Hymenocephalus lethonemus

Schwarzhans, Werner, 2014, Head and otolith morphology of the genera Hymenocephalus, Hymenogadus and Spicomacrurus (Macrouridae), with the description of three new species, Zootaxa 3888 (1), pp. 1-73 : 42-43

Hymenocephalus lethonemus Jordan & Gilbert, 1904

Figs. 18A–G , 21

Hymenocephalus lethonemus Jordan & Gilbert, 1904: 615 (type locality: Sagami Bay , Japan).

Hymenocephalus lethonemus: Gilbert & Hubb ,s 1916: 188; Okamura, 1970: 54.

Material examined. 26 specimens; 1 specimen BSKU 109012 , 105 + mm TL, 28°33’N, 127°02’E, 621 m ; 10 specimens NSMT-P 46740 , 82 + – 145 mm TL, 34°57’N, 138°43’E ; 5 specimens NSMT-P 93138 , 89 + –130+ mm TL, off Izu Island , Japan ; 5 specimens NSMT-P 94418 , 132 + – 170 mm TL, off Yakushima , Japan ; 2 specimens NMST-P 97214 , 128 + – 63 mm TL, 34°45’N, 138°40’E ; 3 specimens USNM 51455 (1658, 1670, 1697), Suruga Bay , Japan, 128– 183 m .

Diagnosis. Pelvic fin rays 11, rarely 12 on one side only; pectoral fin rays 13–15; snout projecting, 20–25% HL; barbel absent; orbit diameter moderate, 30–33% HL; infraorbital width 13–15% HL; preopercular supporter moderately long, with obtuse angle at rear margin, 3–5% HL; gill rakers 20–26; ventral striae reaching to ½ distance from pelvic fin bases to periproct; otolith with moderately high predorsal lobe, colliculi separated, widely placed across collum, terminating at some distance from anterior and posterior rims of otolith; OL:OH = 1.05–1.1; TCL:PCL = 2.4–2.6.

Comparison. Hymenocephalus lethonemus is very similar to H. nascens , with the species differentiated by Gilbert & Hubbs (1920) when they described H. nascens , only by the pelvic fin ray count (11 vs 12–13). The low pelvic fin ray count also defines this species best from other members of the striatulus group, H. billsam and H. striatulus , from which it further differs in the absence of a barbel and the generally lower pectoral fin ray count (13–15 vs 14–18). The wide collum and the short pseudocolliculum of the otoliths add further potentially useful characters for distinguishing H. lethonemus from H. nascens . (TCL:PCL = 2.4–2.6 vs 1.8–2.4). Both species are here tentatively kept separate because of the differences in pelvic fin ray counts and otolith morphology, which appear to be stable.

Description. Head morphology (n = 3) ( Fig. 18A–B ): Snout projecting, long, 20–25% HL, orbit diameter 30–33% HL, interorbital width 50–60% HW. Barbel absent. Head canals well developed, moderately large, infraorbital width 13–15% HL, supraorbital canal with 5 segments, width 10–13% HL, supratemporal canal narrow, above segment 4 of supraorbital canal, preopercular canal width 10–14% HL, postorbital-preopercular interspace 6–7% HL. Infranasal supporter moderately large; infraorbital supporter short, expanding only beyond rear part of orbit, 50–70% OD; preopercular supporter moderately long, 3–5% HL, rear margin with obtuse angle.

Otolith morphology (n = 6) ( Fig. 18C–G ): Otolith moderately large; OL:OH = 1.05–1.1; OH:OT = 3.5. Dorsal rim with a distinct, broad predorsal lobe, entire dorsal rim undulating, with incision behind predorsal lobe; posterior tip shifted above sulcus termination; ventral rim regularly curved, smooth, deepest anterior of the middle; anterior rim high, subvertical to nearly vertical. Inner face slightly convex, with median sulcus. Ostial and caudal colliculi small, widely placed across collum, terminating at some distance from anterior and posterior tips of otolith; pseudocolliculum short. CCL:OCL = 0.8–1.1; TCL:PCL = 2.4–2.6. Dorsal depression small; ventral furrow distinct, close to ventral rim, anteriorly and posteriorly turning upwards to the sulcus.

Distribution ( Fig. 21 ). Hymenocephalus lethonemus is common off southern Japan from Sagami Bay to the Ryukyu Islands.

Figures

FIGURE 18. Hymenocephalus lethonemus: A–B—BSKU 94418, A—Lateral view of head, B—Dorsal view of head. C–G—Otoliths, C–E—Inner faces, F—Ventral view, G—Anterior view, C—NSMT P 46740, D—NSMT P 93138, E–G—BSKU 109012.

FIGURE 21. Geographical distribution of Hymenocephalus billsam, H. lethonemus, H. nascens and H. striatulus.