ドクウロコイボダイ科　Tetragonuridae                         Squaretails

 





SNFR 16603, 340 mm SL.


45 ツマリドクウロコイボダイ

Tetragonurus atlanticus Lowe, 1839             Bigeye Squaretail

 

Specimen examined: 1 (SNFR 16603; 340 mm SL), 338–400 m depth.

Counts: D XVI-I, 11; A I, 10; P1 17; P2 I, 5; GR 7+11=18; LLS ca. 90; V 25+23=48. Measurements: head length 25.3% SL; body depth 16.5% SL; eye diameter 26.5% HL; interorbital width 23.4% HL; snout length 29.7% HL; upper-jaw length 37.4% HL; lower-jaw length 35.0% HL; caudal-peduncle length 25.7% SL; caudal-peduncle depth 4.9% SL; pectoral-fin length 12.0% SL; pelvic-fin length 6.9% SL.

 

計数形質：背鰭16棘-1棘，11軟条；臀鰭1棘，10軟条；胸鰭17軟条；腹鰭1棘，5軟条；鰓耙数7+11=18；側線鱗数　約90；脊椎骨数25+23=48．計測形質：頭長25.3% SL；体高16.5% SL；眼径26.5% HL；両眼間隔23.4% HL；吻長29.7% HL；上顎長37.4% HL；下顎長35.0% HL；尾柄長25.7% SL；尾柄高4.9% SL；胸鰭長12.0% SL；腹鰭長6.9% SL．

特　徴　体は細長く，円筒形に近い．頭部は中庸大で，体長の約1/4を占める．吻はやや短く，その先端は丸い．口を閉じると下顎先端部は上顎の口内に収納される．両顎には基部が長く先端部が湾曲した歯が密に1列に並ぶ．鋤骨と口蓋骨にも小型の歯がある．眼は大きく，眼径は吻長よりもやや短い．涙骨は大きく，上顎の大部分を被う．鰓蓋骨の縁辺に棘や鋸歯はなく，円滑で厚い．鰓耙は柔軟で，幅広く短い．背鰭は2基で，背鰭棘は短く，棘条部の基底は軟条部の基底よりも長い．臀鰭始部は第2背鰭始部よりも後ろにあり，短い棘が1本ある．胸鰭と腹鰭はともに短い．尾鰭は深く二叉する．肛門は第2背鰭中央部の直下付近に位置する．尾柄部は太くて長い．鱗で形成された硬い骨質隆起が尾鰭基底に2対ある．鱗は小さい菱形で硬く，規則的に並び，剥がれにくい．吻部，頭頂部，および顎骨周辺は無鱗で，表面に微少な孔が密に分布する．側線鱗はやや不明瞭．喉部に食道嚢をもつ．体色はほぼ一様に黒色ないし黒褐色．

分　布　中層遊泳性．太平洋，インド洋，大西洋の温帯から熱帯域（Haedrich, 1967; Parin and Piotrovsky, 2004）．

備　考　本種は頭部と眼が大きく，脊椎骨数が少ないことで同海域に生息するドクウロコイボダイTetragonurus cuvieri Risso, 1810から識別できる．ドクウロコイボダイ属魚類の成魚は中層域に生息するが，幼魚は表層のクラゲに付随していることがある．本属の種はクラゲ類（刺胞動物やクシクラゲ類などの有櫛動物）を専食していると考えられている．本種の初期生活史に関する情報はインド洋からはないが，太平洋産の仔稚魚については詳しく記載されている（Ahlstrom et al., 1976; 岡本ほか，2001）．

（岡本）

 

Description  Body elongate and cylindrical. Head large, length about 1/4 of SL. Snout somewhat short and rounded. Lower jaw almost hidden within upper jaw when mouth closed. Teeth on both jaws similar, each with elongate base and curved tip. Small teeth present on vomer and palatine. Eye round and large, its diameter slightly shorter than snout length. Lachrymal bone large, covering upper jaw. Opercular region smooth and thick. Gill rakers soft, broad, and short. Two dorsal fins, spines short, first dorsal-fin base longer than second. Anal-fin origin behind second dorsal-fin origin, one short spine present. Pectoral and pelvic fins short. Caudal fin deeply forked. Anus located vertically below mid-point of second dorsal fin. Caudal peduncle long and wide. Modified scales forming two pairs of hard keels on caudal-fin base. Small and rhomboidal ctenoid scales arranged regularly on body, not deciduous. Snout, top of head, and jaws naked, small pores densely distributed on surface. Lateral line inconspicuous. Pharyngeal sac present. Body uniformly black to blackish brown.

Distribution  Mesopelagic. Tropical to temperate waters of Pacific, Indian and Atlantic oceans (Haedrich, 1967; Parin and Piotrovsky, 2004).

Remarks  This species can be distinguished from its congener from the same area, T. cuvieri Risso, 1810 by having a large head and eye, and fewer vertebrae. Although adult Tetragonurus are mesopelagic, young occur near the surface layer, often with jellyfish. Tetragonurus probably feeds exclusively on jellyfish (cnidarians and ctenophores). There is no information about the early life stages of the present species from the Indian Ocean, but those from the Pacific have been described in detail (Ahlstrom et al., 1976; Okamoto et al., 2001).

(M. Okamoto)