The oxygen need and oxygen uptake of the southern bluefin tuna are directly related. As the tuna increases its metabolic need by swimming faster, water flows into the mouth and over the gills more quickly, increasing the oxygen uptake. Additionally, since there is no energy required to pump the water over the gills, the tunas have adapted an increased energy output to swimming muscles. The oxygen and nutrient uptake in the circulatory system is transported to these swimming muscles rather than to tissues required to pump water over the gills in other teleost fish.