Hymenocephalus yamasakiorum Nakayama, Endo & Schwarzhans, 2015

Nakayama, Naohide, 2020, Grenadiers (Teleostei: Gadiformes: Macrouridae) of Japan and adjacent waters, a taxonomic monograph, Megataxa 3 (1), pp. 1-383 : 221-222

Hymenocephalus yamasakiorum Nakayama, Endo & Schwarzhans, 2015

[Japanese name: Oguro-sujidara]

( Fig. 146 ; Appendix 3-8F)

Hymenocephalus yamasakiorum Nakayama, Endo & Schwarzhans, 2015a:505 , figs. 1–4 (original description; holotype: BSKU 113692, from Tosa Bay off Aki, in 320–400 m; new Japanese name: “Oguro-sujidara”); Motomura 2020:39 (listed; Japan).

Diagnosis. A species of Hymenocephalus with 9 pelvic-fin rays; barbel very long, length 73% PRL, its tip extending beyond vertical through hind margin of orbit when depressed; snout high, not depressed, length 37% PRL; orbit diameter 43% PRL; interorbital space 31% PRL; first dorsal-fin rays II,10; pectoral-fin rays i16–i17; gill rakers on first arch (outer/inner) 14/20, on second arch 20/19; pyloric caeca 15; trunk blackish dorsally, but the dark area abruptly ending posterior to first dorsal-fin base; dorsal half of tail prominently darker posteriorly, but ventral half completely lacking black melanophores; caudal vertebrae barely visible externally when fresh; ostial and caudal colliculi of otolith not fused.

Material examined. 1 specimen. Holotype of Hymenocephalus yamasakiorum: BSKU 113692 (1, 26.5 mm HL, 151+ mm TL), off Aki , Kochi Pref., Tosa Bay, Japan, 320–400 m, F/ V Kosei-maru, bottom trawl, coll. N. Nakayama et al., 8 Apr. 2014.

Counts and measurements. Counts: first dorsal-fin rays II,10; pectoral-fin rays i16–i17; pelvic-fin rays 9; gill rakers on first arch (outer/inner) 14/20, on second arch 20/19; pyloric caeca 15.

The following measurements are in % of HL, followed by those in % of PRL in parentheses: snout length 28 (37); orbit diameter 33 (43); postorbital length 45 (59); postrostral length 76; orbit–preopercle distance 40 (52); suborbital width 9 (12); upper-jaw length 51 (67); length of rictus 46 (60); length of premaxillary tooth band 42 (55); preoral length 12 (16); distance between tip and lateral angle of snout 16 (21); snout width 26 (35); internasal width 18 (24); interorbital width 24 (31); body width over pectoral-fin bases 46 (61); body depth at first dorsal-fin origin 63 (82); body depth at anal-fin origin 49 (64); prepelvic length 105 (138); preanal length 162 (212); isthmus–pelvic distance 49 (64); isthmus–anal distance 105 (138); pelvic–anal distance 60 (79); pelvicfin length 78 (102); pectoral-fin length 53 (70); predorsal length 102 (133); height of first dorsal fin 82 (107); length of first dorsal-fin base 35 (46); interdorsal length 77 (101); length of gill slit 30 (39); length of posterior nostril 6 (8); barbel length 55 (73); length of pyloric caecum 14 (19).

Size. At least 15 cm TL.

Distribution. So far known only from Tosa Bay off Aki at a depth of about 320–400 m (Appendix 3-8F). Very rare.

Remarks, relationships, and comparisons. Hymenocephalus yamasakiorum was recently described from a single specimen collected from Tosa Bay, Japan ( Fig. 146 ), and it belongs to the H. iwamotoi group as defined by Schwarzhans (2014) ( Nakayama et al. 2015a). In Japanese waters, the species might be confused with H. longibarbis by sharing a long chin barbel (73% and 53–72% PRL in H. yamasakiorum and H. longibarbis respectively), but they are readily separable by a difference in physiognomy: in H. yamasakiorum , the tip of the snout is high and moderately protrudes beyond the upper jaw giving a slightly pointed appearance to the snout ( Fig. 146A ), whereas the snout of H. longibarbis is low and greatly depressed, barely extending beyond the upper jaw ( Fig. 139A ). Body pigmentation is also different between the two species; in H. yamasakiorum , the dorsal half of the tail is abruptly darker posteriorly, but is uniformly dusky in H. longibarbis . Regarding meristic and morphometric characters, H. yamasakiorum differs from H. longibarbis , in having more pelvic- (9 vs. 8) and pectoral-fin rays (i16– i17 vs. i12–i16), fewer pyloric caeca (15 vs. 18–25), a longer snout (37% PRL vs. 26–33%), a broader interorbital space (31% PRL vs. 18–27%), a shorter pelvic fin (102% PRL vs. 109–153%), and a narrower pelvic–anal distance (79% vs. 87–115%). See the original description given by Nakayama et al. (2015a) for further morphological details and comparisons with other congeners.

Figures

FIGURE 139. Fresh specimens of Hymenocephalus longibarbis. (A) BSKU 92382, 44.7 mm HL, 239+ mm TL, Mimase fish market, Kochi Pref., Pacific; (B–C) BSKU 112441, 51.1 mm HL, 267+ mm TL, Tosa Bay off Susaki, Kochi Pref., Pacific. (A) Lateral view; (B) dorsal and (C) ventral views of the head and trunk. [Photos: N. Nakayama]

FIGURE 146. Holotype of Hymenocephalus yamasakiorum. BSKU 113692, 26.5 mm HL, 151+ mm TL, Tosa Bay off Aki, Kochi Pref., Pacific, 320–400 m depth. (A) Lateral view; (B) dorsal and (C) ventral views of the head and trunk. Fresh condition. [Photos: reproduced from Nakayama et al. (2015a: fig. 1) with permission from the Ichthyological Society of Japan]