Young Japanese eels enter rivers in small shoals from February to May, and ascend to the upper reaches of rivers and mountain lakes. After several years in freshwater and with the onset of sexual maturity, the eels migrate downstream and enter the sea from August to October. However, some Japanese eels have never entered freshwater (sea eels); in fact three different categories of eels are found, each with a different migratory history: river eels, estuarine eels and sea eels. Anguilla japonica has a flexible pattern of migration, with an ability to adapt to various habitats and salinities. Anguillid eel migrations into freshwater are clearly not an obligatory migratory pathway, and this form of diadromy should be defined as facultative catadromy, with the sea eel as one of several ecophenotypes.
Adults feed on benthic crustaceans, bony fish and insects. Japanese eels have very similar migratory and recruitment styles to European eels. The spawning of A. japonica occurs near some of the seamounts in the West Mariana Ridge. The Japanese eel is found widely around Japan and China. The northern limits are the island of Hokkaido, the coast of the Bohai Sea and the Liao Ho River. The southern limits are the island of Hainan and the Gulf of Tonkin. The distribution extends towards the Benin Islands.