Gilbert's halosaurid fish

A marine fish belonging to the order Cynodontidae and family Cynodontidae of the class Osteichthyes. It is widely distributed in the warm waters of the world (excluding the northeastern Atlantic and Mediterranean Sea), mainly in the Indian and Pacific Oceans, including the Pacific coast south of Iwate Prefecture, the Okinawa Trough, southern Taiwan, the east coast of the Korean Peninsula, the Hawaiian Islands, southern Australia, the Maldives, Ceylon, Zanzibar, and the west coast of India. The body is long and slender, gradually tapering toward the rear. The tail is significantly laterally compressed. The snout is vertically flattened and significantly protrudes. The mouth is small and opens on the ventral surface of the snout, and the rear end of the maxilla reaches below the front edge of the eye. There is a tooth band consisting of fine granular teeth on both the upper and lower jaws, the palatine bone, and the pterygoid bone. The dorsal fin is located near the center of the front half of the body, and the base is significantly short. The base of the anal fin is remarkably long, beginning further forward than the middle of the body and extending to the tip of the tail. The pectoral fins are located above the lateral line, and the pelvic fins are located slightly forward of the beginning of the dorsal fin. There is no caudal fin. The lateral line runs straight down the ventral side of the body. The body is covered with round scales that fall off easily. The lateral line scales are enlarged and are about twice the size of the body scales. The body is silvery gray to dark purple-brown, and the anal fin is dark brown. The mouth and gill covers are blackish. It swims on the deep sea floor at depths of 650 to 2,500 meters, digging the seabed with its flat snout and preying mainly on small invertebrates such as crustaceans and shellfish. During the breeding season, males develop large olfactory organs, which are thought to detect pheromones released by their mates. The larvae go through a leptocephalus stage. They can reach a maximum length of about 55cm. They are sometimes caught by deep sea trawls, but have also been seen from deep sea boats. The Japanese name comes from the flat top of the head, which resembles a lizard's head. There is one other species in this family known from Japan, the black-banded lizardfish, Halosauropsis macrochir , but this species can be distinguished from this species by its shorter snout, black lateral line area, and the first dorsal fin ray being the same length as the second dorsal fin ray.

[Hirotoshi Asano and Kunio Amagaoka February 18, 2019]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

硬骨魚綱ソコギス目トカゲギス科に属する海水魚。岩手県以南の太平洋沿岸、沖縄舟状海盆(しゅうじょうかいぼん)（トラフ）、台湾南部、朝鮮半島東岸、ハワイ諸島、オーストラリア南部、モルジブ諸島、セイロン島、ザンジバル島、インド西岸などインド洋・太平洋を中心とした世界の温暖海域（大西洋北東部、地中海を除く）に広く分布する。体は細長く、後方に向かってだんだんと細くなる。尾部は著しく側扁(そくへん)する。吻部(ふんぶ)は縦扁し、著しく突出する。口は吻の腹面に開き、小さくて、上顎(じょうがく)の後端は目の前縁下に達する。上下両顎、口蓋(こうがい)骨、翼状骨に微細な粒状歯からなる歯帯がある。背びれは体の前半部の中央付近にあり、基底は著しく短い。臀(しり)びれの基底は著しく長く、体の中央よりも前方から始まり、尾端まで伸びる。胸びれは側線よりも上方にあり、腹びれは背びれ始部すこし前に位置する。尾びれはない。側線は体の腹側面を直走する。体は円鱗(えんりん)をかぶり、はげ落ちやすい。側線鱗は肥大し、体鱗のおよそ2倍。体は銀灰色～暗紫褐色で、臀びれは濃褐色。口部と鰓蓋(さいがい)部は黒みを帯びる。水深650～2500メートルの深海底を遊泳し、平たい吻を使って海底を掘り、主として甲殻類や貝類など小さな無脊椎(むせきつい)動物を捕食する。繁殖期には雄は嗅覚(きゅうかく)器官を大きく発達させて、仲間の出すフェロモンを感知すると考えられている。仔魚(しぎょ)はレプトセファルス（葉形(ようけい)幼生）期を経る。最大全長は55センチメートルほどになる。ときどき、深海トロールでとれるが、深海艇でも観察される。和名は平たい頭頂部がトカゲの頭を連想させることに由来する。本科には日本からもう1種クロオビトカゲギスHalosauropsis macrochirが知られているが、この種は吻が短く、側線の区域は黒く、第1背びれ軟条は第2背びれ軟条と同長であるなどの特徴で本種と区別できる。

［浅野博利・尼岡邦夫　2019年2月18日］

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