Fishes have a two-chambered heart because they do not have lungs - instead, they have gills. The way their respiration works is by counter-current exchange. The water flows one way over the gills, and because of the partial pressure of oxygen is lower in the blood, as the water passes over the gills, molecules of oxygen are basically "pulled" into the blood, oxygenating the blood. This blood then flows in a linear circuit and eventually returns to the heart, which then re-pumps the blood to the gills to be re-oxygenated. Because of this unique principle of counter-current exchange, the two-chambered heart is the optimal heart chamber configuration for a fish.
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