Fish have blood that is carried through their bodies by their circulatory systems. Fish typically have less blood per gram of their body weight than other animals.
Fish have erythrocyte- and leukocyte-rich blood. Erythrocytes carry oxygen from the gills and distribute it throughout the body from the bloodstream while disposing of carbon dioxide through the opposite pathway. These cells rely on hemoglobin to work effectively. Leukocytes contain a variety of cells that contribute to a fish's bodily defense systems, fighting bacteria or parasites and clotting blood when necessary.
Some fish, such as the icefish, have blood that does not contain hemoglobin. They survive in oxygen-rich water by dissolving gases in their blood plasma. The lack of hemoglobin allows them to conceal themselves from predators.