Leptochiton costatoacus, Sirenko, 2020

Sirenko, Boris I., 2020, New shelf and bathyal Philippine leptochitons (Mollusca: Polyplacophora Lepidopleurida), Zootaxa 4878 (3), pp. 467-500 : 485-487

Leptochiton costatoacus n. sp.

( Figs. 21–23 )

Type material. Holotype (MNHN-IM-2007-35506) now disarticulated consisting of SEM stub of valves I, III, IV, V, VIII, part of perinotum and radula, mount of part of perinotum and radula and vial with other valves.

Type locality. Philippines, Bohol / Sulu seas, 8°43.7’N, 123°14.0’E, 105–109 m (PANGLAO 2005, stn CP2374) .

Etymology. From the Latin costatus, ribbed, and acus, needle, referring to the armature of the girdle in this species, which has ribbed needles.

Material examined. Known only from the holotype.

Distribution. Only known from type locality.

Diagnosis. Animal small, BL 5.5 mm, low elevated, valves rounded, not beaked; tail valve mucro posterior, postmucronal slope slightly concave; tegmentum sculptured with small granules arranged in longitudinal rows in pleural areas of intermediate and tail valves, more or less in radial rows in head valve and quincuncially in other areas, including jugal area; each granule with one megalaesthete and 12 micraesthetes; dorsal scales sculptured with 11 to 12 fine, partly double riblets; marginal and sutural needles with ribs; central teeth of radula wide, major lateral tooth of radula unicuspidate, with very week knob on inner margin of head.

Description. Holotype (BL 5.5 mm) elongate oval in outline. Shell rounded, low elevated (elevation ratio 0.33), valves not beaked. Tegmentum white in colour.

Head valve semicircular, tegmentum sculpture with small, roundish granules arranged in more or less radial rows. Intermediate valves rectangular, lateral areas not raised, anterior and posterior margins nearly straight. Pleural areas sculptured with round or oval granules arranged in longitudinal rows, jugal and lateral areas with granules arranged in quincunx. Tail valve narrower than head valve, mucro posterior, postmucronal slope slightly concave. Antemucronal area sculptured like central area of intermediate valves, postmucronal area sculptured like lateral areas. Each granule with one megalaesthete and 10–12 micraesthetes.

Articulamentum moderately developed, apophyses small, widely separated, subtriangular in valves II–III, more or less trapezoidal in tail valve.

Girdle very narrow, dorsally densely beset with elongate, slightly curved, pointed scales, sculptured with eleven to twelve fine riblets (60 x 43 μm), long sutural scales up to 110 μm, with 9 riblets, sutural needles long and ribbed, (250 x 20 μm), ribbed marginal needles (240 x 25µm) same size but wider. Two rows of ventral scales near margin (71 x 30 μm) with two short ribs in upper half, other ventral scales lanceolate, flat, smooth (90 x 23 μm).

Holotype with ten gills on each side extending from valve VI to near anus.

Radula of the holotype 2.8 mm long with 26 transverse rows of mature teeth. Central teeth wide with trapezoidal blade. Major lateral teeth with sharply pointed, unidentate head with very weak knob on inner margin.

Gut containing intact foraminifera and detritus.

Remarks. In Leptochiton costatoacus n. sp., the granules are located in longitudinal rows in the central areas of the intermediate valves and in the antemucronal area of the tail valve. Regarding the rest of the features of girdle, radula and other areas of the shell (jugal area, postmucronal area of the tail valve and the most part of head valve) the new species is quite close to species of the L. belknapi group. Of all the species of this group the new species most closely resembles Leptochiton fijiensis Sirenko, 2016 , but differs by having a posterior mucro (vs. anterior mucro in L. fijiensis ), 11–12 ribs in dorsal scales (vs. 6–7 ribs in L. fijiensis ), ribbed sutural needles (vs. smooth needles in L. fijiensis ) and longitudinal rows of granules in pleural areas (granules arrange in quincunx in L. fijiensis ).

Figures

FIGURE 21. Leptochiton costatoacus n. sp., Philippines, holotype (MNHN-IM-2007-35506), BL 5.5 mm: A. Valve I, dorsal view; B. Valve III, dorsal view; C. Valve IV, dorsal view; D. Valve VIII, dorsal view; E. Valve IV, detail of tegmentum in central and lateral areas; F. Valve III, rostral view; G. Valve VIII, lateral view; H. valve V, ventral view.

FIGURE 22. Leptochiton costatoacus n. sp., Philippines, holotype (MNHN-IM-2007-35506), BL 5.5 mm: A. Dorsal and ventral scales and marginal needles; B. Radula; C. Sutural scales and needles; D. Central and first lateral teeth of radula.

FIGURE 23. Leptochiton costatoacus n. sp., Philippines, holotype (MNHN-IM-2007-35506), BL 5.5 mm: A. Sutural needle; B. Dorsal scale; C. Marginal needle; D. Ventral scale near margin; E. Ventral scale from middle hyponotum; F. Heads of major lateral teeth of radula; G. Central and first lateral teeth of radula; H. Aesthete group. Scale bars 100 µm.