Leptochiton peruvianus, Sirenko, Boris, 2015

Sirenko, Boris, 2015, Shallow and deep-sea chitons of the genus Leptochiton Gray, 1847 (Mollusca: Polyplacophora: Lepidopleurida) from Peruvian and Chilean waters, Zootaxa 4033 (2), pp. 151-202 : 184-188

Leptochiton peruvianus n. sp.

( Figs 8 E, 30–33)

Types. Holotype (ZISP 2209), now disarticulated consisting of SEM stub of valves I, IV, V, VIII, part of perinotum and radula, mount of part of perinotum and radula and vial with other valves, and two paratypes (ZISP 2210, 2211).

Type locality. Peru-Chile Trench, RV Akademik Kurchatov, cruise 4, st. 274, 12°29’8’’S, 77°32’1’’W, 560 m. Etymology. Named after Peru.

Material examined. Peru-Chile Trench, RV Akademik Kurchatov, cruise 4, st.274, 12°29’8’’S, 77°32’1’’W,

560 m, holotype, BL—5.0 mm and 1 paratype ( ZISP 2210), BL—5.0 mm, 24.10.1968; Peru, RV Dmitriy

Mendeleev, cruise 20, st. 1645, 08°48.3’S, 79°59.0’W, 518 m, paratype ( ZISP 2211), BL— 6.5 mm, 13.03.1978. Distribution. Peru-Chile Trench, 518– 560 m.

Diagnosis. Animal small, up to 6.5 mm. Valves lowly elevated, not beaked. Intermediate valves short and wide, anterior and posterior margins straight, lateral area not elevated. Tail valve wider than head valve, with anterior mucro, antemucronal and postmucronal slopes straight. Tegmentum with 40 longitudinal rows of weakly raised round granules on central areas, quincuncially arranged on lateral areas. Each granule with one megalaesthete and six micraesthetes in front. Girdle dorsally densely covered with flattened, obtusely pointed scales with 10 distinct ribs. Radula with very small and numerous teeth, central teeth narrow, first lateral teeth elongated, major lateral teeth with tridentate, sharp cusps, central denticle largest. Eleven gills per side, arranged from valve V to anus.

Description. Holotype 5.0 mm BL. Shell series widening posteriorwards (animal ovoid in outline), lowly elevated (elevation ratio 0.26 in valve V), valves rounded. Color of tegmentum white.

Head valve semicircular, notably narrower than tail valve (ratio of width of tail valve to width of head valve 1.1). Intermediate valve rectangular, short and wide, anterior and posterior margins straight, lateral margins rounded. Tail valve large with anterior mucro, antemucronal and postmucronal slopes straight.

Tegmentum sculptured with weakly raised round granules arranged in longitudinal rows on central areas of intermediate valves and on antemucronal areas of tail valve elsewhere quincuncially arranged. Forty longitudinal rows on central areas of valve V. Each granule with one megalaesthete and six micraesthetes in front. Aesthete pores of megalaesthete twice as large as pores of micraesthetes.

Articulamentum moderately developed, apophyses short, width of apophyses less than width of jugal sinus.

Girdle narrow (about 0.4 mm wide near valve V in holotype, 11% of total width) dorsally densely covered with obtusely pointed scales (55 x 48 µm) with 10 distinct ribs. Intersegmental area with long needles (160 x 11 µm) embedded in chitinouse cups in the form of “ring-shaft needles” of Thiele (1908). Marginal spicules of two kinds: some similar to intersegmental needles (136 x 11 µm), and elongate bluntly pointed scales with 4 ribs (70 x 18 µm). Ventrally girdle covered with elongate, pointed, smooth scales (62 x 23 µm).

Radula of holotype 2.2 mm long with 160 transverse rows of very small mature teeth. Central teeth narrow, first lateral teeth elongate, major lateral teeth with tridentate cusp, central denticle largest.

Eleven gills per side arranged from valve V to anus.

Remarks. The new species differs from most other species of Leptochiton by having the tail valve wider than head valve and by having an extensive row of gills arranged from anus to valve V. Leptochiton peruvianus differs from L. linseae also by having seven pores in the aesthete group (three in L. linseae ) and 10 ribs on the dorsal scales instead 16–17 in L. linseae . The new species differs from L. mutschkeae , which also has tail valve wider than head valve, in having a tridentate cusp of major lateral tooth (vs. unidentate in L. mutschkeae ), in the number of ribs in dorsal scales (10 in L. peruvianus and 2 in L. mutschkeae ) and the number of aesthete pores in the granules (7 in L. peruvianus and 5 in L. mutschkeae ). Leptochiton peruvianus resembles L. longibranchiae in the extended gill rows and wide tail valve, but differs from latter in having a tridentate cusp of the major lateral tooth (vs. unidentate in L. longibranchiae ), 10 ribs on dorsal scales instead of a single one in L. longibranchiae , more numerous transverse rows in the radula (160 in L. peruvianus and 33 in L. longibranchiae ). Leptochiton peruvianus differs from L. laurae , L. sykesi and L. medinae in having a tricuspid dental cusp of the major lateral tooth (bidentate cusp in the latter three species), seven pores in aesthete group (three to five in the latter three species), obtusely pointed, wide dorsal scales (sharply pointed, narrow dorsal scales in the latter three species).

Figures

FIGURE 8. Preserved specimens of three species. A. Leptochiton laurae Schwabe & Sellanes, 2010, (ZISP 2259). Chile, near Valdivia, BL— 16.0 mm, dorsal and lateral views; B. L. laurae, (ZISP 2260). South Orkney Ids, RV Evrika, dredge 32, BL— 13.0 mm, dorsal and lateral views; C. L. laurae, (ZISP 2261). South Shetland Ids., RV Polarstern, st. 149, BL— 10 mm, dorsal and lateral views; D. Leptochiton longibranchiae n. sp., paratype, (ZISP 2213) BL— 9.0 mm, ventral view; E. Leptochiton peruvianus n. sp., paratype, (ZISP 2211), BL— 6.5 mm, dorsal and lateral views. Scale bars 1.0 mm.