(Japanese name: Hebigenge)
( Figs. 9–13; Table 3)
Lycenchelys hippopotamus Schmidt, 1935: 35 ( nomen nudum); Taranetz, 1937: 161 ( nomen nudum).
Lycenchelys hippopotamus Schmidt, 1950: 106, fig 4, pl. IX (original description, type locality: east coast of Sakhalin Island, Sea of Okhotsk); Andriashev, 1955: 354, 361, figs. 2, 7–8 (description); Matsubara, 1955: 774 (key to species); Fedorov, 1976: 8, tables 3–4 (description); Toyoshima, 1983: 267, 332, pl. 154 (description); Toyoshima, 1984: 293, pl. 273-O (brief description); Toyoshma, 1985: 149, 169, figs. 6–7, 24–25, 31, table 1 (description); Hatooka, 1993: 902, unnumbered fig. (key to species); Anderson, 1994: 65, 113, 117 (osteological comments); Amaoka et al., 1995: 239, pl. 401 (brief description); Anderson, 1995: 76 (description); Koyanagi, 1997: 538, fig. 2 (brief description); Hatooka, 2000: 1032, unnumbered fig. (key to species); Hatooka, 2002: 1032, unnumbered fig. (key to species); Mecklenburg et al., 2002: 701, unnumbered figs. (brief description); Anderson & Fedorov, 2004: 17 (species list); Shinohara & Anderson, 2007: 64 (key to species); Amaoka et al., 2011: 315, unnumbered fig. (brief description); Balushkin et al., 2011: 980, 1024 (catalog of specimens); Hatooka, 2013: 1226, unnumbered fig. (key to species); Nakabo & Hirashima, 2015: 217 (species list and etymology of scientific name).
Lectotype: ZIN 24826, female, 206.8 mm SL, east coast of Sakhalin Island, Okhotsk Sea ( 53°09.5’N, 149°52.1’E), 1150 m depth, 7 Aug. 1932.
Paralectotype ( 2 specimens): ZIN 24826, 1 male and 1 female, 167.7–194.9 mm SL, collected with lectotype.
Other specimens ( 29 specimens): HUMZ 77571, 77573, 77774, 119986–87, 120005, 120285, 120329, 120345, 121155, 121157, 121159 –60, 121162, 121164, 121193, 121452, 121463, 123979, 124056, 124115, 126116, 126186, 126218, 126223, 126228, 20 males and 9 females, 117.9–211.9 mm SL, northeastern Hokkaido Island, Okhotsk Sea.
Diagnosis. Vertebrae 23–24 + 105–115 = 128–138; head length 12.5–15.0% SL; interorbital pores 1–2; occipital pores 2; postorbital pores 3–4; suborbital pores usually 8 + 1; 1st suborbital pore located just below nostril tube; preoperculomandibular pores 8–9; vomerine teeth 3–7; palatine teeth 1–7, arranged in single row; opercular flap well-developed; pelvic-fin base positioned below lower edge of gill opening; lateral line complete and positioned ventrally; scales present or absent on pectoral fin and absent on its base; body uniformly dark chocolate-brown when fresh.
Description. Counts and proportional measurements in Table 3.
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Body very elongate, cross section oval anteriorly, compressed laterally near tail; its width at anal-fin origin 2.3–4.6% SL (unknown for lectotype). Head moderately long, ovoid; dorsal profile of head gently sloping from posterior edge of eye to above about last postorbital pore. Head of adults generally longer in males than in females. Snout rather short, 127.2–199.8 (172.3)% of eye diameter. Eye ovoid, relatively small. Interorbital space narrow, width 20.0–37.0 (27.8)% of eye diameter. Nostril tube short, reaching 1st suborbital pore when depressed. Mouth subterminal. Posterior edge of upper jaw about reaching vertical through anterior margin of eye in adult males, not reaching vertical through anterior margin of eye in females and juveniles. Labial lobe of lower jaw weak. Teeth on jaws sharp; upper jaw with single row, anterior teeth large and posterior teeth small; 4 females, including lectotype and 1 paralectotype, with additional small teeth behind anteriormost tooth; lower jaw with 2–3 irregular rows anteriorly and single row posteriorly, anterior teeth large and those posteriorly small; vomer and palatine small and conical; vomerine teeth irregularly arranged; palatine teeth in single row. Lower edge of gill opening reaching lower end of pectoral-fin base. Opercular flap well developed. Gill rakers short; those on upper limb triangular, many triangular and some blunt on lower limb ( Fig. 10). Pseudobranch filaments relatively long. Lateral line deciduous, complete and positioned ventrally; originating posterior to last postorbital pore and terminating on tail. Scales small and cycloid, present on body, tail and about 40–80 % of vertical fins basally. Scales present or absent on basal portion of pectoral-fin rays (present in lectotype). Head, nape, pectoral axil and pectoral-fin base without scales.
Dorsal-fin origin posterior to vertical through pectoral-fin base; 1st dorsal-fin pterygiophore between neural spines of 4th to 6th (between 5th and 6th) vertebrae. Anal-fin origin below 17th to 19th (17th) dorsal-fin ray; 1st anal-fin pterygiophore posterior to parapophysis of ultimate to antepenultimate abdominal (penultimate) vertebra. Last dorsal-fin pterygiophore between neural spines of 2nd to 5th (between 3rd and 4th) preural vertebrae. Last anal-fin pterygiophore between hemal spines of 2nd to 6th (between 3rd and 4th) preural vertebrae. Caudal fin with 2 epural, 4–5 (4) upper hypural and 3–4 (4) lower hypural rays. Pectoral fin moderately short, reaching to about middle of abdomen; its posterior margin rounded dorsally and having notches ventrally. Upper end of pectoral-fin base about on lateral horizontal midline of body. Pelvic fin relatively long; its base below lower edge of gill opening; its posterior margin reaching vertical through pectoral-fin base.
Head pores well developed and distinct. Nasal pores 2; anterior pore in front of nostril tube, posterior pore above 2nd suborbital pore ( Fig. 11A, B). Postorbital pores usually 3 (3) ( Fig. 11A, B); distance between 1st and 2nd pores longest of those between adjacent pores; 1 additional small pore present just behind 1st pore on both sides in HUMZ 121157. Suborbital pores usually 9 (9), 8 pores located below lower margin of eye and 9th behind eye; when 10 pores, 9 pores below eye and last behind eye on left side in HUMZ 77571 and 121160, and both sides in HUMZ 126194; 5th pore below vertical through anterior margin of eye; last pore of those below eye slightly posterior to vertical through 1st postorbital pore ( Fig. 11A). Preoperculomandibular pores 8–9 (8); 4 on lower jaw, 1–2 (1) on junction of lower jaw and preopercle, and 3 on preopercle; last preoperculomandibular pore located posterior to lower margin of eye ( Fig. 11A, C). Interorbital pores 1 or 2 (1); when 2, anterior pore located above 4th suborbital pore, and posterior pore between center or slightly posterior to center of eyes ( Fig. 11B); when 1, anterior pore absent and posterior pore located slightly posterior to center of eyes. Occipital pores 2, pores on either side of dorsal midline anterior to 2nd postorbital pore ( Fig. 11B).
Color in alcohol. Lectotype (based on color photograph; Fig. 13) with dark brown head, pectoral fin and margin of vertical fins, slightly paler body and vertical fins; purplish gray abdomen. Non-type specimens similar to lectotype, except HUMZ 77571, 77573 and 77774, which have uniformly pale brown head and body and black dots on pectoral fin (HUMZ 121157 and 121159).
Color when fresh (based on color photograph of HUMZ 124056; Fig. 9). Head and pectoral fin blackish. Body and vertical fins uniformly dark chocolate brown.
Distribution. Southern Okhotsk Sea to the northwestern Bering Sea, at depths of 408–1800 m ( Schmidt, 1950; Andriashev, 1955; Matsubara, 1955; Fedorov, 1976; Toyoshima, 1983, 1984, 1985; Hatooka, 1993, 2000, 2002, 2013; Anderson, 1994, 1995; Amaoka et al., 1995, 2011; Koyanagi, 1997; Mecklenburg et al., 2002; Anderson & Fedorov, 2004; Shinohara & Anderson, 2007; Balushkin et al., 2011; this study).
Size. Maximum length 223 mm TL ( Anderson, 1995; Mecklenburg et al., 2002). The largest specimen examined for this study was 211.9 mm SL ( 216.3 mm TL).
Remarks. Lycenchelys hippopotamus is similar to L. makushok, L. melanostomias and L. rassi in having more than 100 total vertebrae, 1–2 interorbital pores, 2 occipital pores, 3–4 postorbital pores, single ventral lateral line and no distinct spots or blotches on the body (vs. lacking this combination of characters in other species of Lycenchelys) (e.g., Schmidt, 1950; Andriashev, 1955; Toyoshima, 1983, 1985; Fedorov & Andriashev, 1993; Hatooka, 1993, 2000, 2002, 2013; Anderson, 1995; Shinohara et al., 1996; this study). Lycenchelys hippopotamus is distinguished from these three species by the position of the 1st suborbital pore. In L. hippopotamus, it is located just below the nostril tube ( Fig. 12A), while it is located behind the nostril tube in L. makushok, L. melanostomias and L. rassi ( Fig. 12B, C, D). In addition, L. hippopotamus has 1–2 interorbital pores, and when only the middle pore is present, it is located between the middle of the eyes or slightly posterior. In contrast, L. makushok, L. melanostomias and L. rassi, always have only 1 pore in the interorbital space, and it is located anterior to the middle of the eyes.