Hagfish - Nutaunagi (English spelling) hag fish

A general term for fish in the Myxinidae family, order Myxinidae, class of hagfish, and one of these. Until 2006, fish in the Myxinidae family were classified in the order Myxiniformes, family Myxinidae, but because the name contained discriminatory words, the Japanese Society of Ichthyology renamed them to the current standard Japanese name in January 2007. Although the body is eel-shaped, it belongs to the superclass Agnathosupera, and, together with lampreys, is an academically interesting vertebrate that appeared more than 400 million years ago. Its eyes are buried under the skin, it has no lenses or irises, and its optic nerves are degenerate. Its mouth is slit-like with four pairs of barbels, and it has one or six pairs of gill slits that open outward. It excretes foul-smelling mucus from numerous mucus pores arranged in one or two rows on the lower side of its body. It has two rows of radular teeth on the left and right of the front end of its tongue. They have no fins other than a tail fin, and the largest ones can reach a body length of about 80 centimeters.

They are nocturnal, and have the habit of hiding in the mud on the seabed during the day. They usually swim slowly, meandering, but when stimulated, they will twist their body into the shape of the number eight. When they attach to their prey, they create knots like knotted string. They have the habit of gathering around carrion, piercing their prey's body with their tongue teeth, then retracting it to its original position, scraping off the flesh and internal organs. In addition to fish, they also eat shellfish, shrimp, crabs, and polychaetes that live in the mud. They make holes in the skin of slow-moving fish and squids that get caught in fishing nets, or enter through the mouth or gills to eat the flesh and internal organs. They are disliked by fishermen because they damage the nets with a lot of mucus during this process. They spawn from summer to autumn. The eggs are large and acorn-shaped, and each fish carries only a small number of eggs, less than 50.

More than 13 species are known to exist in the deep seas of the Pacific and Atlantic Oceans. Five species are found in Japan, and they can be broadly divided into two groups: one with six to eight external gill slits, and the other with just one.

The inshore hagfish, Eptatretus burgeri , has six external gill slits and is found in various parts of Japan south of the Chubu region of Honshu, the East China Sea, and the southern part of the Korean Peninsula. It is also called Isomekura because it lives in waters less than 10 meters shallower. In some areas, it is also called Anago. It is about 60 centimeters long. Its body is brownish-brown. The spawning season is from August to October. The eggshell is 22 millimeters long and 9 millimeters in diameter, with numerous thread-like projections at both ends. Each fish can hold around 20 eggs. It is somewhat delicious in summer. In addition to this species, the group also includes the deep-sea black hagfish Eptatretus atami and the purple hagfish Eptatretus okinoseanus , but they can be distinguished by the fact that the black hagfish's external gill slits are close together and not lined up in a row, while the purple hagfish's external gill slits are lined up in a row of eight.

The group with one external gill slit includes the narrow hagfish Myxine garmani (formerly known as the mekura eel) and the Okinawa narrow hagfish Myxine paucidens (formerly known as the Okinamekura eel). The narrow hagfish has 12 or fewer teeth in the front and back rows of its radula. Its body color is blue-purple. The Okinawa narrow hagfish is similar in body color to the narrow hagfish, but has 6 and 7 teeth in the front and back rows of its radula. Both the narrow hagfish and the Okinawa narrow hagfish are about 50 centimeters long. Both are rare species that live on the deep sea floor, and their ecology is unclear.

[Akira Ochiai and Kunio Amano]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

ヌタウナギ綱ヌタウナギ目ヌタウナギ科Myxinidaeの魚類の総称、およびそのなかの1種。2006年（平成18）までMyxinidaeの魚類はメクラウナギ目メクラウナギ科に分類されていたが、差別的語を含むため、日本魚類学会が2007年1月に現在の標準和名に改名した。体はウナギ形をしているが無顎上綱に属し、ヤツメウナギ類とともに、4億年以前に出現した学術上興味ある脊椎(せきつい)動物である。目が皮下に埋没し、レンズや虹彩(こうさい)がなく、視神経も退化的である。口は裂孔状で4対のひげがあり、外に開く鰓孔(さいこう)は1対または6対である。体側の下部に1～2列に並んだ多数の粘液孔から悪臭のする粘液を出す。舌の前端の左右に2列の舌歯がある。尾びれ以外のひれがない。体は大きなもので80センチメートルぐらいになる。

　夜行性であり、昼間は海底の泥の中に潜む習性がある。普通、ゆっくりと蛇行して泳ぐが、刺激を受けると体を「8」の字によじる。餌物(えもの)に付着すると、紐(ひも)を結んだような結節をつくる。腐肉に集まる習性があり、舌歯を餌物の体内に突き刺し、これを元の位置へ引っ込めるようにして肉や内臓をそぎ取る。魚類のほか、泥の中にすむ貝類、エビ・カニ類、多毛類なども食べる。漁網に入った動きの鈍い魚類、イカ類の皮膚に穴をあけたり、口や鰓孔から侵入して肉や内臓を食べ、また、このとき多くの粘液で網を損じるので漁業者に嫌われる。夏から秋にかけて産卵する。卵はどんぐり形を呈して大きく、1尾がはらむ卵の数は少なく、50個以下である。

　太平洋、大西洋の深海に13種余りが知られている。日本には5種がいて、外鰓孔が6～8個のグループと、1個のグループとに大別できる。

　和名ヌタウナギinshore hagfish／Eptatretus burgeriは6個の外鰓孔をもち、本州中部地方以南の日本各地、東シナ海、朝鮮半島南部などに分布する。10メートル以浅にも生息するので、イソメクラともいわれた。地方によってはアナゴともいう。体長60センチメートルぐらい。体は茶褐色。産卵期は8～10月。卵殻は長さ22ミリメートル、径9ミリメートル、多数の糸状突起が両極にある。1尾が抱く卵の数は20個前後。夏にやや美味である。このグループには本種のほかに深海性のクロヌタウナギEptatretus atamiとムラサキヌタウナギEptatretus okinoseanusがいるが、クロヌタウナギの外鰓孔は互いに接近して1列に並ばず、ムラサキヌタウナギの外鰓孔は1列に8個並ぶことで区別できる。

　外鰓孔が1個のグループには、ホソヌタウナギ（旧和名メクラウナギ）Myxine garmaniとオキナホソヌタウナギ（旧和名オキナメクラ）Myxine paucidensがいる。ホソヌタウナギの舌歯の前列と後列の歯はそれぞれ12本またはそれ以下である。体色は青紫色。オキナホソヌタウナギは、体色はホソヌタウナギに似るが、舌歯の前列と後列の歯がそれぞれ6本と7本である。ホソヌタウナギ、オキナホソヌタウナギとも全長は50センチメートルぐらい。いずれも深海底に生息する希種で、生態は明らかでない。

［落合　明・尼岡邦夫］

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