Genus Hymenocephalus Giglioli, 1884

[Japanese name: Sujidara-zoku]

Hymenocephalus Giglioli, 1884:228 (type species: Hymenocephalus italicus Giglioli, 1884, by original designation).

Mystaconurus G̹nther, 1887:139 [as subgenus of Macrurus; type species: Hymenocephalus italicus Giglioli, 1884, by subsequent designation by Jordan & Evermann (1898)].

Papyrocephalus Gilbert & Hubbs, 1920:539 (as subgenus of Hymenocephalus; type species: Hymenocephalus aterrimus Gilbert, 1905, by original designation).

Diagnosis. Anus immediately anterior to anal-fin origin, surrounded by narrow periproct. Ventral striae, consisting of fine, parallel black lines over silvery layer on each side of isthmus, shoulder girdle, most of chest, and posterior to pelvic fins. Gular region covered with fine striations of black epithelium. Light organ developed as long tube, extending from anus to slightly anterior to pelvic-fin bases; small lens at each end of light organ. Second spinous ray of first dorsal fin smooth along its leading edge. Pelvicfin rays 7–15, outer ray slender, not expanded or ribbonlike distally. Teeth all small, in narrow villiform bands in both jaws. Trunk moderately deep, laterally compressed. Head bones paper-thin, fragile in most species; head skin membranous, translucent. Nasal bones weakly ossified, not forming stout, dorsoventrally flattened processes. Chin barbel present or absent. Body scales large, highly deciduous; radial grooves developed anteriorly; spinules rudimentary, very short, erect, needle-like or narrowly triangular, arranged in quincunx order; reticulate structure absent. Branchiostegal rays 7. Most species small, usually less than 20 cm TL. Body with silvery reflection when fresh (in most species). [From Iwamoto & Graham (2001), Iwamoto et al. (2011), McMillan & Iwamoto (2014), and Schwarzhans (2014).]

Remarks. Hymenocephalus comprises 27 species, six of which are known from Japanese and adjacent waters (Nakayama et al. 2015a). Most species are small and less than 20 cm TL (Iwamoto et al. 2015). Due to the fragile nature of the head and body, specimens of Hymenocephalus are easily damaged when trawled, which often makes species identification difficult. Species of the genus typically occur at upper slope depths of the world’s oceans, but are strangely absent from the northeastern Pacific (McMillan & Iwamoto 2014; Schwarzhans 2014; Nakayama et al. 2015a). Hymenogadus Gilbert & Hubbs, 1920 and Spicomacrurus Okamura, 1970 were previously recognizedassubgeneraof Hymenocephalus (e.g., Okamura 1970a; Iwamoto 1990), but these taxa are now considered to deserve full generic status (Iwamoto et al. 2011, 2015; Schwarzhans 2014). Accordingly, Hymenocephalus currently includes two subgenera, viz., Hymenocephalus and Papyrocephalus Gilbert & Hubbs, 1920 .

Key to species of Hymenocephalus from Japan and adjacent waters

1a Pelvic-fin rays 7–9; ventral half of tail immaculate or only slightly peppered with small black melanophores, dorsal half usually pale, darker in some specimens; chin barbel variously developed, length longer than 1/3 orbit diameter (14–73% PRL) ..................................................................................... 2

1b Pelvic-fin rays 11–12; dorsal and ventral halves of tail uniformly peppered with small black melanophores; chin barbel absent or rudimentary, if present, length distinctly shorter than 1/3 orbit diameter (less than 5% PRL)............. 5

2a Barbel length greater than 1/2 PRL (53–73% PRL), its tip extending to about vertical through posterior margin of orbit or beyond; orbit diameter 35–43% PRL .............................. 3

2b Barbel length less than 1/2 PRL (14–52% PRL), its tip not reaching vertical through posterior margin of orbit; orbit diameter 43–59% PRL ......................................................... 4

3a Pelvic-fin rays 8; snout depressed, its tip barely protruding beyond upper jaw; dorsal half of tail pale to dusky; interorbital width 18–27% PRL......................................... H. longibarbis

3b Pelvic-fin rays 9; snout not depressed, its tip protruding distinctly beyond upper jaw; dorsal half of tail abruptly darker posteriorly; interorbital width 21% PRL................................ ................................................................... H. yamasakiorum

4a Barbel length 36–52% PRL; pelvic-fin length 95–103% PRL; ostial and caudal colliculi of otolith separated....................... ....................................................................... H. hachijoensis

4b Barbel length 14–23% PRL; pelvic-fin length 70–90% PRL; ostial and caudal colliculi of otolith fused ............................. ....................................................................... H. striatissimus

5a Gill rakers on second arch (outer/inner) 24–26/22–25; suborbital width 16–17% PRL....................... H. lethonemus

5b Gill rakers on second arch (outer/inner) 18–22/15–21; suborbital width 22–26% PRL........................ H. papyraceus