Hymenocephalus aterrimus Gilbert, 1905

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 : 33-35

Hymenocephalus aterrimus Gilbert, 1905

Figs. 13A–L , 16

Hymenocephalus aterrimus Gilbert 1905: 666 (type locality: off Kauai , Hawaii).

Hymenocephalus aterrimus: Marshall & Iwamoto, 1973: 607 ; Iwamoto & Merrett,1997: 516; Merrett & Iwamoto 2000: 757. Hymenocephalus (Papyrocephalus) aterrimus Gilbert & Hubbs, 1920: 539 .

Material examined. 33 specimens; 3 specimens AMS I.29753-001, 105–125 mm TL, 31°53’S, 153°16’E, 878–933 m; 1 specimen AMS I.30304-006, 105+ mm TL, 32°13’S, 153°06’E, 820–857 m; 1 specimen BSKU 111021 , 148 + mm TL, 28°23’S, 113°01’E, 855 m ; 2 specimens LACM 57120-1 , 96 + and 110+ mm TL, north off Maui , TC-61-65 ; 5 specimens MCZ 51405, 119– 135 mm TL, off Havana , 860 m ; 2 specimens MNHN 1994-0872 , 122 and 129+ mm TL, 23°07’S, 166°51’E, 850 m ; 4 specimens MNHN 1994-0876 , 74 + – 118 mm TL, 12°31’S, 174°22’W, 715–730 m ; 2 specimens MNHN 1997-0670 , 85 + and 110 mm SL, 21°10’S, 165°53’E, 786–800 m ; 1 specimen MNHN 2000-1067 , 08°57’S, 140°15’W, 708–738 m ; 3 specimens USNM 216161 , 195– 205 mm TL, 24°40’N, 80°00’W, 732–860 m ; 4 specimens USNM 405734 , 122–130 + mm TL, 16°58’N, 87°53’W ; 3 specimens USNM 405736 , 80 + – 154 mm TL, 16°58’N, 80°07’W, 732 m ; 2 specimens USNM 51677 , 117 + –140+ mm TL, off southern Oahu , Hawaii, 386 m .

Diagnosis. Black-colored large-headed fish (head size 21–25% TL); pelvic fin rays 13–14; pectoral fin rays

13–16; snout projecting, 20–24% HL; head bones papery thin, often distorted; barbel absent; orbit diameter small, 20–24% HL; interorbital width 60–70% HW; infraorbital width 20–25% HL; postorbital-preopercular interspace 10–14% HL; preopercular supporter very small, forked; gill rakers 21–25; ventral striae reaching to about ⅓ from pelvic fin bases to periproct; otolith small with rounded outline, no predorsal lobe developed; colliculi separated; OL:OH = 1.0–1.2; TCL:PCL = 2.2–2.8.

Comparison. Hymenocephalus aterrimus is readily recognized by the large head, black color, small eyes, wide infraorbital space, small preopercular supporter, relatively small otolith without predorsal lobe, and clearly separated colliculi. It closely resembles H. barbatulus , from which it differs primarily in the larger number of pelvic fin rays (13–14 vs 7) and pectoral fin rays (13–16 vs 10), and H. sazonovi n.sp., from which it differs in the head and otolith shape and the larger number of gill rakers (21–25 vs 19–20).

Description. Head morphology (n = 3) ( Fig. 13A–C ): Head large, with very soft bones, snout bluntly rounded, long, (snout length 20–24% HL), orbit diameter 20–24% HL, interorbital width 60–70% HW. Barbel absent. Head canals well developed, large, infraorbital width 20–25% HL, supraorbital canal with 4 to 5 segments, width 13–17% HL, supratemporal canal not identified, preopercular canal width 19–23% HL, postorbital-preopercular interspace 10–14% HL. Infranasal supporter small; infraorbital supporter short, expanding only beyond rear part of orbit, 30–60% OD; preopercular supporter short, forked (3–4% HL).

Otolith morphology (n = 24) ( Fig. 13D–L ): Otolith smaller than usually in the genus, with regularly curved outline and with low or indistinct predorsal lobe; OL:OH = 1.0–1.2; OL:OT about 3. All rims smooth and regularly curved; dorsal rim somewhat reduced, ventral rim broad, shallow. Inner face almost flat, with median sulcus. Ostial and caudal colliculi small, terminating at moderate distance from anterior and posterior tips of otolith; pseudocolliculum short. CCL:OCL = 0.8–1.2; TCL:PCL = 2.2–2.8. Dorsal depression indistinct; ventral furrow thin, moderately close to ventral rim.

Discussion. Hymenocephalus aterrimus seems to reside in deeper water than most other species of the genus with most of the catches from 700 to nearly 1000 m water depth. As already mentioned, this deeper habitat may have resulted in certain functional adaptations such as the reduction of eye and otolith size, the enlargement of the head canal system and the development of very thin, papery head bones. As a result of the fragile head bones, very few specimens have been found in collections that were adequate for a detailed study of the head morphology. Hymenocephalus aterrimus is also an unusually widely distributed species in the genus (see below), and because of this, I investigated specimens from different locations to find out whether regional differences could be seen. However, as a result of the generally poor preservation of the specimens my studies were primarily focused on meristic values and otoliths. Meristic values of pectoral fin rays and gill rakers show a rather wide variability, but no pattern of regional differentiation. Otoliths do not exhibit any valuable regional variation. It was therefore concluded that H. aterrimus represents a single, well-defined species with a very wide geographical distribution range, except for specimens on the Nazca and Sala y Gomez ridges in the SE Pacific, which represent a related, but separate species— H. sazonovi n.sp. (see below).

Distribution ( Fig. 16 ). The geographical distribution pattern of H. aterrimus , although wide, is somewhat patchy and discontinuous. In the Atlantic it is known from the Caribbean and the continental slope of northern South America. It has not been recorded from the eastern Atlantic or the Indian Ocean. In the Pacific it has been caught off subtropical western and eastern Australia, off several islands in the southern Pacific ( New Caledonia, Vanuatu, Wallis & Futuna, Marquesas Islands), and off Hawaii in the central northern Pacific.

Figures

FIGURE 13. Hymenocephalus aterrimus: A—USNM 216161, lateral view of head. B—USNM 51677, cotype, lateral view of head. C—USNM 216161, dorsal view of head. D–L—Otoliths, E, G–L—Inner faces, D—Anterior view, F—Ventral view, D–F—MNHN 1997-0670, G—USNM 405736, H—BSKU-ED915, I—MNHN 1994-0876/2, J—MCZ 51405/1, K—AMS I.30384-006, L—MNHN 1994-0872.

FIGURE 16. Geographical distribution of Hymenocephalus aterrimus, H. barbatulus, H. nesaeae, H. papyraceus and H. sazonovi.