Many sea creatures drink water, but their methods vary significantly. Marine life has evolved diverse strategies to manage water balance in the salty ocean. The ocean’s salt content presents a constant challenge to maintaining internal hydration, leading different groups of animals to employ distinct physiological mechanisms.
Bony Fish: The Ocean Drinkers
Marine bony fish, also known as teleosts, constantly face dehydration in seawater. Their internal body fluids are less salty than the surrounding ocean, making them hypotonic. This osmotic gradient causes water to continuously move out of their bodies, primarily across their gills. To counteract this water loss, these fish actively drink large quantities of seawater.
They are estimated to drink about 1% of their body weight each hour to compensate for water lost through osmosis. After ingesting seawater, they absorb both water and salt through their intestines.
To eliminate excess salt, they possess specialized chloride cells in their gills. These cells actively pump out sodium and chloride ions, effectively desalinating the consumed water. Their kidneys also contribute to osmoregulation by producing small volumes of concentrated urine, primarily to excrete divalent ions like magnesium and sulfate, while conserving water.
Cartilaginous Fish: Unique Internal Balance
Cartilaginous fish, such as sharks, rays, and skates, employ a different osmoregulation strategy. Unlike bony fish, they are largely isotonic or slightly hypertonic to seawater, meaning their internal osmotic concentration is similar to or slightly higher than the surrounding ocean.
They achieve this balance by retaining high concentrations of urea and trimethylamine N-oxide (TMAO) in their blood and tissues. Urea is a waste product, but acts as an osmolyte, increasing their internal osmotic pressure. TMAO helps counteract the harmful effects of high urea concentrations on cellular proteins.
This internal osmotic balance significantly reduces the osmotic gradient, minimizing water loss. As a result, cartilaginous fish do not need to actively drink seawater for hydration.
They also possess a specialized rectal gland. This gland actively secretes a highly concentrated salt solution, primarily sodium chloride, into the gut for excretion, removing excess salt.
Marine Mammals and Other Sea Animals: Water from Food and Efficiency
Marine mammals, including whales, dolphins, seals, and sea lions, generally do not drink seawater for hydration. Their primary water source comes from the food they consume. Prey like fish and squid contain significant water, providing much of the necessary hydration.
Additionally, these animals produce metabolic water as a byproduct of breaking down fats and proteins. Marine mammals have highly efficient kidneys capable of producing extremely concentrated urine, allowing them to excrete excess salt ingested with their food while conserving water.
Similarly, marine birds like albatrosses and penguins, and marine reptiles such as sea turtles and sea snakes, also rely on water from their food and metabolic processes. These animals have specialized salt glands, which are highly effective at excreting concentrated salt solutions. This extrarenal salt excretion mechanism allows them to process the salt content of their diet without dehydrating.