Leptochiton milwaukensis, Sirenko, 2024

Sirenko, Boris, 2024, The chitons of the family Leptochitonidae (Mollusca: Polyplacophora) from the Emperor Seamounts, Pacific Ocean, Zootaxa 5474 (5), pp. 522-532 : 524-528

Leptochiton milwaukensis n. sp.

( Figures 2–5 )

Type material. Holotype (ZIN 2473), damaged, without tail valve, now disarticulated consisting of SEM-mounted valves I, II, IV, V, parts of perinotum and radula, and vial with other valves.

Type locality. Pacific Ocean, Emperor Seamounts, Milwaukee Seamount, 32°28’0’’N, 171°55’0’’E, 675 m (R/ V Vitjaz, stn. 6259-1, Ocean grab) .

Etymology. Named after the Milwaukee Seamount.

Diagnosis. Chiton small, BL 3.4 mm. Valves rounded, not beaked, sides weakly convex. Tegmentum sculptured with rounded, flattened granules quincuncially arranged in all areas except lateral areas of some intermediate valves where granules arranged in radial rows. Each granule with 1 megalaesthete and 2 micraesthetes in front. Pores of megalaesthete round and those of micraesthetes narrowed. Girdle narrow, densely covered with wide scales with 10–11 longitudinal ribs. Sutural needles long and ribbed. Radula with 34 transverse rows of mature teeth, central tooth, rather long, widest in the proximal half, pinched in the middle, with small, strongly rounded blade, major lateral teeth with an unidentate cusp. Four gills on each side.

Description. Holotype small, elongate oval. Valves rounded, moderately elevated, dorsal elevation of valve V 0.40, side slopes convex, not beaked, color of tegmentum white.

Head valve semicircular, posterior margin widely V-shaped, notched in the middle. Intermediate valves broadly rectangular, anterior margin jugally slightly excavated in valve II, and slightly concave in other intermediate valves, posterior margin concave at both sides of the slightly beaked apex, lateral areas not raised. Tail valve unknown.

Tegmentum sculptured with flattened, rounded granules (32–35 µm) arranged quincuncially distinct in all areas except lateral areas of most intermediate valves where granules arranged in radial rows. Each granule with 1 round pore of megalaesthete and 2 slit-shaped pores of micraesthetes in front.

Articulamentum white, apophyses short, wide, apophyses of intermediate valves sloping gently towards the jugal sinus forming a short rudiment similar to that of the genera Hanleyella Sirenko, 1973 and Terenochiton Iredale, 1914 ( Sirenko 1973 b, 2019a).

Girdle narrow, about 150–200 µm wide near valve V (width of valve V 1.6 mm), dorsally densely covered with slightly curved scales (48–54 x 27 μm) with 10–11 distinct, longitudinal ribs. Intersegmental area with long needles (270 x 10 μm) embedded in chitinous cups in the form of ring-shaft needles. Margin armed with two different kinds of elements: numerous long pointed scales (82 x 14 μm) with 2–5 ribs on one side, and one row scales (52 x 25 µm) with 3–4 short narrow ribs in the upper part. Ventrally girdle covered with elongate, obtusely pointed, smooth scales (50 x 20 μm).

Radula of holotype 1.6 mm long with about 34 transverse rows of small mature teeth. Guitar-shaped central tooth with longitudinal rib in middle and small blade on narrowed upper part. The first lateral tooth wing-shaped especially in front working and therefore erased part of the radula. Major lateral teeth with unidentate cusp with obtuse top. Major uncinal teeth are slightly elevated.Four gills per side arranged from valve VI to anus.

Remarks. Leptochiton milwaukensis n. sp. possesses unique flattened tegmentum granules with a rounded pore of megalaesthetes and two slit-shaped pores of micraesthetes, as well as a characteristic shape of the radula teeth. Morphological closest to the new species are four species ( L. gloriosus Kaas, 1985 , from western Indian Ocean; L. guadelupei Sirenko, 2018 , from western Atlantic; L. spiniferus, Sirenko 2020 , from Philippines; L. tahitiensis Sirenko, 2020 , from Tahiti), from which the new species differs by having unique granules on the tegmentum, only 3 pores of aesthetes in the granules (vs. 5–11 pores in all four mentioned species) and also by other features listed below.

L. milwaukensis n. sp. differs from L. gloriosus by having 10–11 ribs on dorsal scales (vs. 6–8 ribs in L. gloriosus ).

The new species differs from L. guadelupei by having the granules of the central area of intermediate valves being arranged quincuncially (vs. granules of central area arranged in longitudinal rows in L. guadeloupei ), 10– 11 ribs on dorsal scales (vs.16–17 ribs in L. guadelupei ), gentle slopes of apophyses on the inner side (vs.gentle slopes of apophyses absent in L. guadelupei ). L. milwaukensis n. sp. differs from L. spiniferus and L. tahitiensis by having gentle slopes of apophyses on the inner side (vs. gentle slopes of apophyses absent in L. spiniferus and L. tahitiensis ).

Figures

FIGURE 2. Leptochiton milwaukensis n. sp., Pacific Ocean, Emperor Seamounts, Milwaukee Seamount, 675 m, holotype (ZIN 2473) BL 3.4 mm. A. Valve I, dorsal view. B. Valve V, dorsal view. C. Valve IV, dorsal view. D. Valve II, dorsal view. E. Valve V, detail of tegmentum in central area. F. Valve V, rostral view. G. Whole specimen, dorsal and lateral views.

FIGURE 3. Leptochiton milwaukensis n. sp., Pacific Ocean, Emperor Seamounts, Milwaukee Seamount, 675 m, holotype (ZIN 2473) BL 3.4 mm. A. Dorsal scales, marginal needles. B. Dorsal scales, marginal needles and scales. C. Sutural needles. D. Dorsal and ventral scales.

FIGURE 4. Leptochiton milwaukensis n. sp., Pacific Ocean, Emperor Seamounts, Milwaukee Seamount, 675 m, holotype (ZIN 2473) BL 3.4 mm. A. Sutural needle, dorsal and ventral scales. B. Central, first lateral and major lateral teeth in non-working part of radula. C. Teeth in working (above) and in non-working (below) parts of radula. D. Central, first lateral and major lateral teeth in working part of radula.

FIGURE 5. Leptochiton milwaukensis n. sp., Pacific Ocean, Emperor Seamounts, Milwaukee Seamount, 675 m, holotype (ZIN 2473) BL 3.4 mm. A. Sutural needle. B. Dorsal scales, dorsal and lateral views. C. Marginal needle. D. Marginal scale. E. Ventral scale. F, G. Aesthete group (which area). H. Head of major lateral teeth. I. Central and first lateral teeth of radula in non working part. J. Central and first lateral teeth of radula in working part of radula. Scale bar 100 µm.