Siamese fighting fish

Betta splendens, also known as the Siamese Fighting Fish is a member of the Anabantidae family of fish, and it possesses the characteristic labyrinth organ. This is located just behind the head and it is a small chamber with many folds covered in tissue with many blood vessels that allow it to breath atmospheric air.

This is useful in the oxygen poor waters where it lives, and it is so dependant upon it that a Betta splendens will die if it cannot reach the water surface to breathe. They build small bubble nests for their fry. The males are extremely aggressive and in Asian countries they are are used in a sport similar to cockfighting.