クロタチカマス科　Gempylidae            Snake Mackerels

 

38 ナガタチカマス
Thyrsitoides marleyi Fowler, 1929     Black Snoek

 

Specimen examined: 2 (SNFR 19352, 19353; 1015–1016 mm SL), 723–824 m depth.

Counts: D XVII-I, 15–17; A I, 16; P₁ 15; P₂ I, 5; GR 5–15+4–15=9–30; V 19+16=35. Measurements: head length 26.0–26.2% SL; body depth 12.6–13.7% SL; eye diameter 16.7–17.4% HL; snout length 42.4–43.5% HL; interorbital width 18.6–20.5% HL; upper-jaw length 45.6–46.2% HL; lower-jaw length 64.8–65.3% HL; caudal-peduncle length 4.5–5.6% SL; caudal-peduncle depth 3.3–3.5% SL; longest pectoral-fin length 11.7–12.4% SL; longest pelvic-fin length 6.1–6.4% SL.

 

計数形質：背鰭17棘-1棘，15–17軟条；臀鰭1棘，16軟条；胸鰭15軟条；腹鰭1棘，5軟条；鰓耙数5–15+4–15=9–30；脊椎骨数19+16=35．計測形質：頭長26.0–26.2% SL；体高12.6–13.7% SL；眼径16.7–17.4% HL；吻長42.4–43.5% HL；両眼間隔18.6–20.5% HL；上顎長45.6–46.2% HL；下顎長64.8–65.3% HL；尾柄長4.5–5.6% SL；尾柄高3.3–3.5% SL；胸鰭最長軟条長11.7–12.4% SL；腹鰭最長軟条長6.1–6.4% SL．

特　徴　体は大きく延長し，側扁する．頭は大きく，体長の約1/4．両眼間域はわずかに凹む．目は小さく，眼径は吻長の約1/3．鼻孔は2個でやや離れ，前鼻孔は小円形，後鼻孔は裂孔状．口は大きく，開口時に突出しない．上顎後端は眼の前縁下に達する．両顎の先端に肉質の小突起がある．上顎前端に3対の大きな牙状歯があり，1対の小牙状歯が下顎先端にある．両顎の側歯は強い犬歯状歯．鋤骨に歯は無く，口蓋骨には1列の小犬歯が並ぶ．鰓耙は隅角部のやや大きな1本を除き，2叉または3叉する小棘状．第1背鰭基底長は第2背鰭基底長の約2.5倍．胸鰭は小さく，吻長とほぼ等しい．腹鰭は胸鰭の1/2よりやや大．側線は上枝と下枝に2分岐し，分岐点は背鰭第4棘の下方に位置する．上枝は背鰭基底と平行に背鰭第15棘まで達する．下枝はやや急に後下方に曲がり，背鰭第6棘下付近から体側中央を直走し，尾鰭基底部に達する．鱗は小さく，脱落しやすい．体背部は濃褐色，体腹部は銀白色を帯びる．第1背鰭鰭膜は上部が黒，下部は淡色．他の各鰭は黒色を帯びる．

分　布　インド・西部太平洋の温帯から熱帯域 (Nakamura and Parin, 1993)．

備　考　Thyrsitoidesには本種のみが知られる．本種は，両顎の先端に肉質の小突起を持つこと，側線が2分岐し下枝は体側中央を走ること，背鰭棘条数が17–19であること等の特徴を持つことにより他のクロタチカマス科魚類から識別できる（Nakamura and Parin, 2001）．

（日��）

 

Description  Body greatly elongated and compressed. Head large, its length about 1/4 of standard length. Interorbital region slightly concave. Eye small, its diameter about 1/3 of snout length. Two nostrils, somewhat widely separated from each other, anterior one round and posterior one slit-like. Mouth large, not protractile when opened. Posterior margin of maxilla reaching to anterior margin of eye. A dermal process at tip of each jaw. Three pairs of large fangs at front of upper jaw, and a pair of fangs at front of lower jaw. A single row of strong canine teeth in both jaws. No teeth on vomer and small uniserial canine teeth on palatine. Gill rakers on first gill arch bifid or trifid spinescent except　one or two at corner enlarged with a long spine. First dorsal-fin base 2.5 times longer than second dorsal-fin base. Pectoral fin short, its length almost as long as snout length. Pelvic fin well developed, its length slightly longer than half of pectoral fin. Lateral line bifurcated, upper branch following dorsal contour of body, lower one originating below fourth dorsal-fin spine or slightly behind it, running mid-laterally. Scales very small and deciduous. Body dark brown dorsally, silvery white ventrally. Fins blackish brown, dorsal fin element black.

Distribution  Tropical to temperate waters of Indo-West Pacific (Nakamura and Parin, 1993).

Remarks  This genus is monotypic and can be distinguished from the other gempylid genera by having dermal processes on the jaws, the bifurcated lateral line with the lower branch running along the middle of body, and dorsal-fin spines XVII to XIX (Nakamura and Parin, 2001).

(K. Hidaka)