Grenadiers (Teleostei: Gadiformes: Macrouridae) of Japan and adjacent waters, a taxonomic monograph Author Nakayama, Naohide 0000-0001-9556-1537 Department of Marine Biology, School of Marine Science and Technology, Tokai University, 3 - 20 - 1 Orido, Shimizu, Shizuoka 424 - 8610, Japan ® naonakayama @ tsc. u-tokai. ac. jp or ® gadiformes@gmail.com text Megataxa 2020 2020-11-05 3 1 1 383 journal article 56026 10.11646/megataxa.3.1.1 1ece557d-068b-465b-809e-0fc1110035b3 2703-3090 6422776 7A95A1DD-0372-4FAC-BA3B-1896386BC710 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). 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] 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.