Bullfrog - Bullfrog (English spelling)

A frog of the family Ranidae in the order Anura in the class Amphibians. It is a large species, also known as the edible frog because it is eaten. Its original distribution area is roughly the eastern half of the United States, but it has been artificially introduced to various parts of the world and has become naturalized. It was first introduced to Japan in 1918 (Taisho 7), and is now widely distributed from southern Hokkaido to Okinawa. In recent years, it has become less common to eat it, and there is an increasing demand for it as an animal for experiments.

The dorsal surface is green or dark brown with scattered irregular black spots, and the ventral surface is white with similar light brown spots. The eardrum is large, especially in males. It is 10 to 18 cm long and lives in relatively deep water such as ponds, swamps, large ditches, and slow-flowing rivers, but juveniles can also be seen in shallow water. They are often found in open areas in lowlands, but not in mountainous areas or forests. The spawning season is early summer, and males each establish a territory, floating on the water surface and making a sound similar to that of a cow. The eggs are small, about 2 mm in diameter, and more than 10,000 eggs spread on the water surface to form a large membranous egg mass. The larvae overwinter on the bottom of the water, and by the following summer they have grown to about 12 cm in length and metamorphosed. Both juveniles and adults rarely leave the water's edge, but they can be found on roads on nights of heavy rain.

[Mitsuru Kuramoto]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

両生綱無尾目アカガエル科のカエル。大形種で、食用にするため食用ガエルともよばれる。分布域は本来アメリカ合衆国のほぼ東半分であるが、世界各地に人為的に移入され野生化している。日本には1918年（大正7）に初めて移入され、現在では北海道南部から沖縄まで広く分布する。最近では食用とすることが少なくなり、実験用動物としての需要が増えている。

　背面は緑色または暗褐色で不規則な黒斑(こくはん)が散在し、腹面は白色地に同様の淡褐斑がある。鼓膜が大きく、とくに雄で著しい。体長10～18センチメートルで、池や沼、大きな溝、流れの緩い河川などの比較的深い水域に生息するが、幼体は浅い水域にもみられる。低地の開けた場所に多く、山間部や森林内ではみられない。産卵期は初夏で、雄は1匹ずつ縄張り（テリトリー）をつくり、水面に浮かんでウシに似た鳴き声を発する。卵は小形で径約2ミリメートル、1万個以上の卵が水面に広がって大きな膜状の卵塊となる。幼生は水底で越冬し、翌年の夏には全長12センチメートルほどに成長して変態する。幼体、成体とも水辺を離れることは少ないが、大雨の夜には道路上にも出没する。

［倉本　満］

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