Belknapchiton, Sirenko & Saito & Schwabe, 2022

Sirenko, Boris, Saito, Hiroshi & Schwabe, Enrico, 2022, A redescription of Leptochiton belknapi Dall, 1878 (Mollusca: Polyplacophora Leptochitonidae), the type species of the new genus Belknapchiton, Zootaxa 5205 (2), pp. 101-124 : 103-104

Genus Belknapchiton new genus

Type species. Leptochiton belknapi Dall, 1878 .

Etymology. The genus name is a combination of the family name of Rear Admiral George Eugene Belknap (1832–1903), who was captain on the U.S. S. Tuscarora , which at this time collected the deepest chiton, namely the present type species, and the Greek word χιτών (= chiton), itself the first polyplacophoran genus and commonly used for representatives of this molluscan class (gender: masculine).

Diagnosis. Small to large sized leptochitonids with BL up to 42 mm, elongate oval, moderately to highly elevated (dorsal elevation about 0.40–0.62). Head valve usually wider than tail valve. Tail valve generally with a distinct centrally to posteriorly situated mucro. Tegmentum allover with quincuncially arranged oval to elongate oval raised granules, exceptionally tending to a longitudinal pattern in remaining parts. Granules with 3–12 aesthetes. Perinotum with bluntly pointed rather narrow scales with up to 16 longitudinal ribs, and long slender needles scattered among them. Radula short, number of mature teeth rows between 18 and 32; central tooth wide, around 1.5 times longer than wide, first lateral teeth distinctly shorter than central, second lateral teeth with strong unidentate head occasionally with small denticle-like appendage at inner edge. Number of gills in adult specimens between 9 to 24, ranging from anus to valves vii, sometimes vi–v. Predominantly in deep water, bathymetrically ranging from about 100 to 4600 m. Feeding type mainly detrivorous, plant association not observed. Brooding not observed.

Remarks. Members of the new genus were until now all included in the genus Leptochiton Gray, 1847 . However, the polyphyletic composition of this genus was recently shown by a molecular study ( Sigwart et al. 2011).

An attempt to unravel the systematic position of the yet accepted species within this heterogenous genus leads first to the reinstatement of the genus Terenochiton Iredale, 1914 by Sirenko (2019). Leptochiton , as commonly still used in the widest (traditional) sense, is poor on characteristics. This has made it challenging to split separate lineages at a genus level but, once Sirenko (2004) started to group species, it was eventually revealed that some taxa share peculiarities hypothesized to be the basis for future generic separations.

The following species, which we herein transfer to the new genus, were formerly attributed to the Leptochiton belknapi -group ( Sirenko 2015, and subsequent additions), and contain collectively the most abundant, diverse, and widespread group of species conventionally assigned to Leptochiton :

Belknapchiton aequispinus ( Bergenhayn, 1933) , Japan, 92–600 m.

B. alveolus (M. Sars MS, Lovén, 1846) , Bergen , Norway; Bohuslän, Sweden, 270–540 m.

B. ater ( Saito, 1997) , Suruga Bay , Japan, 140–400 m.

B. belknapi (Dall, 1878) , off Aleutian Islands, 53˚08’N, 171˚19’W, 1840 m.

B. benthus ( Haddon, 1886) , Pacific Ocean, to East from Honshu 35˚41’N, 157˚42’E, 4206 m.

B. bergenhayni ( Saito, 2011) , Omurodashi Bank , Japan, 34˚29.8‘N, 139˚29.3‘E, 92–95 m.

B. costatoacus ( Sirenko, 2020), Philippines, Bohol / Sulu seas, 8°43.7’N, 123°14.0’E, 105–109 m.

B. fijiensis (Sirenko, 2016) , Fiji, 18°19’S, 178°05’E, 234–361 m.

B. giganteus ( Nierstrasz, 1905) , Banda Sea, Indonesia, 2798 m.

B. halistreptus halistreptus ( Dall, 1902) , off Acapulco, Mexico, 1188–3382 m.

B. halistreptus abbreviatus ( Dall, 1908), off Acapulco, Mexico, 900–1200 m.

B. japonicus ( Thiele, 1909) , Enoshima and Kajiyama, Japan, 300 m , Suruga Bay 130–400 m , Tosa Bay , 144– 302 m. B. kaasi ( Sirenko, 1990) , Iturup , Kurile Islands, 44°52‘N, 149° 27‘7‘‘E, 910– 920 m.

B. macleani ( Sirenko, 2015) , Peru-Chile Trench, 23°50’0’’S, 71°06’0’’W, 4600 m.

B. mutschkeae ( Sirenko, 2015) , Peru-Chile Trench, 23°50’0’’S, 70°56’6’’W, 2900 m.

B. okamurai ( Saito, 2001) , Tosa Bay , Japan, 400 m.

B. opiparus ( Iredale & Hull, 1925) , off Cape Wiles , Great Australian Bight, 34˚56’44.3’’S, 135˚41’3.3’’E, 180 m. B. sp. (= Leptochiton cf. opiparus of Sirenko 2020), New Zealand, Kermadec Islands , NNE of Herald Islets, Raoul Island, 29°12.00’S, 177°49.30’W, 1188–1225 m .

B. seishinmaruae ( Saito, 1997) , Suruga Bay , Japan, 870–950 m.

B. sigwartae ( Sirenko, 2015) , Peru, 06°41’S, 80°48’W, 822 m.

B. similis (E. A. Smith, 1894) off Colombo, Indian Ocean, 06°32’N, 79°37’E, 1215 m.

B. simplex ( Nierstrasz, 1905) , Makassar Strait , Indonesia, 0°34.6’N, 119°8.5’E, 1301 m .

B. taiwanensis ( Sirenko, 2018) , Taiwan, Bashi Channel , 22˚01.9‘N, 120˚36.4‘E, 246 m.

The new genus differs from other representatives of the family Leptochitonidae by having an elongate oval shape of body, small rather narrow, sharply pointed scales and scattered long, smooth, needles, the combination of a wide central tooth with short first lateral teeth, and large head of major lateral teeth of radula occasionally with an additional small denticle-like appendage at inner edge.

A significant character of the new genus is the unidentate head of the major lateral tooth. This character however is also found in the so called Leptochiton asellus -group ( Leptochiton asellus ( Gmelin, 1791) , L. arcticus (G. O. Sars, 1878) , L. cancellatus (Sowerby II, 1840)) , and in the rugatus -group ( Leptochiton rugatus (Carpenter in Pilsbry, 1892), L. alascensis ( Thiele, 1909) , L. assimilis ( Thiele, 1909) , L. cascadiensis Sigwart & Chen, 2017 , L. incubatus Sirenko, 2017 , L. subrugatus Sirenko & Sigwart, 2021 , L. surugensis Saito, 1997 ). Both of the latter groups, however, have longitudinally arranged granules in the jugal areas and slender central teeth. It should be noted that the presently accepted name of the type species of Leptochiton Gray, 1847 is L. asellus .

Species of the genus Belknapchiton superficially resemble species belonging to the group of Leptochiton vitjazae ( Sirenko, 1977) , as recently summarized by Sirenko & Schwabe (2019). However, the latter differ from the former by narrower central teeth of the radula, the tridentate head of the major lateral tooth, narrow dorsal girdle spicules, and a restricted gill number (4–5). In addition, the species of the group of L. vitjazae belong to xylophages (i.e., wood-eating) ( Sirenko 2004), while members of Belknapchiton feed mainly on detritus and foraminifera.

Genus distribution. Pacific, Indian and Atlantic oceans, Pliocene ( Dell’Angelo et al. 2021 for B. alveolus )– Recent.