Pycnodontochiton sinensis Sirenko, Zhang & Sigwart 2025, sp. nov.

Pycnodontochiton sinensis Sirenko, Zhang & Sigwart sp. nov. Materials Type status: Holotype. Occurrence: catalogNumber: MBM 229047; individualCount: 1; lifeStage: adult; occurrenceID: 95C13B24-441F-5EBB-A332-74086077580A; Taxon: scientificName: Pycnodontochiton sinensis Sirenko, Zhang & Sigwart; kingdom: Animalia; phylum: Mollusca; class: Polyplacophora; order: Lepidopleurida; family: Leptochitonidae; genus: Pycnodontochiton; specificEpithet: sinensis; taxonRank: species; scientificNameAuthorship: Sirenko, Zhang & Sigwart; nomenclaturalCode: ICZN; Location: higherGeography: Western Pacific Ocean; locality: Haima cold seeps; verbatimDepth: 1392 m; verbatimCoordinates: 16°43′45″N, 110°28′23″E; verbatimLatitude: 16°43′45″N; verbatimLongitude: 110°28′23″E; verbatimCoordinateSystem: degrees minutes seconds; decimalLatitude: 16.729167; decimalLongitude: 110.473056; Identification: identifiedBy: Julia D. Sigwart, Boris Sirenko; dateIdentified: 2024; Event: eventID: H 707 TV 6-3; eventDate: 7 / 7 / 2022; year: 2022; month: 7; day: 7; Record Level: institutionCode: MBM; basisOfRecord: PreservedSpecimen Type status: Paratype. Occurrence: catalogNumber: MBM 229048; individualCount: 1; lifeStage: adult; occurrenceID: 1E9EAE51-45BB-58EB-883F-EFB037FB1F82; Taxon: scientificName: Pycnodontochiton sinensis Sirenko, Zhang & Sigwart; kingdom: Animalia; phylum: Mollusca; class: Polyplacophora; order: Lepidopleurida; family: Leptochitonidae; genus: Pycnodontochiton; specificEpithet: sinensis; taxonRank: species; scientificNameAuthorship: Sirenko, Zhang & Sigwart; nomenclaturalCode: ICZN; Location: higherGeography: Western Pacific Ocean; locality: Haima cold seeps; verbatimDepth: 1392 m; verbatimCoordinates: 16°43′45″N, 110°28′23″E; verbatimLatitude: 16°43′45″N; verbatimLongitude: 110°28′23″E; verbatimCoordinateSystem: degrees minutes seconds; decimalLatitude: 16.729167; decimalLongitude: 110.473056; Identification: identifiedBy: Julia D. Sigwart, Boris Sirenko; dateIdentified: 2024; Event: eventID: H 707 TV 6-3; eventDate: 7 / 7 / 2022; year: 2022; month: 7; day: 7; Record Level: institutionCode: MBM; basisOfRecord: PreservedSpecimen Type status: Paratype. Occurrence: catalogNumber: SMF 380828; individualCount: 1; lifeStage: adult; previousIdentifications: Leptochiton sp.; occurrenceID: 7B64F365-BF4E-50FB-8D77-D7620E06B757; Taxon: scientificName: Pycnodontochiton sinensis Sirenko, Zhang & Sigwart; kingdom: Animalia; phylum: Mollusca; class: Polyplacophora; order: Lepidopleurida; family: Leptochitonidae; genus: Pycnodontochiton; specificEpithet: sinensis; taxonRank: species; scientificNameAuthorship: Sirenko, Zhang & Sigwart; nomenclaturalCode: ICZN; Location: higherGeography: Western Pacific Ocean; locality: Haima cold seeps; verbatimDepth: 1385 m; verbatimCoordinates: 16°43.937'N, 110°27.681'E; verbatimLatitude: 16°43.937'N; verbatimLongitude: 110°27.681'E; verbatimCoordinateSystem: degrees decimal minutes; decimalLatitude: 16.732283; decimalLongitude: 110.46135; Identification: identifiedBy: Julia D. Sigwart, Boris Sirenko; dateIdentified: 2024; Event: samplingProtocol: suction sampler on ROV HAIMA, on board R / V HAIYANG 6 cruise HYLH 201902; eventDate: 5 / 5 / 2019; year: 2019; month: 5; day: 5; Record Level: institutionCode: SMF; basisOfRecord: PreservedSpecimen Type status: Paratype. Occurrence: catalogNumber: SCSMBC 240288; individualCount: 1; lifeStage: adult; previousIdentifications: Leptochiton sp.; occurrenceID: F00A3CBA-7173-5494-A5FF-6B22B79BE5ED; Taxon: scientificName: Pycnodontochiton sinensis Sirenko, Zhang & Sigwart; kingdom: Animalia; phylum: Mollusca; class: Polyplacophora; order: Lepidopleurida; family: Leptochitonidae; genus: Pycnodontochiton; specificEpithet: sinensis; taxonRank: species; scientificNameAuthorship: Sirenko, Zhang & Sigwart; nomenclaturalCode: ICZN; Location: higherGeography: Western Pacific Ocean; locality: Haima cold seeps; verbatimDepth: 1385 m; verbatimCoordinates: 16°43.937'N, 110°27.681'E; verbatimLatitude: 16°43.937'N; verbatimLongitude: 110°27.681'E; verbatimCoordinateSystem: degrees decimal minutes; decimalLatitude: 16.732283; decimalLongitude: 110.46135; Identification: identifiedBy: Julia D. Sigwart, Boris Sirenko; dateIdentified: 2024; Event: samplingProtocol: suction sampler on ROV HAIMA, on board R / V HAIYANG 6 cruise HYLH 201902; eventDate: 5 / 5 / 2019; year: 2019; month: 5; day: 5; Record Level: institutionCode: SCSMBC; basisOfRecord: PreservedSpecimen Description Holotype (body length 17.0 mm) elongate oval, valves subcarinated, moderately elevated (dorsal elevation 0.35), not beaked, side slope slightly convex, apex damaged, tegmentum white in colour. Head valve semicircular, hind margin with notch; intermediate valves rectangular, valves V and VI widest, anterior margin convex in valve II and concave in other intermediate valves, posterior margin slightly convex, lateral area weakly raised; tail valve is 1.1 times wider than head valve, mucro anterior, postmucronal slope first concave then convex, antemucronal area convex (Fig. 12 A, B). Tegmentum sculptured with small elongated granules (160–200 x 60–70 µm) closely arranged in longitudinal rows in central areas of intermediate valves and in antemucronal areas of tail valve and in quincunx patterns or in a random manner in other areas. Each granule with one aesthete or sometimes two aesthetes, irregularly placed within low granules (Fig. 13 A, C). Head valve, lateral areas of intermediate valves and postmucronal area of tail valve with 5–6 concentric terraced growth rings. Articulamentum well developed, white, apophyses small, widely separated from each other, more or less triangular in intermediate valves or trapezoidal in tail valve. Ratio of width of jugal sinus to width of apophyses 1.3. Girdle at valve V, 5.5 times narrower than valve width, dorsally covered with bluntly pointed, slightly flattened, long spicules (160 x 50 µm) with 10–12 vague longitudinal ribs around spicule. Marginal needles (200–210 µm) and ventral spicules (100–110 x 20–30 µm) also with vague longitudinal ribs, slightly square in cross section (Fig. 13 D, F, G). Holotype with gills extending from valve V to near anus. Paratype 1 with 19 gills on each side (actual number 17, estimated 19 due to defect), paratype 2 with 22 gills. Radula 5 mm long, with 126 transverse rows of mature teeth, central tooth large and unusually wide (over 130 µm), with round dorsal edge; first lateral tooth narrow, roughly L-shaped, with a small blade; major lateral teeth large, with bidentate heads; first uncinal tooth unusually narrowly elongated, major uncinal tooth long with well-developed blade (Fig. 13 B, E). Width of valves (holotype): I- 8.0 mm, II- 9.2 mm, III- 10.0 mm, IV- 10.9 mm, V- 11 mm, VI- 11.0 mm, VII- 10.2 mm, VIII- 8.9 mm. Schwabe organ large, dark brown-black (Fig. 12 C). Type material Holotype (MBM 229047), now disarticulated, consisting of SEM stub of valves I, II, V, VIII, part of perinotum and radula, mount of part of perinotum and radula and vial with other valves. Paratype 1 (MBM 229048) now disarticulated and the parts of the dorsal spicules, marginal spicules and aesthetes are provided. Paratype 2 (SMF 380828), now partially disarticulated, consisting of ethanol-preserved body, air-dried dissected valves I, II, III, VIII, radula preparations (in ethanol + dry on SEM stub) and SEM stub with bleached fragment of valve III and perinotal spicule preparation. Paratype 3 (SCSMBC 240288), partially disarticulated. Material examined Only known from the type material. Type locality Western Pacific Ocean, Haima cold seeps, in the western part of the South China Sea. Collected at two sites: 16°43′45″N, 110°28′23″E, depth 1392 m and 16°43.937'N, 110°27.681'E, depth 1385 m. Diagnosis As for genus. Etymology Named for its occurrence off the coast of China. Distribution Only known from the type locality. Taxon discussion As already noted, P. sinensis gen. et sp. nov. is closely related to the congener P. tenuidontus. Pycnodontochiton sinensis gen. et sp. nov. differs from P. tenuidontus in having an even wider central tooth of radula and without its narrowing in distal part and having bidentate heads of the major lateral teeth and also by its ribbed marginal needles and by ribbed ventral spiculae. The new species is distinguished from all other species of chitons by the remarkably long and flat central tooth of radula and an unusually long first uncinal tooth. The radula of P. sinensis gen. et sp. nov. is its most distinctive feature. Pycnodontochiton tenuidontus, from hydrothermal areas, has a similar radula that was described as unique because of its flat, overlapping teeth (Saito and Okutani 1990), similar to P. sinensis gen. et sp. nov. and both species have indistinct granular sculpture. The radula of P. tenuidontus also has a prominent, wide and round central tooth and a distinctive inner small lateral (between the major lateral and sweeper teeth), two features that are absent in P. sinensis gen. et sp. nov. Pycnodontochiton tenuidontus has three aesthetes in a bundle on each granule, but P. sinensis gen. et sp. nov. has one or two aesthetes per granule. The valves in P. tenuidontus are also round-backed, compared to subcarinate in P. sinensis gen. et sp. nov. Two other species with densely-packed, flat-headed major lateral teeth in the radula are Leptochiton kerguelensis (Haddon, 1886), from Antarctic and sub-Antarctic regions and Leptochiton ferreirai Sirenko & Sellanes, 2016 from deep water near Chile. Both those species are much smaller in body size than Pycnodontochiton spp. and live in distant regions and other features of the radula differ. The radula of L. kerguelensis has large, broad sweeper teeth not like the elongate first uncinal seen in P. sinensis gen. et sp. nov. The major lateral teeth in the radula of L. ferreirai are distinctly tricuspid and pointed, not square and bicuspid as in P. sinensis gen. et sp. nov. Methods Paratypes SMF 380