Leptochiton spiniferus, Sirenko, 2020

Sirenko, Boris I., 2020, New shelf and bathyal Philippine leptochitons (Mollusca: Polyplacophora Lepidopleurida), Zootaxa 4878 (3), pp. 467-500 : 492-494

Leptochiton spiniferus n. sp.

( Figs. 28–32 )

Type material. Holotype (MNHN-IM-2013-67050), specimens damaged (head valve missing), now disarticulated consisting of valves III, VII, VIII, part of perinotum and radula, mount of part of perinotum and radula mounted on SEM stub and vial with other valves;.

Type locality. Philippines, Balicasag Id., 9°36.6’N, 123°45.4’E, 80–90 m (PANGLAO 2004, stn L45, 03.07.2004) .

Etymology. From the Latin spiniferus to bear prinkle, referring to the intersegmental needles.

Material examined. The holotype.

Distribution. Known only from the holotype.

Diagnosis. Small, oval, shell rounded. Tail valve mucro anterior. Tegmentum sculptured with small, aciniform granules arranged in quincunx. Dorsal scales bent, obtusely pointed, with thickened base and 9–11 longitudinal partly double ribs. There are 2–3 long (up to 310 µm), bent intersegmental needles arranged under posterior margin of valves in middle between apex and side margin of valve. Marginal elements of two types: long ribbed needles and long ribbed sharply pointed scales. Central tooth of radula small, somewhat pinched in the middle and pointed in base; major lateral tooth with a strong cusp bearing one sharp denticle.

Description. Small (probably adult), BL about 2.0 mm, oval, moderately elevated (elevation ratio in valve III 0.40). Valves rounded, not beaked. Color of tegmentum dirty white.

Intermediate valves broadly rectangular, short and wide, anterior margin weakly concave in pleural area, posterior margin nearly straight, not beaked, lateral margins rounded. Lateral areas weakly raised. Tail valve high, transversely elliptical with anterior mucro, anterior margin weakly concave, antemucronal area short, convex, postmucronal area deeply concave.

Tegmentum sculptured with small, aciniform granules arranged in quincunx. Each granule has one megalaesthete and 6–8 micraesthetes.

Articulamentum well developed, white; apophyses small, triangular and widely separated.

Girdle very narrow, covered with wide, bent, obtusely pointed dorsal scales (48–50 x 50 µm), with thickened base and 9–11 longitudinal partly double ribs. Intersegmental areas deeply penetrated between valves with 2–3 long (310 x 17 μm), smooth bent needles. Needles arranged under posterior margin of valves in middle between apex and side margin of valve. Marginal elements of two kind: long ribbed needles (140 x 10 µm) and long ribbed sharply pointed scales (80 x 15 µm). Two rows of large ventral scales near margin have 5–7 short, weak ribs on the distal half at scales’ ventral side, remaining ventral scales shorter, smooth and obtusely pointed (50 x 16 μm).

Radula 0.7 mm long with 38 transverse rows of mature teeth. Central tooth of radula small, somewhat pinched in the middle and pointed at base; major lateral tooth with strong cusp bearing one sharp denticle.

Gills not counted as specimen was dry.

Remarks. The new species has an unique feature: The intersegmental areas are deeply penetrated between valves with 2–3 long, smooth bent, calcareous needles. The needles are arranged beneath the posterior margin of the valves in the middle between apex and side margin of valve. This feature combined with the aciniform granules well distinguishes Leptochiton spiniferus n. sp. from all other known species of genus Leptochiton including those of the subgenus Pilsbryella Nierstrasz, 1905 . The only species of the latter subgenus Leptochiton (P.) setigera ( Nierstrasz, 1905) has silky hairs but they do not extend as far to the apex as the intersegmental calcareous needles of the new species.

I am sure that L. spiniferus n. sp. differs from L. (P.) setigera . If we take both descriptions ( Nierstrasz 1905; Kaas & Van Belle 1985) as a basis, the differences between the two species are more than sufficient. The sculpture of the tegmentum, the scales and the spicules of the perinotum differ considerably from each other. Moreover, the head of major lateral teeth of the radula, the calcareous needles and chitinous hairs between the valves differentiate both taxa from each other. But I think that type lot of L. (P.) setigera contains different species: The lectotype (st. 88) represents L. (P.) setigera , but two paralectotypes (st. 12) are juveniles of Nierstraszella . I have studied these paralectotypes of L. (P.) setigera . The lectotype and both paralectotypes formed the base for the description of the species L. (P.) setigera by Nierstrasz (1905) and by Kaas & Van Belle (1985). However, the paralectotypes belong to the genus Nierstraszella , most likely to N. andamanicus ( Kaas & Van Belle 1985, figs 74, 1–4, 6–9). This identification of the paralectotypes is based on the 1) arrangement of false tegmentum granules in quincunciale pattern, 2) subcarinated valves, 3) elongated dorsal spicules-scales, 4) a tridentate cusp of the major lateral tooth, which is typical for juvenile individuals of Nierstraszella ( Sirenko 1992) and 5) the paralectotypes were found on a piece of sunken wood which is very typical for Nierstraszella . The new species L. spiniferus can be compared with those parts of the descriptions and figures given by Nierstrasz 1905 (p. 11–13, Taf. I, fig. 7, Taf. III, figs 60–63) and Kaas & Van Belle 1985 (p. 161–163, fig. 74. 5) which refer to the lectotype of L. (P.) setigera . The new species differs from L. (P.) setigera by having aciniform granules of the tegmentum (vs round granules in L. (P.) setigera ) and calcareous intersegmental needles (vs. chitinous hairs in L. (P.) setigera ). Nierstrasz (1905) clearly distinguished spines and hairs (“Gürtel stark mit Stacheln besetzt und überdies mit Haaren versehen...” page 13, translated: “girdle strongly covered by spines and also by hairs…”), he characterized the hairs as being chitinous (“Diese Haare sind chitinös und von brauner Farbe.” page 11; translated: “These hairs are chitinous and of brown color.”). Therefore, these hairs cannot be considered to represent hard spines. By the way, two species of chitons: Leptochiton longisetosus Sigwart & Sirenko, 2012 and L. dykei Sigwart & Sirenko, 2012 ( Sigwart & Sirenko 2012) have long silky intersegmental bristles very similar to the hairs described by Nierstrasz for L. (P.) setigera .

Figures

FIGURE 28. Leptochiton spiniferus n. sp., Philippines, holotype (MNHN-IM-2013-67050), BL 2.2 mm: A. Whole specimen, dorsal view; B. Whole specimen, lateral view; C, D. Position of sutural needles.

FIGURE 29. Leptochiton spiniferus n. sp., Philippines, holotype (MNHN-IM-2013-67050), BL 2.2 mm: A. Valve III, dorsal view; B. Valve VII, dorsal view; C. Valve VIII, dorsal view; D. Valve III, rostral view; E. Valve III, detail of tegmentum in central area; F. Valve VII, detail of tegmentum in central area; G. Valve VIII, lateral view.

FIGURE 30. Leptochiton spiniferus n. sp., Philippines, holotype (MNHN-IM-2013-67050), BL 2.2 mm: A. Dorsal, marginal and ventral scales; B. Dorsal and marginal scales and marginal needles; C. Dorsal scales; D. Sutural needles and dorsal scales.

FIGURE 31. Leptochiton spiniferus n. sp., Philippines holotype (MNHN-IM-2013-67050), BL 2.2 mm: A. Radula; B. Central and first lateral teeth of radula.

FIGURE 32. Leptochiton spiniferus n. sp., Philippines (MNHN-IM-2013-67050), BL 2.2 mm: A. Sutural needle; B. Dorsal scale; C. Marginal needle; D. marginal scale; E. Ventral scale near margin; F. Ventral scale from middle hyponotum; G. Head of major lateral tooth of radula; H. Central and first lateral teeth of radula; I. Aesthete group; J. Position of sutural needles. Scale bars (A, B-I) 100 µm, (J) 1 mm.