Leptochiton longisetosus, Sigwart, Julia D. & Sirenko, Boris I., 2012
Sigwart, Julia D. & Sirenko, Boris I., 2012, Deep-sea chitons from sunken wood in the West Pacific (Mollusca: Polyplacophora: Lepidopleurida): taxonomy, distribution, and seven new species, Zootaxa 3195, pp. 1-38: 20-22
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Leptochiton longisetosus n. sp.
Figures 4 View FIGURE 4 E, 13, 14
Leptochiton n. sp. 5 Sigwart 2008 b: 2.34–2.37, figs 2.6, 2.30–2.35; Sigwart 2009 a: table 2, fig. 2; Sigwart et al. 2011: table 3, fig. 2; Yearsley & Sigwart 2011: table 1.
Type material. Holotype ( MNHN 23699) disarticulated, consisting, of mounts of shell, perinotum and radula, 40 paratypes ( MNHM 23700) and 1 paratype ( ZISP). The type material consists of specimens collected as a single lot.
Type locality. Vanuatu, Sanma, Espiritu Santo Island, Big Bay, 15 ° S 166.86 ° E, 551–750 m; Boa 1 sta. CP 2429.
Etymology. The name is Latin for long setae, in reference to the elongate intersegmental bristles and especially the unusually long marginal fringe.
Material examined. 126 specimens in 8 lots from Vanuatu, 64 specimens in 12 lots from the Solomon Islands.
Distribution. Locally abundant and common species in Vanuatu (492–777 m), and the Solomon Islands (197– 650 m and 1000–1109 m). Found living and feeding on sunken terrestrial plant remains.
Diagnosis. Animal small, up to 13.2 × 8.3 mm. Valves low elevated, with pronounced keel; not beaked. Head valve wider than tail valve. Intermediate valves side margins rounded, anterior and posterior margins straight, lateral areas not raised but diagonal slightly inflated. Tail valve mucro subcentral, postmucronal slope concave. Tegmentum with oval to round granules in quincunx, forming central diagonal rows pointed inward and to the anterior. Girdle wide, dorsally densely covered in round-topped ribbed spicules, distinctive chitinous bristles in intersegmental areas and marginal fringe. Four gills per side. Central teeth of radula with long blades; major lateral teeth with tridentate cusps.
Description. Holotype 9.5 × 6.2 mm. Shell strongly carinated, with a pronounced keel, low elevated (elevation ratio 0.29–0.45). Valves are not beaked but with a distinct apex.
Head valve semicircular, wider than tail valve. Anterior slope is convex.
Intermediate valves broadly rectangular, lateral areas are not raised and very slightly depressed near apices. Diagonal ridge slightly inflated, differentiating lateral areas. Anterior margin is straight, except in valve II where the anterior margin is convex. Posterior margin is straight. Side margins are rounded. Apex distinct but not projecting.
Tail valve with subcentral (slightly anterior) mucro, anterior margin of jugal area is convex and antemucronal area keeled; postmucronal slope is concave to a flat thin margin.
Tegmentum densely sculptured with raised, rounded to oval granules (47 µm) arranged quincuncially. Granules on central area are oval and tending to form diagonal rows oriented from the central anterior to lateral posterior. Aesthete caps are not projecting. Colour of tegmentum white, older parts of valves (near apex) sometimes with grey to brown deposit.
Aesthete pores arranged on each granule with one megalaesthete and two micraesthetes in front. The three aesthetes in a group are of approximately equal size. Central megalaesthete is approximately 9 µm diameter.
Articulamentum is weakly developed; apophyses small, short, widely separated, trapezoidal in valves II–VIII. Articulamentum not forming a thickened ridge on outer margin of terminal valves.
Girdle wide relative to valves, about 1.0 mm wide near valve V in holotype often inflated in patches. Dorsally densely covered in elongate, round-topped spicule-scales (98 × 21 µm), with 4 to 5 strong ribs on each face. Intersegmental areas with longer scales (up to 140 µm) and groups of chitinous bristles (up to 2 mm) in long chitinous cups. Marginal fringe with same chitinous bristles (up to 700 µm), very long (but not as long as intersegmental tufts), longest particularly around head and tail valves, marginal spicules up to 140 × 21 µm. Ventrally, girdle covered with elongate, smooth flat narrow-scales (90 × 18 µm).
Radula of holotype 2.3 mm long, with 29 transverse rows of mature teeth. Central teeth with long blades. Major lateral teeth with tridentate head; three denticles of equal width, interior denticle shortest.
There are four gills per side, of equal length, all with laminae on a single side, except the second gill from the posterior with double laminae.
Gut contents containing particulate cellulose woody material of pale colour.
Remarks. Leptochiton longisetosus is superficially similar to L. boucheti and L. dykei . The marginal fringe in L. boucheti (100 Μm) is much shorter than in L. longisetosus (700 Μm); the intersegmental bristles in L. dykei (1000 Μm) are much longer than in either L. boucheti or L. longisetosus (both up to 140 Μm). The aesthete arrangement in L. longisetosus is also distinctive in having only two micraesthetes per megalaesthete.
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