A new genus and two new species of brooding chitons (Mollusca: Polyplacophora) from the North-Western Pacific Author Sirenko, Boris I. text Zootaxa 2024 2024-08-07 5492 4 505 529 http://dx.doi.org/10.11646/zootaxa.5492.4.2 journal article 10.11646/zootaxa.5492.4.2 1175-5326 13268886 9EAFBF80-9862-480A-8CAA-9DB544AE614E Genus Kaasichiton new genus Type species. Leptochiton lukini Sirenko, 1990 . Genus distribution. Kurile Islands , the Sea of Okhotsk and the Bering Sea , east Kamchatka , Recent , 10–1763 m . Etymology. The genus name is a combination of the family name of Pieter Kaas ( November 14 th 1915 – June 8 th 1996 ) and the Greek word χιτών (= chiton). Pieter, or more familar Piet,was a famous malacologist who described dozens of species and genera of chitons and was one of the authors of six volumes of the “Monograph of Living Chitons”, for which he, among others, compiled beautiful drawings. Diagnosis. Chitons of small size with BL up to 6.8 mm , usually 3.0–4.0 mm, elongate, valves low to moderately elevated (dorsal elevation about 0.36–0.49 [ Table 1 ]). Head valve wider than tail valve. Tail valve generally with a wide, flattened, posterior mucro shifted to the postmucronal area. Ratio of length of antemucronal area to length of postmucronal area 1.17–2.10. Tegmentum smooth to the naked eye or with very low, rare granules arranged in vague longitudinal rows in central areas of intermediate valves and in antemucronal areas of tail valve and in a random manner in other areas. Each granule or microsculpture element with one aesthete pore. Perinotum very narrow, 9–10 times narrower than valve V, covered with bluntly pointed, short or long scales with 3–5 distinct or vague longitudinal ribs. Radula long, about half of body length, with 60–123 rows of mature teeth; central tooth narrow, first lateral teeth wing-shaped, major lateral teeth narrow, with uni-,bi- or tridentate cusps. Number of gills in adult specimens between 5 to 8, ranging from valves VI–VII to anus. Living predominantly on shelf, bathymetrically ranging from 10 to 1763 m . Feeding type mainly detrivorous. All four species are brooding . TABLE 1. Comparison of some important features of shell, girdle and radula of four species of Kaasichiton n. gen.

Species	Ratio of	Ratio of length	Ratio of width of	Dorsal	Length of	Number of	Number of
width of head of antemucronal to width of area to length of		jugal sinus to width elevation of dorsal scales rows of radula denticles in cusp of apophyses in valve V of girdle of major lateral
tail valve	postmucronal area valve V		(µm)	tooth of radula
K. lukini	1.08–1.16	1.32	2.3–2.4	0.39–0.40	55–90	90–123	2
K. incubatus	1.03–1.17	1.33–2.10	1.32	0.36–0.49	61–67	100	1
K. ionensis n. sp.	1.05–1.12	1.17–1.22	1.65–2.07	0.38–0.45	65–70	60–64	3
K. kamchatkensis n. sp . 1.09		1.19	1.6	0.39	66–118	60–84	2

FIGURE 1. Kaasichiton lukini , Kurile Islands, Chirpoy Island, 20 m, holotype (ZIN 1919), BL 4.0 mm (A–C, E–G), Kaasichiton lukini , Kurile Islands, Urup Island, Van der Linde Cape, 20 m, paratype (ZIN 1920), BL 3.0 mm (D): A. Valve I, dorsal view; B. Valve V, dorsal view; C, D. Valves VIII, dorsal views; E. Valve V, detail of tegmentum surface of central area; F. Valve V, rostral view; G. Valve VIII, lateral view. Remarks. The species-rich genus Leptochiton in its traditional composition contained species with a wide range of morphological characters. Recently several genera ( Hanleyella Sirenko, 1973 , Ferreiraella Sirenko, 1988 , Nierstraszella Sirenko, 1992 , and Belknapchiton , Sirenko, Saito & Schwabe, 2022 ) could be separated from Leptochiton . The genus Terenochiton Iredale, 1914 could be maintained ( Sirenko 1973 , 1988, 1993; Sirenko et al. 2022) and several groups of morphologically similar species were recognized ( Sirenko 2015 ). The genus Leptochiton in its traditional composition is polyphyletic according to molecular studies ( Sigwart et al. 2011 ). The new genus can be compared with the remaining Leptochiton species forming groups of species that are similar to each other. The new genus Kaasichiton n. gen. resembles the group of Leptochiton rugatus (Carpenter in Pilsbry, 1892 ). This group of chitons consisting of six species inhabits the northern part of the Pacific Ocean ( Sirenko & Sigwart 2021 , 2022), including the areas inhabited by species of the new genus. Kaasichiton n. gen. differs from the L. rugatusgroup by having a head valve, which is always wider than the tail valve ( vs head valve is narrower than the tail valve in species of L. rugatus- group), a wide, flattened posterior mucro on the postmucronal area ( vs a narrow mucro, commonly on the anterior postmucronal area in species of the L. rugatus- group), a very narrow perinotum, which is 9–10 times narrower than the width of the fifth valve ( vs the perinotum is only 4–5 times narrower than the valve in species of the L. rugatus- group), a weak tegmentum sculpture ( vs distinct sculpture in species of the L. rugatusgroup ), and wing-shaped first lateral teeth ( vs narrow, oar-shaped first lateral teeth in species). Unlike species of the L. rugatus- group, all species of the new genus are brooding eggs and juveniles in their pallial groove. The new genus is well distinguished from the other species of the genus Leptochiton by morphological characters listed in the genus diagnosis FIGURE 2. Kaasichiton lukini , Kurile Islands, Urup Island, Van der Linde Cape, 20 m, paratype (ZIN 1920), BL 3.0 mm: A, C. Dorsal spicules and needles; B, D. Radula. Leptochiton kerguelensis Haddon, 1886 , widespread in the seas of the Southern Ocean, is also a brooding species, and is morphological closest to the species of Kaasichiton n. gen. However, it differs from them by having an anterior mucro, a narrow, oar-shaped first lateral tooth of the radula and twice as many longitudinal ribs on the dorsal scales of the perinotum.