Arctic lamprey

This fish belongs to the order Petromyzidae and family Petromyzidae of the class Petromyzidae. It is found in Ibaraki Prefecture on the Pacific coast, and north of Shimane Prefecture on the Sea of ​​Japan coast, as well as on the continent surrounding the Arctic Ocean. It reaches a body length of 60 cm, and has a long, slender, eel-shaped body. There are seven gill slits in a row behind the eyes. There are two teeth on the dental plate of the upper mouth, and three pairs of teeth on the lateral lip. It resembles the Siberian lamprey because of the black tip of its caudal fin, but can be distinguished by the black tip of its second dorsal fin and the sharp teeth on the dental plate of its lower mouth. From spring to summer, males build spawning beds on the gravel bottom upstream of rivers. The male attaches himself to the female's head and wraps his body around it to lay eggs, after which both the male and female die. The eggs laid are about 1 mm in diameter. It is unique in that it undergoes complete cleavage type egg development. From spring to summer, eggs are laid on the gravel bottom of the upper reaches of rivers. After hatching, the larvae, called ammocoetes, resemble amphioxus fish, and spend four years as larvae before descending to the sea to spend two to three years. During this time, they grow by sucking the blood of salmon, trout, flounder, and cod. They are caught using bag nets, forked hand nets, gill nets, and lamprey traps, and are used for grilled eels and other dishes. The species is designated as an endangered species of Class II on the Ministry of the Environment's Red List (2013), which indicates that it is at increasing risk of extinction.

[Akira Ochiai and Kunio Amano]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

頭甲綱ヤツメウナギ目ヤツメウナギ科に属する魚。太平洋側は茨城県、日本海側は島根県以北、北極海を取り巻く大陸に分布する。体長60センチメートルに達し、体は細長くウナギ形。目の後ろに7個の鰓孔(さいこう)が1列に並ぶ。上口の歯板の歯は2本で、側唇の歯は3対。尾びれの先端が黒いことでシベリアヤツメに似るが、第2背びれの先端が黒いこと、下口の歯板の歯は鋭いことなどで区別する。春から夏に河川の上流の砂礫(されき)底に雄が産卵床をつくる。雄は雌の頭に吸い付き、体を巻き付けて産卵し、産卵後は雌雄とも死ぬ。産み付けられた卵は直径約1ミリメートル。全割型の卵発生を行うことで特異である。春から夏に河川の上流の砂礫底に産み付けられた卵は、孵化(ふか)後4年間をナメクジウオに似たアンモシーテスammocoetesとよばれる幼生生活ののち、海に下って2～3年間過ごす。その間、サケ・マス類、カレイ類、タラ類の血を吸って成長する。袋網、叉手(さで)網、刺(さし)網、ヤツメ筌(うけ)で漁獲し、蒲焼(かばや)きなどにされる。環境省のレッド・リスト（2013）で、絶滅の危険が増大している、絶滅危惧種Ⅱ類に指定されている。

［落合　明・尼岡邦夫］

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