The Ariake-kai Sea is a body of salt water surrounded by Fukuoka, Saga, Nagasaki, and Kumamoto Prefectures, all of which lie on the island of Kyushu in Japan. It is the largest bay in Kyushu. Its deepest point is only about 50 m deep, and extreme tides exceed 4 m. It is used for aquaculture, with nori being a major product. Various species of fauna including Mutsugoro (mudskipper), Pen shell (Atrina pectinata), and fiddler crab live in the Ariake-kai Sea. In autumn, there will be red-grass along the sea shore. Recent years have brought increasing pollution, with resultant red tide. Isahaya-wan Bay is a branch of the Ariake-kai Sea. Across the Amakusa Islands lies the Yatsushiro-kai Sea. Five ferry routes cross the Ariake Sea.
Mutsugoro (mudskippers) are members of the subfamily Oxudercinae (tribe Periophthalmini), within the family Gobiidae (Gobies). They are completely amphibious fish, fish that can use their pectoral fins to walk on land. Being amphibious, they are uniquely adapted to intertidal habitats, unlike most fish in such habitats which survive the retreat of the tide by hiding under wet seaweed or in tidal pools. Mudskippers are quite active when out of water, feeding and interacting with one another, for example to defend their territories. They are found in tropical, subtropical and temperate regions, including the Indo-Pacific and the Atlantic coast of Africa.
Mutsugoro in Ariake-kai Sea were survivors when Kyushu Island used to be a part of continent for 10, 000 years ago.