ヤセムツ科 Epigonidae
Deepwater
Cardinalfishes
27 コハクヤセムツ（新称）
Epigonus angustifrons
Abramov and Manilo, 1987 Amber Deepwater Cardinalfish (New English name)
Specimens
examined: 6 (
Counts:
D VII-I-I, 10–11; A II, 9; P1 19–20; P2 I, 5; GR
11–12+23–24=34–35; LLS 47–50+3–5 (caudal fin); scales above lateral line 3;
scales below lateral line 9; V 10+15=25. Measurements: head length 29.6–32.5%
SL; body depth 17.7–19.0% SL; eye diameter 34.7–37.8% HL; interorbital
width 24.2–26.4% HL; snout length 22.4–24.2% HL; upper-jaw length 45.2–48.7%
HL; lower-jaw length 49.8–55.9% HL; caudal-peduncle length 23.9–28.4% SL;
caudal-peduncle depth 7.9–8.6% SL; pectoral-fin length 18.0–20.2% SL;
pelvic-fin length 14.7–17.5% SL; spine length of second dorsal-fin 4.8–5.8% SL;
second anal-fin spine length 4.7–5.8% SL; pelvic-fin spine length 8.2–10.1% SL;
first dorsal-fin base length 11.1–12.5% SL; second dorsal-fin base length
9.8–11.0% SL; anal-fin base length 8.6–9.8% SL.
計数形質：背鰭7棘-1棘-1棘，10–11軟条；臀鰭2棘，9軟条；胸鰭19–20；腹鰭1棘，5軟条；鰓耙数11–12+23–24=34–35；側線鱗数 47–50+3–5（尾鰭）；側線上方鱗数3；側線下方鱗数9；脊椎骨数10
+ 15 = 25．計測形質：頭長29.6–32.5%
SL；体高17.7–19.0%
SL；眼径34.7–37.8%
HL；両眼間隔24.2–26.4%
HL；吻長22.4–24.2%
HL；上顎長45.2–48.7%
HL；下顎長49.8–55.9%
HL；尾柄長23.9–28.4%
SL；尾柄高7.9–8.6%
SL；胸鰭長18.0–20.2%
SL；腹鰭長14.7–17.5%
SL；第2背鰭の棘長4.8–5.8%
SL；第2臀鰭棘長4.7–5.8%
SL；腹鰭棘長8.2–10.1%
SL；第1背鰭基底長11.1–12.5%
SL；第2背鰭基底長9.8–11.0%
SL；臀鰭基底長8.6–9.8%
SL．
特 徴 体は細長く，側扁する．体高は体長の約1/5．頭は中庸大で，やや幅が広い．吻は丸くて短い．眼は円形で大きく，眼径は吻長よりも大きい．主鰓蓋骨に骨質の棘はない．口は大きい．小型の円錐歯が上顎に１列，下顎では側面に１列と縫合部に2–3列をなす．鋤骨と口蓋骨には絨毛状歯帯があるが，舌上に歯はない．背鰭は2基で離れており，その間に短い独立した1棘が存在する（まれに皮下に埋没）．胸鰭と腹鰭はともに短く，肛門に達しない．肛門は第2背鰭始部の直下にある．尾鰭の後縁は深く湾入する．鱗は弱い櫛鱗で剥がれやすい．側線は1本で，明瞭．体は茶褐色で，口内は黒色．
分 布 インド洋南西部（Okamoto
and Aungtonya, 2013; 本研究），大西洋南東部（Abramov
and Manilo, 1987）．
備 考 本種は，オオヤセムツE. telescopus
(Risso, 1810)に似るが，鰓耙数が33–36（vs.
オオヤセムツ23–26），体高が体長の約1/5（vs.
体長の約1/4），体長は40cmを越えないこと（70
cmに達する）によって識別できる．その他の本海域に出現する同属種とは，第1背鰭と第2背鰭の間に独立した1棘をもつこと，口腔内が黒いことによって異なる[ハゲヤセムツE. denticulatus
Dieuzeide, 1950，E. exodon
Okamoto and Motomura, 2012，マルヤセムツE. robustus
(Barnard, 1927)には独立した背鰭棘はなく，口腔内は淡色]．Abramov
and Manilo (1987)は本種の舌上には小型の歯があると報告したが，これは微少な乳頭状突起の誤り．Epigonus trunovi
Parin and Prokofiev in Parin
et al., 2012は本種の新参シノニムである（Okamoto
and Aungtonya, 2013）．
（岡本）
Description
Body
elongate and laterally compressed. Body depth about 1/5 of SL. Head moderate in
size and rather wide. Snout round, short. Eye large and circular, its diameter
greater than snout length. Opercular spine absent. Mouth large. Minute conical teeth arranged in a
single row on upper jaw and lateral part of lower jaw, in 2–3 rows at symphysis of lower jaw. Villiform teeth in bands present on vomer
and palatine, but tongue toothless. Two separate dorsal fins, an isolated spine
present between first and second dorsal fins (rarely hidden under skin). Pectoral and pelvic fins short, not reaching anus. Anus located below second dorsal-fin origin. Caudal fin
deeply forked. Scales weakly ctenoid, deciduous.
Lateral line single, conspicuous. Body dark brown, oral cavity black.
Distribution Western
South Indian Ocean (Okamoto and Aungtonya, 2013;
present study), southeastern
Remarks This
species is similar to E. telescopus (Risso, 1810), but
differs in having 33–36 gill rakers (vs. 23–26 in E. telescopus),
slender body depth (1/5 of SL vs. 1/4 of SL in E. telescopus), and smaller size (not
reaching 40 cm SL vs. reaching 70 cm SL in E.
telescopus). Epigonus angustifrons differs from other
congeners in the area by having an isolated dorsal-fin spine between the first
and second dorsal fins and the black oral cavity [vs. an isolated dorsal-fin
spine absent and the oral cavity pale in E.
denticulatus Dieuzeide,
1950, E. exodon
Okamoto and Motomura, 2012, and E. robustus (Barnard, 1927)]. Abramov and Manilo (1987)
reported that E. angustifrons
has small teeth on the tongue, but they misidentified the minute papillae as
teeth. Epigonus trunovi Parin and Prokofiev in Parin et al., 2012 is a junior synonym of the
present species (Okamoto and Aungtonya, 2013).
(M.
Okamoto)