カラスザメ科　Etmopteridae

カラスザメ科　Etmopteridae Lantern Sharks

SNFR 19237, 226mm TL.

3 カラスザメ属の1種

Etmopterus sp.

Specimens examined: 2 (SNFR 19236–19237; 227–334 mm TL; 231–267 mm body length, BDL), 618–792 m depth. Measurements: (% BDL=% body length, tip of snout to origin of upper lobe of caudal fin): total length 119.9–125.1; snout–vent length 62.8–65.2; head length 26.8–27.0; pre-dorsal-fin length 36.4–37.8; pre-second dorsal-fin length 64.8–70.1; first dorsal-fin anterior margin 4.8–4.9; length of first dorsal-fin base 5.2–5.6; second dorsal-fin anterior margin 10.5–11.3; length of second dorsal-fin base 7.9–8.2; dorsal margin of caudal fin 24.7–27.3; pectoral-fin anterior margin 10.4–10.9; length of pectoral-fin base 6.1–6.4; interdorsal space 31.6–35.6; eye length 5.6–6.1; eye height 4.3–4.5; prenarial length 3.7–3.9; preoral length 13.1–13.4.

計測形質（％BDL）：全長 119.9–125.1；総排泄孔前長62.8–65.2；頭長 26.8–27.0；第1背鰭前長36.4–37.8；第2背鰭前長 64.8–70.1；第1背鰭前縁長 4.8–4.9；第1背鰭基底長 5.2–5.6；第2背鰭前縁長 10.5–11.3;第2背鰭基底長 7.9–8.2；尾鰭上葉長24.7–27.3；胸鰭前縁長 10.4–10.9；胸鰭基底長 6.1–6.4；背鰭間隔 31.6–35.6；眼径 5.6–6.1；眼高 4.3–4.5；鼻前長 3.7–3.9；口前長 13.1–13.4．

特　徴　体は非常に細く，尾部は長い．腹鰭基底後端から尾鰭下葉始部までの長さは吻端から胸鰭始部の長さとほぼ等しく，背鰭間の距離よりやや短い．吻端から第1背鰭棘までの長さは第1背鰭棘から第2背鰭基底後端の長さとほぼ等しい．第1背鰭基底は腹鰭基底より胸鰭基底にかなり近くに位置する．頭幅は口前長とほぼ等しい．噴水孔から吻端の長さは噴水孔から胸鰭始部の長さよりやや長い．鰓孔は非常に狭く，噴水孔の幅とほぼ等しく，また眼径の1/4とほぼ等しいかそれより小さい．上顎歯は3列で5尖頭，下顎歯は1列で単尖頭．第1背鰭始部は胸鰭内縁直上にある．背鰭間隔は短く，胸鰭前長よりやや長い．第2背鰭は第1背鰭より大きく，約2倍の大きさ．第2背鰭始部から尾鰭上葉始部の長さは胸鰭前長とほぼ等しいかやや短い．臀鰭はない．尾鰭は細長い。尾鰭上葉外縁の長さは吻端から胸鰭始部の長さとほぼ等しい．体側の楯鱗は細く鉤状で体表の鱗列は規則正しく並ぶ．吻の大部分は楯鱗に覆われる．

体上部は茶色. 吻下面と腹部は黒色，細長い黒色斑が腹鰭上部から前方と後方に延びる. 尾鰭基底部に体軸に沿って細長い黒色斑がある．

分　布　インド洋南西部（本研究）．

備　考 本標本はフジクジラE. lucifer Jordan and Snyder, 1912とホソフジクジラE. brachyurus Smith and Radcliffe, 1912に類似するが，腹鰭上方および臀鰭始部の黒色斑がやや太いこと（ホソフジクジラでは非常に細い），黒色斑の後方に延びる分枝が前方に延びる分枝よりも長いかほぼ等しいこと（フジクジラでは後方に延びる分枝の方が短い）(Compagno, 1984) で区別できる．フジクジラとホソフジクジラはこれまで西部太平洋のみから知られている（Compagno, 1984; Ebert et al., 2011）．本標本は未記載種の可能性があるが，さらなる調査を要する．

（青沼）

Description A very slender-bodied lantern shark with a long tail, distance from pelvic insertion to ventral caudal-fin origin about as long as from tip of snout to pectoral-fin origin, and slightly shorter than interdorsal space. Distance from snout tip to first dorsal-fin spine about equal to distance from first dorsal-fin spine to second dorsal-fin insertion, first dorsal-fin base considerably closer to pectoral-fin base than to pelvic-fin base. Head width about equal to preoral length. Pre-spiracular length slightly greater than distance from spiracles to pectoral origins. Gill openings very narrow, about as wide as spiracle, 1/4 eye length or less. Teeth rows 3/1, upper-jaw teeth with less than three pairs of cusplets, lower-jaw teeth with no cusplet. Origin of first dorsal fin above inner margins of pectoral fins. Interdorsal space short, slightly greater than distance from snout tip to pectoral origin. Second dorsal fin much larger than first and about twice its area; distance between second dorsal-fin origin and upper caudal-fin origin about equal to or slightly less than pre-pectoral length. Anal fin absent. Caudal fin long and slender, length of dorsal margin about equal to distance from snout tip to pectoral-fin origin. Dermal denticles with slender, hooked conical crowns, denticle rows on body regularly arranged. Snout largely covered with denticles. Color brown above, with underside of snout and abdomen abruptly black, with an extremely narrow elongated black mark running above, in front of and behind pelvic fin, and other black marks at caudal-fin base.

Distribution Western South Indian Ocean (present study).

Remarks This lanternshark is close to E. lucifer Jordan and Snyder, 1902 and E. brachyurus Smith and Radcliffe, 1912, but differs in rather wide black mark above pelvic fins and caudal-fin base (extremely narrow mark in E. brachyurus) and posterior branch of the black mark longer than or subequal to anterior branch (posterior branch shorter than anterior branch in E. lucifer) (Compagno, 1984). E. lucifer and E. brachyurus are known to be distributed in the Western Pacific (Compagno, 1984; Ebert et al., 2011). The present specimens may represent an undescribed species, but more investigations are needed.

(Y. Aonuma)