Hymenocephalus hachijoensis Okamura, 1970

Nakayama, Naohide, 2020, Grenadiers (Teleostei: Gadiformes: Macrouridae) of Japan and adjacent waters, a taxonomic monograph, Megataxa 3 (1), pp. 1-383 : 208-209

Hymenocephalus hachijoensis Okamura, 1970

[Japanese name: Hachijo-sokodara]

( Figs. 135–136 ; Appendix 3-8A)

Hymenocephalus striatissimus hachijoensis Okamura, 1970a:50 , pl. XVI (original description; as subspecies of H. striatissimus Jordan & Gilbert in Jordan & Starks, 1904; holotype: BSKU 14171, from off Hachijo-jima Island, 40º03′6ʺN, 133º16′5ʺE, in 570 m; 1 paratype collected with holotype; new Japanese name: “Hachijo-sokodara”); Okamura 1970b: table 1 (listed; Japan); Tominaga & Uyeno 1981:489 (listed; Japan).

Hymenocephalus longiceps (not Smith & Radcliffe in Radcliffe 1912): Yatou 1982:141, 246, fig. 85 (brief description; in part, ZMUT KP416; Kyushu-Palau Ridge).

Hymenocephalus hachijoensis: Okamura 1984b:92 , pl. 80, fig. F (compiled); Okamura 1988:92, pl. 80, fig. F (compiled); Sazonov & Iwamoto 1992:62 (description; 3 additional spec. from Emperor Seamounts and Kyushu-Palau Ridge; comparisons with similar congeners); Nakabo 1993:355 (in key; Japan); Sazonov 1994:101 (brief description); Nakabo 2000:419 (in key; Japan); Nakabo 2002:419 (in key; Japan); Nakabo & Kai 2013:495 (in key; Japan); Schwarzhans 2014:49, fig. 23 (description; comparison; in key); Nakayama et al. 2015:505, figs. 1–5 (comparative material; 2 additional spec. from Hachijo-jima Island and Kyushu-Palau Ridge); Motomura 2020:39 (listed; Japan).

Diagnosis. A species of Hymenocephalus with 7 or 8 pelvic-fin rays; barbel moderately long, length 36–52% PRL, its tip not reaching vertical through hind margin of orbit when depressed; snout high, not depressed, length 25–30% PRL; orbit diameter 43–52% PRL; interorbital width 31–34% PRL; first dorsal-fin rays II,9–11; pectoralfin rays i13–i15; gill rakers on first arch (outer/inner) 12– 15/18–22, on second arch 18–20/16–20; pyloric caeca 23; dorsal half of trunk prominently blackish, but the dark area abruptly ending posterior to end of first dorsal-fin base; ventral half of tail immaculate or only slightly peppered with small black melanophores; caudal vertebrae barely visible from outside; ostial and caudal colliculi of otolith not fused.

Material examined. 8 specimens. Holotype of Hymenocephalus striatissimus hachijoensis: BSKU 14171 (32.4 mm HL, 196+ mm TL), northeast of Hachijojima Island , Shichito-Iojima Ridge, Japan, 33.2681ºN, 140.0517ºE, 570 m, coll. O. Okamura, 21 Apr. 1968 . Paratype of H. s. hachijoensis: BSKU 14172 (22.6 mm HL, 128+ mm TL), collected with holotype. Non-types : Japan: ZMUT KP416 (1, 25.0 mm HL, 106+ mm TL), Kyushu-Palau Ridge , date unknown; BSKU 19308 (1, 24.7 mm HL, 133+ mm TL), off Hachijo-jima Island, Shichito-Iojima Ridge, 33.1367ºN, 139.9900ºE, 440 m, FRV Soyo-maru, sta. B3, beam trawl, 8 Dec. 1962 ; * FAKU 202929 (1, 22 mm HL, 140+ mm TL), FAKU 202930 (1, 23.1 mm HL, 141+ mm TL), * FAKU 202944 (1, 19 mm HL, 133+ mm TL), Nishiura fish market, 200– 300 m, bottom trawl, coll. N. Nakayama, 21 May 2016 . Emperor Seamounts : ZMMGU P.18243 (1, 32.2 mm HL, 200 mm TL), ca. 32ºN, 173ºE, Mys Yunony, tr. 86, trawl, 6 Sept. 1979 .

Counts and measurements. Based on 6 specimens (22.6–32.4 mm HL, 106+–200+ mm TL). Counts: first dorsal-fin rays II,9–11; pectoral-fin rays i13–i15; pelvicfin rays 7–8; gill rakers on first arch (outer/inner) 12– 15/18–22, on second arch 18–20/16–20; longitudinal scales 17; transverse scale rows below first dorsal-fin origin 3.5, below first dorsal-fin midbase 3, below second dorsal-fin origin 2; pyloric caeca 23.

The following measurements are in % of HL, followed by those in % of PRL in parentheses: snout length 21–24 (25–30); orbit diameter 34–42 (43–52); postorbital length 44–49 (54–60); postrostral length 80–85; orbit–preopercle distance 40–49 (49–58); suborbital width 8–10 (10–12); upper-jaw length 51–59 (61–70); length of rictus 47–53 (57–62); length of premaxillary tooth band 43–49 (53– 58); preoral length 11–17 (14–20); distance between tip and lateral angle of snout 14–17 (17–21); snout width 24– 31 (29–38); internasal width 19–24 (23–29); interorbital width 26–27 (31–34); body width over pectoral-fin bases 35–46 (43–57); body depth at first dorsal-fin origin 64– 80 (80–94); body depth at anal-fin origin 49–54 (61–67); prepelvic length 102–113 (123–140); preanus length 169 (199); preanal length 166–172 (203–213); isthmus–pelvic distance 49–62 (58–76); isthmus–anal distance 123–124 (151–156); pelvic–anal distance 69–74 (85–88); pelvicfin length 76–87 (95–103); pectoral-fin length 48–61 (56– 75); predorsal length 100–110 (123–135); height of first dorsal fin 78–85 (92–107); length of first dorsal-fin base 33–42 (41–49); interdorsal length 63–88 (79–109); length of gill slit 31–34 (36–42); length of posterior nostril 4 (6); barbel length 30–42 (36–52).

Size. To about 20 cm TL (ZMMGU P. 18243, 200 mm TL, Emperor Seamounts).

Distribution. So far known only from Japan (Appendix 3-8A) and the Emperor Seamounts at depths of 200–710 m ( Sazonov 1994; Schwarzhans 2014; this study). Rare.

Remarks. For further morphological information see the original description ( Okamura 1970a) and a supplementary description given by Schwarzhans (2014).

Hymenocephalus hachijoensis was initially described as a subspecies of H. striatissimus Jordan & Gilbert in Jordan & Starks, 1904 based on two specimens collected from off Hachijo-jima Island, Shichito-Iojima Ridge, Japan, at a depth of 570 m ( Fig. 136 ). Subsequently, it was recognized as a full species by Okamura (1984b, 1988), who considered H. hachijoensis to have a much larger head, a longer barbel (about equal to the orbit diameter), and a smaller orbit as compared with H. striatissimus . Based on examinations of three additional specimens from the Emperor Seamounts and Kyushu-Palau Ridge, Sazonov & Iwamoto (1992:62) and Sazonov (1994) confirmed a clear separation between the two species. Another specimen from the Kyushu-Palau Ridge ( Fig. 135A ) originally recorded as H. longiceps by Yatou (1982) was re-identified as H. hachijoensis by Nakayama et al. (2015a); they also reported one additional specimen from the type locality (BSKU 19308, 24.7 mm HL, 133+ mm TL). Recently, three additional specimens were obtained at the Nishiura fish market in Aichi Pref., southern Japan; these specimens were originally collected from off Izuoshima Island, at a depth of 200–300 m (FAKU 202929– 202930, 202944, 19–23.1 mm HL, 133+–141+ mm TL; Fig. 135B ). Hymenocephalus hachijoensis appears to be restricted to oceanic elevations (such as seamounts and ridges) in the northwestern Pacific.

Relationships and comparisons. Hymenocephalus hachijoensis belongs to the H.antraeus group as defined by Schwarzhans (2014). This group comprises the following four species: H. antraeus Gilbert & Cramer, 1897 endemic to Hawaii; H. hachijoensis ; H. heterolepis ( Alcock, 1889) restricted to the Andaman Sea and the Bay of Bengal; and H. punt Schwarzhans, 2014 recently described from the Gulf of Aden. According to Schwarzhans (2014), H. hachijoensis is readily distinguished from other species of the H. antraeus group by having lower counts of pelvicfin rays (7–8 vs. 10–12) and a longer barbel [30–42% HL (based on this study) vs. ±15%].

In Japan, H. hachijoensis is most similar to H. striatissimus in general appearance, but differs notably in having an otolith with two colliculi (vs. single in H. striatissimus ; see also Schwarzhans 2014: figs. 23C–D and 32D–F). It further differs from H. striatissimus in its longer barbel (36–52% PRL vs. 14–23%) and pelvic fin (95–103% PRL vs. 70–90%). Hymenocephalus hachijoensis somewhat resembles H. yamasakiorum Nakayama, Endo & Schwarzhans, 2015 in physiognomy, but differs in having lower counts of pectoral- (i13–i15 vs. i16–i17) and pelvic-fin rays (7–8 vs. 9), and higher counts of pyloric caeca (23 vs. 15). These two species are also distinguished from each other by the snout length (25– 30% PRL in H. hachijoensis vs. 37% in H. yamasakiorum ) and barbel length (36–52% PRL vs. 73%).

Figures

FIGURE 135. Fresh specimens of Hymenocephalus hachijoensis. (A) ZMUT KP416, 25.0 mm HL, 106+ mm TL, Kyushu-Palau Ridge, Pacific, 685–710 m depth; (B) FAKU 202929, 147+ mm TL, Nishiura fish market, Aichi Pref., Pacific. Lateral views. [Photos: (A) ZMUT; (B) N. Nakayama]

FIGURE 136. Holotype of Hymenocephalus striatissimus hachijoensis (= H. hachijoensis). BSKU 14171, 32.4 mm HL, 196+ mm TL, northeast of Hachijo-jima Island, Shichito-Iojima Ridge, Pacific, 570 m depth. (A) Lateral view; (B) dorsal and (C) ventral views of the head and trunk. Preserved condition. [Photos: N. Nakayama]