Do sea animals drink water, especially given the ocean’s saltiness? Unlike humans, for whom drinking seawater is dangerous due to its high salt content, marine life has developed diverse methods to manage their water and salt balance. These unique physiological strategies allow them to thrive in their aquatic environments, maintaining proper hydration and regulating internal salt levels despite living in a hypertonic solution.
How Fish Handle Saltwater
Ocean fish face a constant challenge due to osmosis. Because seawater has a higher salt concentration than their internal fluids, bony fish tend to continuously lose water through their gills and skin. To counteract dehydration, bony fish like cod and tuna actively drink large quantities of seawater.
After ingesting seawater, bony fish absorb water and salt from their digestive systems. To eliminate excess salt, they possess specialized chloride cells in their gills, which actively pump sodium and chloride ions back into the surrounding water. Their kidneys also produce small amounts of highly concentrated urine to excrete additional salt while conserving water.
Cartilaginous fish, including sharks and rays, employ a different strategy. Instead of actively drinking large amounts of seawater, they retain high concentrations of urea and trimethylamine oxide (TMAO) in their blood and tissues. This makes their internal fluid concentration slightly higher than that of seawater, causing water to passively enter their bodies through osmosis, primarily across their gills. Any excess salt that does enter their system is then excreted by a specialized organ called the rectal gland.
Marine Mammals and Reptiles: Their Unique Approach
Marine mammals like whales, dolphins, and seals generally do not drink seawater. Instead, they obtain most water from the food they consume. The metabolic breakdown of fats and proteins in their prey generates metabolic water, which contributes to their hydration.
These animals also have highly efficient kidneys that produce very concentrated urine. This allows them to excrete excess salts from their diet without losing much water, conserving their internal fluid balance. Their kidneys can excrete urine with a higher salt concentration than seawater itself.
Marine reptiles like sea turtles, sea snakes, and marine iguanas have specialized salt glands to excrete excess salt. Sea turtles, for example, have salt glands near their eyes that produce a salty solution, often making it appear as if they are crying. Marine iguanas have salt glands in their nostrils, enabling them to expel concentrated salt by “sneezing” or blowing out a salty mist. Sea snakes have similar glands under their tongues. These adaptations help them manage salt intake from consuming salty prey or plant matter, and from drinking seawater.
The Water World of Marine Invertebrates
Many marine invertebrates like jellyfish, sea anemones, sponges, and most mollusks are osmoconformers. Their internal body fluid concentrations are similar to the surrounding seawater. With little difference in salt concentration, they do not face the same osmotic challenges as vertebrates. Water and solutes move freely across their body surfaces, maintaining equilibrium with their environment.
Some invertebrates, such as certain crustaceans like crabs and lobsters, exhibit osmoregulation, particularly in estuaries or intertidal zones where salinity fluctuates. While their internal salt concentrations may not precisely match the external environment, their systems manage these variations through active transport of ions. Their excretory systems are simpler than vertebrates’, focusing on maintaining internal balance rather than actively filtering and excreting large volumes of ingested water.