The blood plasma, interstitial fluid, and cytoplasm of cells in southern bluefin tuna are hyposmotic to the surrounding ocean water. This means that the ion concentration within these fluids is low relative to the seawater. The standard osmotic pressure of seawater is 1.0 osmole/L, while the osmotic pressure in the blood plasma of the southern bluefin tuna is approximately half of that. Without the mechanism of osmoregulation present, the tuna would lose water to the surrounding environment and ions would diffuse from the seawater into the fluids of the tuna to establish equilibrium.