From clownfish to catfish, grouper to great white, the diversity of fish in the sea is nothing short of astonishing. Now scientists have managed to account for this wide assortment, at least in part, by tracing the evolution of the organ that allows the creatures to swim at different depths.

To change their buoyancy and move up and down in the water, fish inflate an internal organ called the swim bladder. Some fish, such as herring, must surface and gulp air in order to fill their swim bladders with oxygen. Other fish, which are able to submerge for much longer periods and thus reach greater depths, are able to use oxygen from their blood in order to inflate the swim bladder, thanks to a specific type of protein known as Root-effect hemoglobin.

Michael Berenbrink of the University of Liverpool and his colleagues traced the evolution of this protein in a variety of species, from sharks to dolphinfish, and found that it evolved just once. The emergence of the protein then allowed for the formation of a complex network of veins and arteries, called the rete mirabile, which supported the creatures' retinas and allowed the fish to see better. The rete mirabile also appears to have evolved only once, about 250 million years ago. The capillaries that support the swim bladder and allow oxygen to be delivered to it appeared about 100 million years later.

The swim bladder itself, however, arose independently in four different fish groups, Berenbrink and his collaborators report in the current issue of the journal Science. The team proposes that the evolution of the swim bladder accounts for part of the huge diversity of form and function in living fishes. For example, there are 198 species of Mormyroidea fish, all of which have swim bladders, yet there are only eight species of their close relatives, the Notopteridae, which lack the complex organ.