The question of whether sea creatures drink water has a complex answer that depends entirely on the type of animal. While humans and land animals drink water to survive, being constantly surrounded by it does not automatically mean a marine organism is hydrated. The fundamental challenge for life in the ocean is managing the salt concentration difference between the body and the environment. This need to balance fluid levels has led to diverse and highly specialized physiological strategies across different groups of marine life.
The Osmotic Challenge of Saltwater
Most marine vertebrates have internal body fluids that are significantly less salty than the surrounding seawater. This difference creates a physiological problem governed by osmosis, the movement of water across a semipermeable membrane. Water naturally moves from low solute concentration to high solute concentration to equalize the balance.
Since the internal fluids of most marine animals are less salty than the ocean, water constantly attempts to move out of their bodies. This continuous fluid loss occurs primarily across the gills, which are constantly exposed to seawater for gas exchange. This osmotic gradient forces the animal to develop mechanisms to replace lost fluids and manage the resulting high salt load.
Bony Fish: The Active Drinkers
The vast majority of marine fish, known as teleosts or bony fish, are active drinkers of seawater to combat constant osmotic water loss. Because their internal fluids are hypoosmotic (less salty) than the ocean, they must ingest large quantities of water to replenish the fluid leaving their bodies through the gills. Some bony fish drink an amount equal to about one percent of their body weight every hour.
Once seawater is swallowed, water is absorbed in the gut, leaving the salt behind in the bloodstream. Fish actively excrete this salt using specialized chloride cells located on their gill filaments. These cells use energy-intensive pumps to move chloride and sodium ions directly from the bloodstream out into the seawater. This process allows them to gain water while efficiently ridding their bodies of excess salt, resulting in the production of very little, highly concentrated urine.
Cartilaginous Fish: The Urea Strategy
Cartilaginous fish (elasmobranchs), including sharks, rays, and skates, employ a different osmoregulation strategy. They eliminate the osmotic gradient by retaining a high concentration of organic compounds in their blood. These compounds are primarily urea, a waste product, and trimethylamine oxide (TMAO), which protects their proteins from urea’s harmful effects.
By retaining urea, these fish raise the total solute concentration of their internal fluids to a level that is slightly hyperosmotic (saltier) than the surrounding seawater. This adaptation means that water slightly diffuses into their bodies, eliminating the need to drink seawater for hydration. Their kidneys are specially adapted to reabsorb nearly all the filtered urea back into the bloodstream to maintain this osmotic balance.
Marine Mammals and Reptiles: Specialized Hydration
Air-breathing vertebrates, such as marine mammals and reptiles, have evolved specialized strategies for hydration that do not rely on drinking large amounts of seawater. Marine mammals, like whales and seals, primarily obtain water through their diet, either from the high water content of their prey or from metabolic water. Metabolic water is produced internally when fats and proteins from food are broken down for energy.
These mammals possess highly efficient kidneys that can concentrate urine to a salinity much higher than that of seawater, allowing them to excrete excess salt without losing fresh water. Marine reptiles and birds utilize specialized salt-excreting glands to manage salt intake. These glands, located near the eyes (turtles) or the nostrils (birds), produce a concentrated salt solution that is actively pumped out of the body, allowing them to tolerate incidental ingestion of seawater or salty prey.