Itoyo (English spelling) three spined stickleback

A general term for fish belonging to the order Sticklebacks of the class Osteichthyes and the family Sticklebackidae that have five or fewer spines on their dorsal fin. There are three recognized groups of Japanese three-spined sticklebacks: the Japan Sea group, the Pacific anadromous group, and the Pacific landlocked group, with the addition of the stickleback, for a total of four groups. The scientific names of all fish except for the Pacific anadromous group , Gasterosteus aculeatus aculeatus, have not been confirmed. This fish is famous for collecting aquatic plants to build nests, and is named after the male who secretes a mucus-like thread to solidify the nest. It is widely distributed from the subarctic to temperate zones of the Northern Hemisphere. In Japan, the anadromous type (those that grow and then descend to the sea) is found in various parts of the Sea of ​​Japan north of Shimonoseki and in the Pacific Ocean north of the Tone River, while the landlocked type (those that remain in freshwater even as they grow) lives in spring-fed ponds and small streams in Lake Onuma and Lake Akan in Hokkaido, Aomori Prefecture, the Aizu Basin in Fukushima Prefecture, the Ono Basin in Fukui Prefecture, and Nasu in Tochigi Prefecture. It has three strong spines on its dorsal fin and a pair of strong spines on its pelvic fin, and a continuous line of scale plates from the back of the gill cover to the base of the tail fin. The anadromous type lives in the sea except during the spawning season. It has a long and slender body, and the dorsal fin spines do not have fin membranes, or if they do, they are only found near the base. It grows to about 10 centimeters in length. The landlocked type lives in freshwater all its life. The body is tall, with the dorsal fin membrane reaching the tip of the spine. The body length is only about 7 cm. Both types reach maturity in one year.

The male digs a depression in the bottom of the water, gathers aquatic plants and fallen leaves, and builds a nest by solidifying them with mucus secreted from his kidneys. When he finds a female, he leads her into the nest with a courtship behavior called the zigzag dance, where she lays her eggs. The male immediately enters the nest and releases sperm. He attacks other males who enter, triggered by the red nuptial coloration visible from the throat to the abdomen. He reacts strongly to anything that is red. The male uses his pectoral fins to pump fresh water to the eggs, and protects them from predators until the larvae leave the nest. The female dies after spawning, and the male dies after finishing protecting the larvae. In the anadromous species, the juvenile fish travel down rivers to live in the ocean from July to September, and begin migrating upstream from around February of the following year.

The landlocked species lives in streams and rice paddy ditches where the water temperature is around 15℃, and is designated as a natural monument in various places. Due to the deterioration of the habitat, some areas have designated it as a local population at risk of extinction. On the other hand, in some areas, they are caught during their upstream migration and are fried or fried in tempura.

[Akira Ochiai and Kunio Amano]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

硬骨魚綱トゲウオ目トゲウオ科に属する魚のうち、背びれに5本以下の棘(とげ)をもつ種の総称。日本のイトヨには日本海系、太平洋系降海型および太平洋系陸封型の3系群があり、これにハリヨを加えた4系群が認められている。太平洋系降海型Gasterosteus aculeatus aculeatus以外は学名が確定していない。水草を集めて巣をつくることで有名な魚で、雄が糸状の粘液を出して巣を固めるのでこの名がある。北半球の亜寒帯から温帯にかけて広く分布し、日本では、降海型（成長ののち海に下るもの）は下関(しものせき)以北の日本海各地と利根川以北の太平洋の日本各地に、陸封型（成長しても淡水にとどまるもの）は北海道の大沼と阿寒(あかん)湖、青森県、福島県会津盆地、福井県大野盆地、栃木県那須(なす)などの湧水池や細流にすむ。背びれに3本、腹びれに1対の強い棘があり、鰓蓋(えらぶた)の後から尾びれ基底まで鱗(りん)板が連続して並ぶ。降海型は産卵期以外は海で生活する。体が細長く、背びれ棘(きょく)に鰭膜(きまく)がないか、あっても基部近くに限られる。体長は10センチメートルほどになる。陸封型は一生淡水で生活する。体は高く、背びれのひれ膜は棘の先端まで達する。体長は7センチメートルほどにしかならない。両型ともに1年で成熟する。

　雄は水底にくぼみを掘り、水草や落ち葉を集め、腎臓(じんぞう)から出る粘液でそれらを固めて巣を作る。雌を見つけると、ジグザグダンスと言われる求愛行動で雌を巣に導き入れ、産卵させる。雄は直ちに巣に入り放精する。ほかの雄の進入に対して攻撃を加えるが、その引き金となるのは、のどから腹部にかけてみられる赤色の婚姻色である。赤色であれば、どんなものに対しても強く反応する。雄は胸びれを使って新鮮な水を卵に送り、仔魚(しぎょ)が巣を離れるまで外敵から守る。雌は産卵後、雄は仔魚の保護を終えた後死ぬ。降海型では稚魚は7～9月ごろに川を下りて海洋生活し、翌年2月ごろから遡上(そじょう)を開始する。

　陸封型は水温が15℃前後の小川や水田の溝などにすみ、各地で天然記念物に指定されている。生育環境の悪化で、絶滅のおそれのある地域個体群に指定されている地方もある。その一方で、遡上期のものを捕らえて、から揚げやてんぷらにする地方もある。

［落合　明・尼岡邦夫］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例