Osteodiscus abyssicola, a new snailfish (Cottoidei: Liparidae) collected off the Pacific coast of northern Japan Author Murasaki, Kenta Institute of Oceanic Research and Development, Tokai University, 3 - 20 - 1 Orido, Shimizu-ku, Shizuoka 424 - 8610, Japan Author Kai, Yoshiaki 0000-0002-0991-5173 Maizuru Fisheries Research Station, Field Science Education and Research Center, Kyoto University, Nagahama, Maizuru, Kyoto 625 - 0086, Japan. kai. yoshiaki. 4 c @ kyoto-u. ac. Jp; https: // orcid. org / 0000 - 0002 - 0991 - 5173 Author Endo, Hiromitsu 0000-0001-6916-6322 Laboratory of Marine Biology, Faculty of Science and Technology, Kochi University, 2 - 5 - 1 Akebono-cho, Kochi 780 - 8520, Japan endoh @ kochi-u. ac. Jp; https: // orcid. org / 0000 - 0001 - 6916 - 6322 Author Fukui, Atsushi 0000-0001-5195-9068 School of Marine Science and Technology, Tokai University, 3 - 20 - 1 Orido, Shimizu-ku, Shizuoka 424 - 8610, Japan afukui @ scc. u-tokai. ac. Jp; https: // orcid. org / 0000 - 0001 - 5195 - 9068 text Zootaxa 2021 2021-09-06 5032 1 136 142 journal article 10.11646/zootaxa.5032.1.8 1175-5326 5477750 FB5CA2AF-B3E9-4A19-AB31-33BFB7D24885 Osteodiscus abyssicola sp. nov. New Japanese name: Tsugomori-hariban-kusauo Figs. 1 , 2 ; Table 1 urn:lsid:zoobank.org:act: D12C0636-D7A4-463A-A173-92E234C1D6E5 Holotype . NSMT-P 109986 , 94.7 mm SL, immature female, south-southwest of Kushiro , Hokkaido , northern Japan ; 41° 43.46′ N , 144° 53.63′ E – 41° 42.24′ N , 144° 52.12′ E ; 4,671–4,744 m depth , 4 Aug. 1992 , R / V Hakuho-maru . Diagnosis. A species of Osteodiscus distinguished from all currently recognized congeners by the following combination of characters: vertebrae 49; dorsal-fin rays 44; anal-fin rays 39; principal caudal-fin rays 8; pyloric caeca 5; mouth horizontal; teeth on both jaws simple and sharp, without cusps; upper and lower jaw symphyses without diastema; cephalic pore sizes moderate, similar to or slightly larger than nostril; gill slit extending ventrally to 2 nd pectoral-fin ray base; pectoral fin notched; mandibular symphysis to center of anus 101.6% HL; posterior edge of pelvic disk to center of anus 15.5% HL; epural 1, reduced; epipleural ribs absent. Description. Counts and measurements given in Table 1 . Body slender, anteriorly oval in cross section, tapering gradually and becoming strongly compressed posteriorly, deepest at nape (= vertical depth through center of pelvic disk) ( Fig. 1A, B ). Skin thin, fragile (partly damaged); thin subdermal gelatinous layer present; prickles apparently absent. Head large, dorsal profile rounded from nape to snout. Snout blunt and deep, slightly projecting. Mouth subterminal, horizontal when closed, oral cleft extending to below center of orbit; outer margin of premaxilla completely covered by upper lip; posterior margin of maxilla slightly beyond to vertically below posterior margin of orbit. Lower jaw slightly inferior, anterior part of premaxillary tooth plates visible from ventral aspect of head; posterior half of outer margin of mandible covered by lower lip, anterior tip of lip slightly angled ventrally. Teeth on both jaws simple, sharp, without cusps, in about 20 oblique rows of 5–9 teeth forming bands, inner teeth larger than outer teeth ( Fig. 2A ). Upper and lower jaw symphyses without diastema. Single nostril tube-like, horizontally level with center of orbit. Eye and orbit small, dorsal contour of orbit below dorsal profile of head; pupil round. Cephalic pore sizes moderate, similar to or slightly larger than nostril: nasal pores 2, maxillary pores 6? [6th (= supraorbital) pore apparently present as shown in Fig. 1A , but difficult to discern due to poor skin condition], preoperculo-mandibular pores 7, suprabranchial pore 1; pore pattern 2-6?-7-1. Coronal pore absent. Chin pores (= anteriormost preoperculo-mandibular pores) paired, opening directly on skin surface, well separated from each other. Free neuromasts not apparent in damaged skin. Gill slit relatively short, upper margin horizontally level with lower margin of orbit, lower margin extending ventrally to 2 nd pectoral-fin ray base. Opercular flap angular, pointing posteroventrally, supported by two spines: upper spine (from opercle) and lower spine (from subopercle) extending posterior to vertical through dorsal-fin origin. Dorsal- and anal-fin rays not buried in gelatinous layer. Anteriormost and 2 nd pterygiophores of dorsal fin without rays, inserted between 3 rd and 4 th , and 4 th and 5 th neural spines, respectively. Anal-fin origin below 8 th dorsal-fin ray base. Hypural plates fused with terminal vertebral centrum, upper and lower plates separated by a narrow slit. Single reduced epural present. Pleural ribs and epipleural ribs absent. Pectoral fin moderately or deeply notched (condition unclear due to damage to some rays in notch and lower lobe, as shown in Fig. 1A ). Upper lobe rays slightly protruding from membrane at tip; 2 nd and 3 rd uppermost rays longest, reaching to 4 th anal-fin ray base. Notch and lower lobe rays filamentous, nearly completely free from membrane. Uppermost pectoral-fin base just below level of posterior margin of maxilla; lowermost pectoral-fin base below anterior margin of orbit. TABLE 1 Counts and measurements of Osteodiscus abyssicola sp. nov. , holotype, NSMT-P 109986

Character	Counts	In % of measurements
Standard length	Head length
94.7 mm	24.5 mm
Vertebrae (abdominal + caudal)	49 (10 + 39)
Dorsal-fin rays	44
Anal-fin rays	39
Pectoral-fin rays	21
Caudal-fin rays	9
Principal caudal-fin rays (upper + lower)	8 (4 + 4)
Procurrent caudal-fin rays (upper + lower)	1 (1 + 0)
Branchiostegal rays	6
Pyloric caeca	5
Head length	25.9
Head width	15.2	58.8
Maximum body depth	22.6	87.3
Body depth at anal-fin origin	14.3	55.1
Snout length	10.0	38.8
Maxilla length	13.5	52.2
Mandible length	13.4	51.8
Mouth width	13.6	52.7
Orbit length	4.0	15.5
Eye diameter	4.0	15.5
Interorbital width	10.8	41.6
Bony interorbital width	4.4	17.1
Suborbital depth to maxilla	5.4	20.8
Gill slit length	9.6	37.1
Pectoral-fin ray length in upper lobe	20.9	80.8
Pectoral-fin ray length in lower lobe	-	-
Pectoral-fin ray length at notch	-	-
Pelvic disk length	11.6	44.9
Pelvic disk width	9.6	37.1
Caudal fin length	-	-
Predorsal fin length	27.5	106.1
Snout to anterior edge of pelvic disk	12.9	49.8
Mandibular symphysis to anterior edge of pelvic disk	11.5	44.5
Snout to center of anus	27.7	106.9
Mandibular symphysis to center of anus	26.3	101.6
Posterior edge of pelvic disk to center of anus	4.0	15.5
Preanal fin length	36.7	142.0
Center of anus to anal-fin origin	10.9	42.0
Dorsal-fin origin to anal-fin origin	23.0	89.0

Pelvic disk large, longer than wide, moderately upturned at posterior part; skeletal, musculature greatly reduced, fleshly margin absent ( Fig. 2B ). Basipterygia and six paired pelvic rays (anteriormost spine, posterior five soft rays) covered only by thin skin, internal structures visible through skin. Bases of anteriormost and 2 nd pelvic rays more widely spaced than bases of other rays; anteriormost ray supported by subpelvic process (= part of anterior half of basipterygium), posterior five rays supported by posterior half of basipterygium. Pelvic rays webbed between tips; webs somewhat attenuated posteriorly on disk. Anus well separated from posterior edge of pelvic disk, vertically below dorsal-fin origin. Minute genital papilla-like process at posterior of anus. Stomach and short pyloric caeca located on left side of visceral cavity. FIGURE 1. Osteodiscus abyssicola sp. nov. , holotype, NSMT-P 109986, 94.7 mm SL. (A) drawing; (B) photograph of preserved specimen. FIGURE 2. Teeth on right side of premaxilla (A) and ventral view of head and abdominal region (B) of Osteodiscus abyssicola sp. nov. , holotype, NSMT-P 109986, 94.7 mm SL. Color in alcohol . Body and fins brown, head somewhat darker ( Fig. 1B ). Eye and peritoneum black. Gill cavity dark brown. Stomach, pyloric caeca, and genital papilla-like process pale. Distribution. Western North Pacific, south-southwest of Kushiro, Hokkaido , northern Japan , at a depth of 4,671 –4,744 m . Etymology. The specific name “ abyssicola ”, a noun in apposition, is derived from Latin “abyssus” (abyss) and “cola” (dweller), referring to the deeper habitat of this species compared to congeners. The Japanese name “Tsugomori” is an archaic term in Japanese, modified from “tsuki-gomori” (hiding moon), and refers to a dark night. Comparisons. Osteodiscus abyssicola sp. nov. shares a horizontal mouth with two congeners, O. cascadiae and O. andriashevi ( Stein 1978 ; Pitruk & Fedorov 1990 ; Murasaki et al . 2021 ; this study). However, O. abyssicola sp. nov. clearly differs from O. cascadiae in having 8 principal caudal-fin rays (vs. 6–7), 5 pyloric caeca (vs. 0), the anus well separated from the pelvic disk posterior edge (15.5% HL vs. immediately posterior to or slightly further from disk, 1–4 % HL), moderate-sized cephalic pores, similar to or slightly larger than nostril (vs. notably larger than nostril), absence of diastemata at both jaw symphyses (vs. present), and presence of a reduced epural (vs. absent) ( Stein 1978 , 2012 ; Kido 1988 ; this study). It also differs from O. andriashevi in having 49 vertebrae (vs. 55–60), 44 dorsal-fin rays (vs. 52–54), 39 anal-fin rays (vs. 46–49), 8 principal caudal-fin rays (vs. 7), sharp teeth without cusps (vs. blunt, some with lateral cusps), a notched pectoral fin (vs. unnotched), and the absence of epipleural ribs (vs. present) ( Pitruk & Fedorov 1990 ; Murasaki et al . 2021 ; this study). Additionally, the space between the 1 st and 2 nd pelvic-fin ray bases is greater than between the other fin ray bases in O. abyssicola sp. nov. (vs. space between 2 nd and 3 rd ray bases greater than between others in O. cascadiae and O. andriashevi ) ( Stein 1978 ; Kido 1988 ; Pitruk & Fedorov 1990 ; this study). The remaining species, O. rhepostomias , has a distinctly oblique mouth (vs. horizontal in O. abyssicola sp. nov. ), 9 principal caudal-fin rays (vs. 8), a shorter distance from mandible to anus, 68.2% HL (vs. 101.6% HL), a clear diastema at the lower jaw symphysis (vs. absent), and a large gill slit, extending ventrally to about 9 th pectoral-fin ray base (vs. small, extending to 2 nd pectoral-fin ray base) ( Stein 2012 ; this study).