オオメマトウダイ科Oreosomatidae Oreos
17
ガクガクギョ
Oreosoma atlanticum
Cuvier, 1829
Ox-eyed
Oreo
Specimen examined: 1
(SNFR 19235; 168 mm SL), 725–772 m depth.
Counts:
D VII, 29; A III, 28; P1 19; P2 I, 7; GR 6+22=28; LLS ca.
88–91; V 14+24=38. Measurements: head
length 37.9% SL; body depth 61.3% SL; predorsal
length 56.6% SL; preanal length 61.5% SL; eye
diameter 52.4% HL; interorbital width 37.2% HL; snout
length 27.3% HL; upper-jaw length 47.3% HL; postorbital-head length 22.9% HL;
caudal-peduncle depth 16.6% HL; caudal-peduncle length 33.8% HL; pectoral-fin
length 33.1% HL; pelvic-fin length 57.7% HL.
計数形質：背鰭 7棘，29軟条；臀鰭3棘，28軟条；胸鰭19軟条，腹鰭1棘，7軟条；鰓耙数6+22=28；側線鱗数約 90；脊椎骨数14+24=38．計測形質：頭長37.9
% SL；体高61.3
% SL；背鰭前長56.6
% SL；臀鰭前長61.5
% SL；眼径52.4
% HL；両眼間隔幅37.2
% HL；吻長27.3
% HL；上顎長47.3
% HL；眼後頭長22.9
% HL；尾柄高16.6
% HL；尾柄長33.8
% HL；胸鰭長33.1
% HL；腹鰭長57.7
% HL．
特 徴 体は高く，菱形で，側扁する．頭は大きく，眼窩周辺と下顎域および鰓蓋域に骨質板が発達する．背鰭前方の輪郭は凹み，背鰭始部の前で急に高くなる．眼は非常に大きく，眼径は頭長の約1/2．眼窩の背前縁は張り出す．両眼間隔域はほぼ平坦．吻は短い．前鼻孔は小さく，卵円形；後鼻孔は大きく，スリット状．口は大きく，突出できる．上唇は幅が広い．涙骨は細い．下顎に微細な円錐歯がある．上顎と鋤骨には歯が無い．下顎後端は尖る．背鰭始部は胸鰭の上方にある．背鰭および臀鰭の基底は棘条部で短く，軟条部で長い．背鰭では第1棘と最後の棘が最も短く，第2棘が最も長い．臀鰭始部は背鰭棘条部下にある．臀鰭棘は第1棘が最も長い．背鰭，臀鰭および胸鰭の軟条は不分枝．腹鰭始部は鰓蓋後端直下にある．腹鰭は倒すと臀鰭始部に達する．尾柄は細い．体側は落ちやすい円鱗に覆われる．両眼間隔域，頸部，頬部，峡部，喉部，胸部，腹部ならびに背鰭と臀鰭基底には固着した櫛鱗に覆われる．吻部は無鱗．体は薄茶色．口腔，鰓膜，胸鰭および肛門はこげ茶色．
分 布 南アフリカ，マダガスカル海嶺，オーストラリアおよびニュージーランドの水深600–820
m (James・稲田,
1990; Bray, 2008, 2015; 本研究）．
備 考 オオメマトウダイ科は4属約9–10種からなる（Bray,
1994, 2008; Heemstra, 2002）．ガクガクギョ属（新称）Oreosomaは本種のみからなる．本属は背鰭第1棘が第2棘より短いこと，腹鰭軟条が6–7本であること，背鰭前方の輪郭が凹むこと，体側が剥がれやすい円鱗に覆われること，眼後頭長が眼径の半分以下であることで同じ科の3属から識別される（Karrer,
1986）．
本種の稚魚の体表には顕著な円錐状の瘤が発達する．なお本種の標準和名はタスマン海産マグロ類の吐き出した標準体長61
mmの稚魚に基づいて提唱された（Abe
and Kaji, 1972）．
（篠原）
Description Body
deep, rhomboidal and compressed. Head large, with bony plates developed around
orbit and on mandibular and opercular
regions. Predorsal profile concave, rising sharply
before dorsal-fin origin. Eye very large, its diameter about 1/2 of head
length. Dorsoanterior rim of orbit projected. Interorbital space almost flat. Snout short. Anterior
nostril small, oval; posterior nostril large, slit-like. Mouth large, protrusible. Upper lip broad. Lachrymal narrow. Minute
conical teeth on lower jaw. No teeth on upper jaw and vomer.
Posterior end of mandible pointed. Dorsal-fin origin above pectoral fin. Dorsal
and anal fins spinous portions with very short bases,
much shorter than bases of soft-rayed portions; first and last spines shortest
and second one longest in dorsal fin. Anal-fin origin below spinous
portion of dorsal fin; first spine longest in anal fin. Soft rays in dorsal,
anal and pectoral fins unbranched. Pelvic-fin origin just
below posterior end of opercle. Pelvic fin reaching
anal-fin origin when appressed. Caudal peduncle
narrow. Lateral body covered with deciduous cycloid scales. Interorbital
space, nape, cheek, isthmus, jugular, thorax, belly and bases of dorsal and
anal fins covered with adherent ctenoid scales. Snout
naked. Body light brown. Oral cavity, gill membranes, pectoral fins and anus
dark brown.
Distribution
Remarks
The family Oreosomatidae comprises ca. 9–10 species in four genera (Bray, 1994, 2008; Heemstra, 2002). The genus Oreosoma comprises only O. atlanticum.
This genus is distinguished from the three other genera of the family in having
the first spine of the dorsal fin shorter than the second, pelvic-fin rays 6–7,
convex predorsal profile, sides of body covered with
deciduous cycloid scales, and postorbital head length less than 1/2 of eye
diameter (Karrer, 1986). The juveniles are known to
have prominent cone-like protuberances on the body.
(G. Shinohara)