Leptochiton pumilus 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 : 21-26

Leptochiton pumilus Sirenko et Saito sp. nov.

Figs. 1–6

urn:lsid:zoobank.org:act:A05F95A0-2BAB-408A-99CA-6E1ACE06C85D

Ty p e m a t e r i a l. The holotype ( MNHM IM-2013-2051 ) , and two paratypes ( MNHM IM-2013-49553 , and IM-2013-850 ) .

Ty p e l o c a l i t y. Papua New Guinea, Kranket Island, west side, 05°11.8′ S, 145°48.8′ E, depth 7–15 m (Expédition PAPUA NIUGINI, stn PB12) .

M a t e r i a l e x a m i n e d. The Philippines, Bohol Island, Maribohoc Bay , PANGLAO 2004, stn P1, 09°36′ N, 123°45′ E, depth 90–200 m, tangle nets from local fisherman, paratype ( IM-2013-49553 ), 1 spm, BL 2.0 mm, 30.05.2004, coll. Panglao Marine Biodiversity Project ; Papua New Guinea, Kranket Island, west side, Expédition PAPUA NIUGINI, stn PB12, 05°11.8′ S, 145°48.8′ E, depth 7–15 m, the holotype ( IM-2013-2051 ), BL 2.5 mm, 11.11.2012 ; Madang, east of the airport, Expédition PAPUA NIUGINI, stn PD04 , 05°12.6′ S, 145°47.8′ E, depth 5–18 m, paratype ( IM-2013-850 ) and 1 spm ( IM-2013-850 ), BL 2.0 mm, 07.11.2012 ; south of Sek Island, Expédition PAPUA NIUGINI, stn PD22 , 05°06.3′ S, 145°49.2′ E, depth 12–20 m, 1 spm, BL 2.0 mm, 14– 15.11.2012 .

E t y m o l o g y. From the Latin pumilus , pygmean, because it is very small.

D i a g n o s i s. Animal very small. Shell rather elevated; back rounded. Each granule on central area connected with four nearest granules by crosspieces and provided with five aesthete pores. Girdle dorsally densely covered with small, bent, imbricating, bluntly pointed scales which are ornamented with 11–13 strong bifurcated ribs. Radula with ca. 30 transverse rows of very small teeth; first lateral teeth longer than central tooth; head of major lateral tooth with two denticles.

D e s c r i p t i o n. Animal very small, BL 2.5 mm in holotype (largest specimen); elongate-oval. Valves moderately elevated (dorsal elevation 0.43 in valve V), roundbacked, not beaked. Color of tegmentum white.

Head valve semicircular. Intermediate valve rectangular; lateral margins rounded; anterior margin slightly convex; posterior margin nearly straight; lateral areas not raised. Tail valve high, narrower than head valve; mucro anterior; antemucronal slope short, convex; postmucronal slope long, concave.

Tegmentum of head valve sculptured with 26 radial rows of closely set small, oval, flattened granules. Central areas sculptured with ca. 27 closely set longitudinal rows of oval granules; each granule connected with four nearest granules by crosspieces. Lateral areas sculptured with 6–7 radial rows of granules as head valve. Antemucronal area sculptured as central area with 18 longitudinal rows of granules. Postmucronal area sculptured as head valve with ca. 30 radial rows of oval granules. Each granule with one megalaesthete and four micraesthetes in front. Micraesthete pores slightly smaller than megalaesthete pores.

Articulamentum thin, white. Apophyses small, triangular, widely apart; jugal sinus shallow, wide.

Girdle very narrow, dorsally densely covered with small, up to 45x38 μm (in central part of perinotum) bent, imbricating, bluntly pointed scales, which are ornamented with 11–13 strong, bifurcated ribs; scales longer, narrower with fewer 7 ribs in intersegmental area, rare ribbed needles up to 90x8 μm scattered among dorsal scales. Intersegmental area with long, up to 280x10 μm ribbed needles. Marginal needles 90x13 μm, with 7 ribs on one side. Ventrally girdle covered with elongated flat scales; those near margin, distally obtusely pointed, 70x20 μm, with 5–6 long ribs on the ventral side; those in middle of girdle and near pallial groove shorter, 40x17 μm and smooth.

Gills four short ctenidia per side in studied specimens, arranged from valve VII to anus.

Radula of holotype 0.8 mm long with 31 transverse rows of very small mature teeth. Central tooth elongate, with small blade at top. First lateral teeth longer than central one, bearing elongate, curved blade. Major lateral teeth with bidentate cusp; inner denticle much smaller than outer main one, which has an accessory protuberance on the outer side.

D i s t r i b u t i o n. Philippines and Papua New Guinea, depth 5–90 m.

R e m a r k s. The paratype ( IM- 2013-49553) from the Philippines (90 m) has no accessory protuberance on the outer side of head of major lateral teeth of radula. The protuberance is probably remnant of the third denticle which was generally found in juvenile chiton. Leptochiton pumilus sp. nov. is much similar to Leptochiton muelleri Sirenko et Schwabe, 2011 , but differs from the latter by having bifurcated ribs in dorsal scales (simple ribs in L. muelleri ), four gills (six gills in L. muelleri ), bidentate cusp of major lateral teeth (tridentate cusp in L. muelleri ), and long first lateral teeth (short teeth in L. muelleri ).

Figures

Fig. 1. Leptochiton pumilus, holotype (MNHM IM-2013-2051), Papua New Guinea, Kranket Island, Expédition PAPUA NIUGINI, stn PB12. BL 2.5 mm. A – valve I, dorsal view; B – valve II, dorsal view; C – valve VIII, dorsal view; D – valve IV, ventral view; E – valve II, detail of tegmentum surface of central area; F – valve II, rostral view; G – valve VIII, lateral view.

Fig. 2. Leptochiton pumilus, paratype (MNHM IM-2013-49553), the Philippines, Bohol Island, Maribohoc Bay, PANGLAO 2004, stn P1, BL 2.5 mm (B, D); holotype (MNHM IM-2013-2051). Papua New Guinea, Kranket Island, Expédition PAPUA NIUGINI, stn PB12, BL 2.5 mm (A, C). A – whole animal, lateral view; B – dorsal girdle scales; C – dorsal scales and intersegmental spicule; D – scales around girdle margin.

Fig. 3. Leptochiton pumilus, holotype (MNHM IM-2013-2051), Papua New Guinea, Kranket Island, Expédition PAPUA NIUGINI, stn PB12. BL 2.5 mm. A – dorsal girdle scales; B – central portion of radula, dorsal view; C – ditto, posterior view; D – central portion of radula, close up.

Fig. 4. Leptochiton pumilus, paratype (MNHM-IM-2013-850), Papua New Guinea, Madang, east of the airport, Expédition PAPUA NIUGINI, stn PD4, BL 2.0 mm (A–D); paratype (MNHM-IM-2013- 49553), the Philippines, Bohol Island, Maribohoc Bay, PANGLAO 2004, stn P1, BL 2.0 mm (E). A – valve III, dorsal view; B – valve III, rostral view; C – portion of radula, dorsal view; D – valve III, detail of tegmentum surface of central area; E – central portion of radula, dorsal view.

Fig. 5. Leptochiton pumilus, paratype (MNHM-IM-2013-850), Papua New Guinea, Madang, east of the airport, Expédition PAPUA NIUGINI, stn PD4, BL 2.0 mm. A – scales around girdle margin; B – dorsal scales and intersegmental needle.

Fig. 6. Leptochiton pumilus, holotype (MNHM-IM-2013-2051), Papua New Guinea, Kranket Island, Expédition PAPUA NIUGINI, stn PB12. BL 2.5 mm (A–H, J, L); paratype (MNHM- IM-2013-49553), the Philippines, Bohol Island, Maribohoc Bay, PANGLAO 2004, stn P1, BL 2.0 mm (I); paratype (MNHM IM-2013-850), Papua New Guinea, Madang, east of the airport, Expédition PAPUA NIUGINI, stn PD4, BL 2.0 mm (K). A – intersegmental needle; B – dorsal girdle scale of intersegmental area; C – ribbed dorsal needles; D – dorsal girdle scale; E – marginal needle; F – ventral scale near girdle margin; G – ventral scale; H – central and first lateral teeth; I–K – heads of major lateral teeth; L – arrangements of granules and aesthete pores (schematic drawing). Scale bar 100 µm.