Aquatic animals have several mechanisms for dealing with the effects of osmosis. Without these methods, the cells within these creatures would either shrivel up and die, or swell up and explode. Osmosis is the tendency for a liquid (solvent) with a low concentration of particles (solutes) dissolved in it to move to through a semi-permeable membrane to an area of high solute concentration. The semi-permeable membrane allows the solvent to pass though, while blocking the solutes. The area on the side of the membrane with a higher concentration of solutes is called the hyperosmotic side, and the side with lower solute concentration is called the hypoosmotic side. Osmosis is necessary for the function of the kidneys, which rely on its properties to remove toxins from the blood, and for the re-absorption of water.
Osmosis presents a problem to aquatic life, because through it the concentration of water (H2O, the solvent) or salt (NaCl, the solute) can be affected in dangerous ways. In salt water, the cells of an aquatic animal are hypoosmotic in relation to the surrounding water, so through osmosis water has the tendency to pass through the cell membrane and leave the cell. This causes problems, because water is essential for the function of a cell. Likewise, in a freshwater environment, the interior of cells are hyperosmotic in relation to their surrounding water. Through osmosis, water tends to leave the area of lower solute concentration, and enter the cells. This can also be a problem, because too much water can cause a cell to explode (cytolysis).
In aquatic animals, there are several different methods employed to maintain osmotic homeostasis. Most marine invertebrates are osmoconformers, meaning that they maintain the concentration of solutes in their own body to conform with that of their environment. The downside of this is that it is dependent on a stable water composition.
The next group are the osmoregulators, who regulate the concentration of solutes and solvents in their cells. There are two different kinds of osmoregulators: marine osmoregulators and freshwater osmoregulators. Marine osmoregulators have to deal with high solute concentration in their environment, and a constant tendency to lose water. To cope they drink large amounts of water and use active transport (that which is not caused by a natural tendency, but by expending energy) to remove chloride ions (and subsequently sodium ions) from their body. Freshwater osmoregulators have the opposite problem, which is too much water. They drink very little water, and excrete lots of water in their urine, while obtaining new salts and minerals from food.
Salmon are an interesting case, because they spend most of their life in salt water and return to freshwater later on. When in freshwater, they function as regular freshwater osmoregulators, but they need to deal with extra salt in the ocean. In order to cope, they secrete the steroid hormone cortisol, which stimulates chloride cells to grow. These chloride cells secrete salt out of the body, and maintain osmotic homeostasis.
In order to live in different environments, different animals have different methods for keeping the concentration of water and salts in their bodies at an optimal level. These methods used include osmoconformity, and marine and freshwater osmoregulation. Osmosis can be either a problem that needs to be solved, or a necessary property that functions as a tool, especially in the kidneys. Whatever the case, creation has many ways of correcting for it, and even harnessing it.