Living cells are subject to osmosis, a physicochemical process that causes them to lose or gain water with varying volume.
Throughout the evolutionary process, animals have developed various mechanisms to regulate the osmotic process to which they are subjected. These mechanisms constitute what is called osmoregulation.
Many species of sea animals do not suffer osmosisbecause the tonicity of your cells and body fluids is equivalent to that of salt water. Such animals are called osmoconformes and they need not regulate the concentration of their internal environment. There are animals, however, whose internal tonicity is very different from the tonicity of the place in which they live. Thus, they need to actively control the amount of water entering and leaving the body due to osmosis. They are therefore called osmoregulators.
Osmoregulation in the aquatic environment
Sharks and others cartilaginous fish (rays, dogfish, chimeras, etc.) are able to keep the tonicity of their blood close to that of seawater.
This is achieved by the synthesis and accumulation in the blood of a substance called urea, which constitutes an osmotically important solute. Urea is continuously eliminated by the kidneys so that the animal can control the amount of this solute in the blood. Sharks also have a gland located in the rectum that continually draws excess salts from the bloodby eliminating them by the anus.
Marine bony fish evolved from freshwater ancestors. As a heritage of this origin, the tonicity of its internal liquids is much lower than the tonicity of seawater. Therefore they are continually losing water to the environment due to osmosis.
To compensate for this loss, Marine bony fish drink salt water and are able to eliminate excess salt intake through the surface of the gills.
Seabirds such as gulls and albatrosses have nasal glands that specialize in eliminating excess salt from the body. Sea turtles also have similar glands that open near the eyes.
Marine mammals like dolphins and whalesAlthough they do not drink salt water, they always ingest some seawater with their food. Osmotic balance of these animals is achieved by eliminating salts by the kidneys in the urine.