Those that live in the vicinity of the mouth of a river that opens to the sea are able to constantly adapt to freshwater, brackish water and the seawater, and therefore they spend their lives freely going back and forth between different water environments.
海に注ぐ河口付近に棲息するものは、淡水・汽水・海水に常時適応できるため、自由に行き来して生活する。