The left-mouthed fish is a fish in the family Flounder, known as "flounder" and also called "left-hand butterfly fish".

The left side of the fish often lies on the sandy bottom or lurks in the sand with only its head exposed, using the right side of its body. The color of its left side can change with the color of its habitat.

Originally, the juvenile fish has eyes on both sides of the body. Later, because the right side of the body is lying on the sand, the right eye gradually faces the sun, and then turns to the left side.

The body of fresh left-mouthed fish is dark brown, and the pattern color changes with the environment. Left-mouthed fish will die when it leaves the water and cannot be fed.

They live on the sandy bottom of shallow seas and prey on small fish and shrimps. They are particularly adapted to a benthic life on the seabed. Because of their flat bodies. The eyes are on the upward side of the body, and the color of this side matches the surrounding environment well; the downward side of their body is white. Flounder has extremely fine scales on its body surface. Flounder has only one dorsal fin, which extends from the head almost to the tail fin. They mainly live in temperate waters and are important economic fish in temperate waters.