The ocean offers an endless supply of water that is unusable for maintaining internal hydration because seawater is a highly concentrated salt solution. This environment constantly threatens to pull water out of living tissues. The process of osmoregulation—managing the delicate balance of water and salt concentrations within the body—is essential for survival in this hyperosmotic medium. To thrive in the sea, every marine animal must employ a unique physiological strategy to counter these forces.
The Core Problem: Water and Salt Balance
Marine life faces a fundamental physiological challenge because the ocean’s salt concentration is significantly higher than that within their body fluids. This difference sets up an osmotic gradient across permeable membranes, most notably the gills and the skin. Osmosis dictates that water naturally moves from an area of low solute concentration to an area of high solute concentration to achieve equilibrium.
For most marine vertebrates, water is constantly drawn out of the body into the surrounding seawater. This steady water loss leads to dehydration, which the animal must actively counteract to survive. Salts from the seawater also tend to diffuse inward, creating a dual problem of chronic water loss and salt overload. Correcting this imbalance requires a continuous expenditure of metabolic energy.
Marine Bony Fish: The Constant Drinkers
The vast majority of marine fish, known as teleosts, have an internal salt concentration that is only about one-third the concentration of seawater, making them highly susceptible to water loss. To replace the water constantly lost through their gills, these bony fish must continually drink seawater. This is the primary way they prevent dehydration.
Drinking seawater introduces a massive load of excess salt into their digestive system. The fish’s intestines absorb the water, but they also absorb monovalent ions, such as sodium and chloride. To manage this salt influx, bony fish utilize specialized cells called ionocytes, also known as chloride cells, located within their gills.
These chloride cells actively pump excess sodium and chloride ions out of the bloodstream and back into the surrounding seawater. This active transport mechanism is energetically demanding but allows the fish to rid their bodies of ingested salts. The kidneys also contribute by producing a small volume of urine rich in divalent ions like magnesium and sulfate, which are difficult to excrete through the gills.
Sharks, Rays, and Invertebrates: The Iso-Osmotic Approach
Cartilaginous fish, including sharks, skates, and rays, employ a strategy that avoids the problem of osmotic water loss by making their internal fluids nearly as concentrated as seawater. Instead of using only mineral salts, they retain high concentrations of nitrogenous waste products, primarily urea, in their blood. By keeping urea at levels up to 100 times higher than in other vertebrates, the internal osmotic pressure of the shark’s body fluids is maintained at a level equal to or slightly higher than the surrounding ocean.
This high internal concentration minimizes the osmotic gradient, meaning water does not rush out of their bodies, and they do not need to drink seawater. The downside is that urea can interfere with protein function, a problem countered by the presence of a protective compound called trimethylamine oxide (TMAO). TMAO stabilizes the proteins, allowing them to function normally despite the high urea levels.
Although they avoid massive water loss, these animals still face a slow, steady inward diffusion of salt across the gills and receive additional salt from their diet. To manage this small excess of salt, sharks and rays possess a unique organ called the rectal gland. This specialized salt-secreting structure actively removes sodium and chloride ions from the blood and excretes them as a highly concentrated fluid into the rectum for elimination. Many marine invertebrates, such as jellyfish and sea stars, act as osmoconformers whose internal salt concentrations simply match the seawater, completely removing the osmotic challenge.
Marine Mammals and Reptiles: Highly Specialized Solutions
Air-breathing marine animals evolved from terrestrial ancestors and developed specialized systems for the marine environment. Marine mammals like whales, dolphins, and seals generally do not drink seawater. Instead, they derive most of their fresh water from the metabolic breakdown of fats and proteins in the food they consume.
The oxidation of food releases water as a byproduct, known as metabolic water, which is sufficient for hydration. Marine mammals also have incredibly efficient kidneys containing long loops of Henle. This allows them to produce urine that is significantly more concentrated than seawater. This ability effectively manages the salt load from their diet with minimal water loss.
Marine reptiles, such as sea turtles and sea snakes, and seabirds like albatrosses and gulls, employ a different extrarenal solution. Since their kidneys cannot produce urine concentrated enough to eliminate the full salt load, they utilize specialized salt glands. These glands are located in the head, often near the eyes or nostrils, and secrete a highly concentrated salt solution that can be four to five times saltier than their blood. This mechanism allows these animals to drink seawater or eat salty prey and then excrete the excess salt with minimal loss of body water.