Leptochiton dykei, 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: 17-18
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Leptochiton dykei n. sp.
Figures 4 View FIGURE 4 C, 9, 10
Leptochiton n. sp. 3 Sigwart 2008 b: 2.29–2.32, figs 2.6, 2.21–2.24; Yearsley & Sigwart 2011: table 1.
Type material. Holotype ( MNHN 23695) disarticulated, consisting of mounts of shell, perinotum and radula, 8 paratypes ( MNHN 23696) and 1 paratype ( ZISP).
Type locality. Vanuatu, Sanma, between Espiritu Santo and Malo islands, 15.73 ° S 167.03 ° E, 373–800 m; Boa 1 sta. CP 2412.
Etymology. Named for Dr Gareth Dyke, of University College Dublin.
Material examined. 12 specimens in 3 lots from Vanuatu, 1 specimen from the Solomon Islands.
Distribution. Known from Vanuatu (two records at 670–800 m) and Solomon Islands (854 m). Found living and feeding on sunken terrestrial plant remains.
Diagnosis. Valves low, not beaked. Intermediate valves side margins rounded, anterior margin convex, posterior margin straight, lateral areas not raised. Tail valve with mucro anterior, postmucronal slope concave. Tegmentum with distinctive, large, well-separated, raised, round granules forming 18 longitudinal rows, angled toward central anterior margin. Aesthete caps not projecting. Girdle wide, dorsally covered in round-topped ribbed scales, intersegmental tufts of extremely long chitinous bristles, marginal fringe of same bristles longest at anterior and posterior areas of body, ventrally with large smooth flat scales. Four equal gills on each side. Radula major lateral teeth with tridentate cusps.
Description. Holotype 5.5 × 3.9 mm. Shell carinated, with low elevation (elevation ratio 0.30–0.35 in intermediate valves), valves not beaked.
Head valve semicircular, slightly wider than tail valve. Intermediate valve rectangular, lateral areas not raised, distinctly depressed near apex, anterior margin slightly convex, posterior margins nearly straight to slightly concave. Anterior margin of valve II same as other valves. Sides rounded, apex not projecting.
Tail valve with anterior mucro, anterior margin of jugal area convex, postmucronal slope concave. Whole tail valve little inflated, generally slightly concave.
Tegmentum sculpted with large (70 µm), distinctly separated, raised, round to oval granules arranged in quincunx on lateral and central areas. Granules on pleural area forming approximately nine longitudinal rows per side, angled toward the centre. Granules are larger toward outer and posterior margin, dentating posterior and lateral valve margins. Aesthete caps are not visible. Colour of tegmentum white, apparently not covered with mineral deposit.
Each granule with five or six (but sometimes up to eight) aesthetes of approximately equal size, arranged in an irregular circle on the granule surface. The largest aesthetes approximately 9.5 µm in diameter.
Articulamentum weakly developed; apophyses small, widely separated, trapezoidal in valves II–VIII. Articulamentum thin, not forming flat thickened ridge along outer margin of end valves.
Girdle appears wide, about 0.5 mm wide near valve V, dorsally covered in round-topped scales (40–70 × 18 µm), with approximately four shallow ribs on both faces of each scale. Intersegmental areas with groups (tufts) of long chitinous bristles (up to 1 mm long) with tiny specula (20 × 5 µm) on their tip, embedded in chitinous cups. Marginal fringe with the same but shorter (up to 350 µm) chitinous bristles and long spicules (200 × 18 µm). Ventrally, the girdle is covered with elongate, smooth, flat scales (50–90 × 20 µm), larger than dorsal scales.
Radula of holotype 2.2 mm long, with 36 transverse rows of mature teeth. Central and first lateral teeth short and wide. Major lateral teeth with tridentate head; three denticles of equal width, interior denticle shortest.
Four gills per side, all with laminae on a single side, except the second gill from the posterior with double laminae.
Gut contents containing cellulose woody material of black colour.
Remarks. This species superficially resembles L. boucheti because of the long, articulated intersegmental bristles and in having four gills per side. However, the shell sculpture of this species is distinct, as is the irregular arrangement of the aesthetes. This species has conspicuous long and sharp intersegmental bristles, much longer than in any other species, which give then animal a violent, spiky appearance.
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