malacological Society 24 (1 / 2), pp. 19-36 : 28-33

Terenochiton nomurai Sirenko et Saito sp. nov.

Figs. 7–11

urn:lsid:zoobank.org:act:263E91F1-4BCE-422A-AF51-F7AB886D35C3

Ty p e m a t e r i a l. The holotype (NSMT-Mo 79027), and paratype (NSMT-Mo 79028).

Ty p e l o c a l i t y. Amitori Bay, Iriomote Island, Yaeyama Islands, Okinawa, Japan, 26°11.8′ N, 130°48.8′ E, on reef.

M a t e r i a l e x a m i n e d. Japan, NSMT-Mo 79027, the holotype, BL 6.0 mm, Amitori Bay, Iriomote Island, Yaeyama Islands , Okinawa, Japan, 26°11.8′ N, 130°48.8′ E, on reef, stn H- 6 in the survey for juveniles of crown-of-thorns starfish, Acanthoaster planci , June–October 1985, T. Habe; NSMT-Mo 79028, paratype, BL 3.4 mm, Kuroshima Island , Yaeyama Islands, Okinawa, Japan, 26°11.8′ N, 130°48.8′ E, interstice of dead coral block deeply embedded in sand on reef, stn 2 in the survey for small crustaceans, 21.06.1991, K. Nomura .

E t y m o l o g y. Named after Dr. Keiichi Nomura, former director of the Kushimoto Marine Park, Wakayama, Japan who collected the specimen, and kindly placed it at our disposal.

D i a g n o s i s. Animal small. Shells moderately elevated, carinated. Central and antemucronal areas with rather coarse net-like sculpture formed by connected granules. Each granule with 7(5–9)aesthete pores.Articulamentum well developed, with rudiments of insertion plates in all valves; those in head valve low, nodular. Girdle dorsally densely covered with small, bent, imbricating, round top scales which are ornamented with 6–8 strong bifurcated ribs; intersegmental area between valves with long, spicule which is curved near both ends. Radula teeth thin, with more than 75 transverse rows; head of major lateral tooth unicuspid.

D e s c r i p t i o n. Animal small, BL 6.0 mm in holotype, elongate-oval. Valves moderately elevated (dorsal elevation 0.31 in valve V), carinated, not beaked. Color of tegmentum creamy white.

Head valve semicircular. Intermediate valve rectangular; lateral margins rounded; anterior margin slightly concave between apophyses; posterior margins nearly straight; lateral areas slightly raised. Tail valve rather low, narrower than head valve; mucro central; antemucronal slope convex; postmucronal slope slightly concave.

Tegmentum of head valve sculptured with 31 radial rows of raised, oval, closely set, small granules. Central areas sculptured with ca. 20 longitudinal rows of oval, semi-fused granules, which are also connected with nearest granules in neighboring rows by commarginal riblets, forming a coarse net-like sculpture as a whole. Lateral areas sculptured with 5–6 radial rows of granules as head valve. Antemucronal area sculptured as central area with 14 longitudinal rows of granules. Postmucronal area sculptured as head valve with 25 radial rows of oval granules. Each granule with one megalaesthete and 3 (2–4) micraesthetes on each side of anterior surface. Micraesthete pores slightly smaller than megalaesthete pore, which are occasionally open through short tubular extension ( Fig. 8G , arrow head) in front. Micraesthete pores slightly smaller than megalaesthete pores.

Articulamentum well developed. Head valve with low nodular rudiments of insertion plates. Apophyses of intermediate and tail valves roundly projecting forward with wide jugal sinus, and connecting back in form of rudiments of unslitted insertion plates (on the right side of the tail valve, there is a very narrow slit between the apophysis and rudiment of insertion plate, but it is not detected in the left side).

Girdle narrow, dorsally densely covered with small, up to 75x78 μm, bent, imbricating, round topped scales which are ornamented with 7–9 strong bifurcated ribs, and spreading at base. Intersegmental area with long, up to 390x19 μm, smooth needle, which is curved near both ends. Marginal fringe of narrow scales, up to 73x17 μm, obtusely pointed, with 3–4 narrow ribs. Ventrally girdle covered with oval or nearly rectangular flat scales which are attaining 53x25 μm, smooth on surface or with faintly keeled.

Gills seven ctenidia per side in paratype.

Radula of holotype (BL 2.5 mm) ca. 1 mm long with at least 75 transverse rows of very small, thin teeth. Central tooth elongate, with pointed blade. First lateral teeth longer than central one, bearing elongate, curved blade. Major lateral teeth with unidentate cusp. Major uncinal teeth long with long, wide blade. Outermost marginal teeth wide, short.

D i s t r i b u t i o n. Only known from Yaeyama Islands, Okinawa, Japan, on reef.

R e m a r k s. The genus Terenochiton was recently reinstated by the first author [ Sirenko, 2019] from a synonym of Leptochiton [Van Belle, 1983; Kaas, Van Belle, 1985; 1998]. The peculiar feature of this genus is possession of the rudiments of insertion plates in all valves, by which the present specimens are assigned in this genus.

Terenochiton nomurai sp. nov. closely resembles T. norfolcensis by the sculpture of the tegmentum, girdle armature, and the shape of the radula, but differs from the latter by arrangement of the aesthete pores on the granules of the tegmentum, and less developed rudiments of the insertion plates. The aesthete pores of T. nomurai are arranged in peripheral portion of each granule, whereas they are arranged more or less medially on flatter granule in T. norfolcensis . The rudiments of the insertion plates are low, rather ambiguous in the head valve of T. momurai , but they are much higher and prominent in that of T. norfolcensis .

Figures

Fig. 7. Terenochiton nomurai, A – holotype (NSMT-Mo79027), Japan, Okinawa, Iriomote Island. BL 6.0 mm; B – paratype (NSMT-Mo 79028), Japan, Okinawa, Kuroshima Island. BL 3.4 mm. A, B – whole animals, arrow head indicating intersegmental needle.

Fig. 8. Terenochiton nomurai, holotype (NSMT-Mo79027), Japan, Okinawa, Iriomote Island. BL 6.0 mm. A – valve I, dorsal vies; B – valve IV, dorsal view; C – valve IV, rostral view; D – valve VIII, dorsal view; E – valve VIII, lateral view; F – valve I, detail of tegmentum surface; G – valve IV, detail of tegmentum surface of central area, arrow head showing tubular extension for aesthete pore.

Fig. 9. Terenochiton nomurai, holotype (NSMT-Mo79027), Japan, Okinawa, Iriomote Id. BL 6.0 mm. A – valve I, ventral view; B – close up of square portion inA, showing rudiment of insertion plate; C – valve II, ventral view; D – close up of square portion in C, showing rudiment of insertion plate; E – same rudiment in D, slightly anterior view; F, G – valve VIII, rudiment of insertion plate, ventral and ventro-lateral views. r – rudiment of insertion plate.

Fig. 10. Terenochiton nomurai, A, B, D, E – holotype (NSMT-Mo79027), Japan, Okinawa, Iriomote Island. BL 6.0 mm; C, F – paratype (NSMT-Mo 79028), Japan, Okinawa, Kuroshima Id. BL 3.4 mm. A – dorsal girdle scales; B, E – head of major lateral teeth; C – radula, dorsal view; D – central part of radula; F – marginal part of radula.

Fig. 11. Terenochiton nomurai, holotype (NSMT-Mo79027), Japan, Okinawa, Iriomote Island. 6.0 mm. A – arrangements of granules and aesthete pores (schematic drawing); B – dorsal girdle scale; C – intersegmental needle; D – marginal scale; E – ventral scale; F, G – ventral scale near girdle margin; H – central and first lateral teeth (semi-schematic drawing); I, J – head of major lateral tooth. Scale bars: 10 µm for H–J, 20 µm for B, D–G, 100 µm for C.