ワニトカゲギス科　Stomiidae                             Barbeled Dragonfishes

7 ホウライエソ

Chauliodus sloani Bloch and Schneider, 1801            Sloane’s Viperfish, Sloane’s Fangfish

Specimen examined: 1 (SNFR 15931; 222 mm SL), 524–852 m depth.

Counts: D 6; A 11; P₁ 12; P₂ 6; V (6 unossified anterior centra)+53. Photophores: BR (branchiostegal membranes) 15; IP (ventral series from isthmus to pectoral-fin base) 12; PV (ventral series between pectoral-fin base and pelvic-fin base) 20; VAV (ventral series from pelvic-fin base to anal-fin origin) 27; AC (ventral series from anal-fin base to end of row) 9; IC (all photophores of ventral series) 68; OV (lateral series from posterior portion of opercle to pelvic-fin base) 19; VAL (lateral series from pelvic-fin base to end of row above anal fin) 25. Measurements: head length 13.6% SL; body depth 10.4% SL; dorsal-fin base length 5.7% SL; anal-fin base length 9.2% SL; predorsal length 23.8% SL; pre-anus length 85.5% SL; pectoral-fin length 10.2% SL; pelvic-fin length 15.0% SL; caudal-peduncle depth 2.5% SL; caudal-peduncle length 7.3% SL; eye diameter 19.5% HL; interorbital width 23.5% HL; snout length 29.1% HL; upper-jaw length 91.7% HL; lower-jaw length 92.1% HL.

計数形質：背鰭6軟条；臀鰭11軟条；胸鰭12軟条；腹鰭6軟条；脊椎骨数（前部の軟質の椎体6）+ 53．発光器：鰓条発光器（BR）15；胸鰭前発光器（IP）12；胸部腹側発光器（PV）20；腹部腹側発光器（VAV）27；尾部発光器（AC）9；腹側発光器（IC）68；胸部体側発光器（OV）19；腹部体側発光器（VAL）25．計測形質：頭長13.6% SL；体高10.4% SL；背鰭基底長5.7% SL；臀鰭基底長9.2% SL；背鰭前長23.8% SL；肛門前長85.5% SL；胸鰭長10.2% SL；腹鰭長15.0% SL；尾柄高2.5% SL；尾柄長7.3% SL；眼径19.5% HL；両眼間隔23.5% HL；吻長29.1% HL；上顎長91.7% HL；下顎長92.1% HL．

特　徴　体は細長く，強く側扁する．眼は中庸大で円形，眼径は吻長よりも短い．口は極めて大きく，斜位で，下顎は上顎よりも長い．触鬚はない．固着性で大型の牙状の歯が前上顎骨と下顎に1列に並ぶ．前上顎骨の第3歯は第4歯より短い．主上顎骨の後部には櫛歯状の歯が1列に並ぶ．口蓋骨には円錐歯が1列に並ぶ．鋤骨および舌上に歯はない．背鰭始部は腹鰭基部直上よりかなり前方に位置する．腹鰭は胸鰭よりも長い．眼後発光器は円形．頭部と体は一様に黒茶色で，六角形の鱗模様が4–5列並ぶ（観察標本の体表と尾部は損傷）．

分　布　世界中の温帯域から熱帯域（Gibbs, 1984）．

備　考　ホウライエソ属魚類のうち本種およびC. pammelas Alcock, 1892の2種が，西部インド洋から知られている．本種はC. pammelasよりも多くの腹側発光器（62–68 vs. 58–61 in C. pammelas），腹部腹側発光器（23–28 vs. 21–23），および脊椎骨（前部の軟質の椎体は含まない：53–60 vs. 50–53）をもつことによって識別できる（Gibbs and Hurwitz, 1967）．本種は深層性魚類として知られているが，日周鉛直移動によって夜間は中深層域（200–1000 m）に出現する（Sutton and Hopkins, 1996）．

（岡本）

Description  Body elongate, strongly compressed. Eye moderate in size, round, its diameter shorter than snout length. Mouth extremely large, oblique, lower jaw longer than upper jaw. Barbel absent. Rigid and large fang-like teeth arranged in a single row on premaxilla and lower jaw; 3rd premaxillary tooth shorter than 4th one. Comb-like teeth densely aligned on posterior part of premaxilla. A single row of conical teeth on palatine. Vomer and tongue lacking teeth. Dorsal-fin origin located well anterior to a vertical through pelvic-fin base. Pelvic fin longer than pectoral fin. Postorbital luminous organ round. Body and head uniformly dark brown with 4–5 rows of hexagonal scale patterns (body surface and tail of the examined specimen heavily damaged).

Distribution  Tropical to temperate waters worldwide (Gibbs, 1984).

Remarks  Two Chauliodus species, C. sloani and C. pammelas Alcock, 1892 are known from the western Indian Ocean. Chauliodus sloani can be distinguished from C. pammelas by having more IC photophores (62–68 vs. 58–61 in C. pammelas), VAV photophores (23–28 vs. 21–23), and total vertebrae (not including unossified anterior centra: 53–60 vs. 50–53) (Gibbs and Hurwitz, 1967). Chauliodus sloani is commonly known as a bathypelagic fish, but the species occurs in the mesopelagic zone (200–1000 m) during the night as it undertakes diel vertical migration (Sutton and Hopkins, 1996).

(M. Okamoto)