Because of this, aquatic animals must develop physiological mechanisms to prevent excess flow of water into their bodies. They must also develop mechanisms to prevent the loss of solutes as excess water is excreted. The process by which organisms actively maintain their internal solute concentration is called osmoregulation. Animals must actively transport solutes from their surroundings into their blood against the concentration gradient. The work required for osmoregulation depends on properties of the organism and its environment.
Some animals seem to spend more energy than others to maintain an internal balance.
We can quantify osmotic work in a simple equation in order to understand tradeoffs organisms must make in order to maintain their internal solute concentration. Assuming the animal is only permeable to water, the energy required for osmoregulation can be described by an equation. In this case, we are taking the natural log of a number less than 1, so the osmotic work is negative. By looking at the equation for osmotic work, we can determine what properties of an organism could reduce the amount of work required for osmoregulation. This means an animal like the crayfish, that reabsorbs solutes from urine, has a considerable energy savings.