The root effect haemoglobin, which allows fish to use oxygen in the blood to inflate the swim bladder, appeared to have evolved only once in the evolutionary history of fishes. Possessing this protein allowed the formation of blood vessels which supported the retinas of fishes and allowed them to see better.
According to Berenbrink et al, this network of veins and arteries, called the rete mirabile, also appeared to have evolved only once about 250 million years ago. Swimbladders on the other hand, evolved independently in 4 fish groups, this could account for the diversity of fishes we see today.
"Fish Diversity Tied to Evolution of Diving Ability." By Sarah Graham. Scientific American Science News, 21 Mar 2005, reporting on Berenbrink, M. P. Koldkjær, O. Kepp & A. R. Cossins, 2005. Evolution of Oxygen Secretion in Fishes and the Emergence of a Complex Physiological System. Science, 18 March 2005: 1752-1757.