Lepidochitona pakistana, Sirenko, 2023

Sirenko, Boris, 2023, Chitons of the genus Lepidochitona (Mollusca: Polyplacophora) in the Indian Ocean and ordering of species compositions of some genera of the subfamily Tonicellinae, Zootaxa 5228 (3), pp. 292-316 : 308-312

Lepidochitona pakistana n. sp.

( Figs 19–23 )

Type material. Holotype ( CASIZ 235554 ), now disarticulated consisting of SEM-mounted valves I, II, IV, V, VIII, parts of perinotum and radula, mount of part of perinotum and vial with other valves and 13 paratypes ( CASIZ, 019907, 019915, 019934, 024429), three of them partly disarticulated.

Type locality. Indian Ocean , Pakistan, Karachi area, Goth Jafar, 25°12'24"N, 68°37'19"E, intertidal .

Material examined. 14 spms. Indian Ocean, Pakistan, Karachi area, Goth Jafar, 25°12'24"N, 68°37'19"E, intertidal, rocks, sand, mud, 3 paratypes ( CASIZ 019915 ), BL 3.0– 4.2 mm, 23.10.1973, leg. F. B. Steiner ; 25°12'24"N, 68°37'19"E, intertidal, rocks, sand, weed covered, holotype ( CASIZ 235554 ), BL 4.4 mm and 4 paratypes ( CASIZ 019907 ), BL 2.1–4.4 mm, 24.10.1973, leg. F. B. Steiner ; 25°12'24"N, 68°37'19"E, intertidal, rocks, sand, 3 paratypes ( CAS 019934 ), BL 2.0–5.0 mm, 24– 25.10.1973, leg. F. B. Steiner ; 25°12'24"N, 68°37'19"E, low tide, on dense vegetation, 3 paratypes ( CAS 024429 ), BL 2.5–5.2 mm, 26.10.1973, leg. F. B. Steiner.

Distribution. Only known from the type locality near Karachi, Pakistan.

Diagnosis. Chiton of small size, up to 5.2 mm, valves carinated, little beaked, slightly elevated, mucro anterior. Valves II–VIII with narrow, radial groove near apex; lateral areas slightly raised. Tegmentum smooth, all pores of aesthetes rounded and about the same size. Slit formula 10/1/14. Girdle densely clothed with small, cylindrical spicules with 2–3 weakly noticeable longitudinal grooves in upper half; two rows of tufts of long curved needles between valves, around end valves, and near margin; marginal fringe consists of two kinds of needles: smooth and ribbed; ventral scales sharply pointed, smooth. Major lateral teeth of radula with tricuspid dental caps.

Description. Holotype small, BL 4.4 mm, elongate oval. Shell slightly elevated (dorsal elevation 0.29), back carinated, valves with straight side slopes, little beaked. Color of tegmentum light brown with white triangles in the middle of valves III, IV, VIII.

Head valve semicircular, posterior margin widely V-shaped. Intermediate valves broadly rectangular, front margin anteriorly produced in valve II, somewhat concave in valves III–VII, hind margin concave at both sides of the prominent apex, valves II–VIII with two narrow, radial grooves near apex, lateral areas weakly raised, side margins rounded. Tail valve smaller than head valve, front margin concave, mucro anterior, ratio of length of postmucronal area to length of antemucronal area 2.25, postmucronal slope concave.

Tegmentum of all valves smooth to the naked eye, microscopically ridged. Aesthetes arranged in radial rows on head valve, lateral areas and pleural part of central areas of intermediate valves and postmucronal area of tail valve, jugal areas and antemucronal areas with longitudinally arranged pore rows, several fine, weakly marked growth lines.

Articulamentum moderately developed, translucent, ivory. Apophyses thin, short, broadly triangular with rounded top in intermediate valves, more or less trapezoid in tail valve, separated by a narrow, flat jugal sinus, insertion plates short, slit formula 10/1/14, slits narrow, slit rays present in all valves, teeth short, smooth, eaves narrow, porous.

Girdle narrow, light brown with white spots near tufts of spicules, dorsally densely covered with very small, cylindrical spicules (28 x 14 µm) with 2–3 weakly noticeable longitudinal grooves in upper half; two rows of long bent needles (up to 300 x 28 µm) collected in tufts arranged along the body. One row arranged near valves, and another row arranged closer to girdle margin. Marginal spicules of two kinds: elongate, straight, longitudinally ribbed in dorsal side, sharply pointed needles (200 x 25 µm) and long, smooth needles gradually tapering to a blunt top (200–300 x 28 µm). Two kinds of ventral girdle scales: one row of rectangular scales (70 x 25 µm) near margin and remaining scales (55 x 13 µm), sharp-pointed, with 2–3 narrow ribs in upper half.

Radula of holotype 1.2 mm long with 23 rows of mature teeth. Central tooth short, with narrow base, first lateral teeth elongate, wing-shaped with rounded blade, major lateral teeth with tridentate cusps, denticles sharp, central denticle noticeably larger.

Holotype has 12 long gills per side, from valve III to valve VII. Nephridiopores and gonopores situated between first and second and second and third gills counted from posterior, accordingly.

Remarks. The color of the tegmentum of the new species varies from ivory with small olive and brown spots on the posterior edges of valves I–VII, to light brown with white triangles in the middle of valves III, IV, VIII. The joint slit formula of three type specimens is 10–11/1/11–14. There is an increase in the number of gills with an increase in body length. Individuals with body lengths of 4.3, 4.4 and 5.0 mm had 11, 12 and 13 gills per side, respectively.

Lepidochitona pakistana n. sp. differs from L. africana n. sp. by having cylindrical, round topped dorsal spicules (vs sharply pointed spicules with 2–3 weakly noticeable longitudinal riblets in the upper half in L. africana n. sp), valves II–VII with two narrow, radial grooves near apex (vs grooves lacking in L. africana n. sp.), width of jugal sinus larger width of apophysis (vs width of jugal sinus smaller width of apophysis in L. africana n. sp.), ventral scales smooth (vs ventral scales with ribs in L. africana n. sp.). Moreover, L. pakistana n. sp. has a much longer postmucronal area: the ratio of length of postmucronal area to length of antemucronal area is 2.31 in L. pakistana n. sp. and 1.15 in L. africana n. sp.

The new species differs from the closely related Lepidochitona sp. ( Van Belle 1994) from the Arabian Gulf by having concave front margins of valves III–VII (vs convex in Lepidochitona sp. ), cylindrical dorsal spicules (vs obtusely pointed in Lepidochitona sp. ), marginal needles of two kinds, one of them ribbed (only smooth spicules in Lepidochitona sp. )

L. pakistana n. sp. differs from L. turtoni and L. dicksae by having smooth ventral scales (vs ribbed scales in both latter species), two rows of tufts with long, bent needles (vs one row in L. dicksae and single, short spines in L. turtoni ), cylindrical dorsal spicules (vs obtusely pointed spicules in both species). Moreover, L. pakistana n. sp. has megalaesthete pores equal in size to the micraesthete pores (vs megalaesthete pores are twice as large as micraesthete pores in L. turtoni ).

Figures

FIGURE 19. Lepidochitona pakistana n. sp., Pakistan, W. of Karachi, Goth Jafar, low tide, paratype (CASIZ 019907), BL 4.4 mm.

FIGURE 20. Lepidochitona pakistana n. sp., Pakistan, W. of Karachi, Goth Jafar, low tide, holotype (CASIZ 235554), BL 4.4 mm: A. Valve I, dorsal view; B. Valve II, dorsal view; C. Valve V, dorsal view; D. Valve VIII, dorsal view; E. Valve IV, ventral view; F. Valve V, detail of tegmentum surface of central area; G. Valve V, rostral view; H. Valve VIII, lateral view.

FIGURE 21. Lepidochitona pakistana n. sp., Pakistan, W. of Karachi, Goth Jafar, low tide, holotype (CASIZ 235554), BL 4.4 mm: A. Dorsal spicules, two kinds of marginal needles and two kinds of ventral scales; B. Dorsal spicules; C. Dorsal spicules, two kinds of marginal needles and two kinds of ventral scales; D. Dorsal spicules and sutural needle.

FIGURE 22. Lepidochitona pakistana n. sp., Pakistan, W. of Karachi, Goth Jafar, low tide, holotype (CASIZ 235554), BL 4.4 mm (B), paratype (CASIZ 019915) BL 3.0 mm (A, C) paratype (CASIZ 024429), BL 5.2 mm (D): A. Two kinds of ventral scales; B. Radula; C. Dorsal spicules, two rows of tufts with needles and two kinds of marginal needles; D. Central, first lateral and major lateral teeth of radula.

FIGURE 23. Lepidochitona pakistana n. sp., Pakistan, W. of Karachi, Goth Jafar, low tide, holotype (CASIZ 235554), BL 4.4 mm: A. Needle of tuft; B, C. Dorsal spicules; D. Marginal smooth needle; E. Marginal ribbed needle; F. Ventral rectangular scale; G. Ventral scale; H. Central tooth and first lateral teeth of radula; I. Head of major lateral tooth of radula. Scale bar 100 µm.