Biology
Osmoregulation and Water Balance in Marine Versus Freshwater Teleosts
Quick fact
A marine teleost drinks up to 10% of its body weight in seawater daily to counteract water loss, while a freshwater teleost almost never drinks and instead pumps out up to a third of its body weight in dilute urine each day.
Why this is interesting
A fish in the ocean is constantly losing water to the sea, while a fish in a lake is constantly gaining it. How do they both survive?
Read the full explanation
Understanding Osmoregulation and Water Balance in Marine Versus Freshwater Teleosts
Teleosts, like salmon and cod, live in environments where the salt concentration outside their bodies is very different from inside. In the ocean, the water outside has more salt than the fish's blood, so water tends to leave the fish's body through osmosis—the movement of water from an area of lower salt to higher salt. To compensate, the marine fish drinks huge amounts of seawater. But drinking seawater adds a lot of salt, which it must get rid of. It does this mainly through special cells in its gills that pump excess salt out into the water. In freshwater, it's the opposite: the water outside has less salt, so water constantly enters the fish's body, threatening to over-dilute its blood. Freshwater fish rarely drink; instead, they produce very dilute, copious urine to flush out the excess water, and they actively take up salts from the water through their gills.
A deeper explanation
The key to these opposite strategies lies in active transport—using energy to move salts against their concentration gradient. In marine fish, chloride cells in the gills contain Na+/K+-ATPase, an enzyme that pumps sodium out while bringing potassium in, creating a gradient that drives chloride out as well. This process removes the excess salt ingested with seawater. The fish also produces small amounts of concentrated urine to conserve water. In freshwater fish, gill cells use similar pumps to absorb sodium and chloride from the water into the body. Their kidneys produce large volumes of very dilute urine to eliminate the excess water that enters by osmosis. These adaptations are reversed during migration between salt and fresh water, as seen in salmon, which adjust their gill and kidney function. Understanding this balance reveals how organisms use cellular pumps and membrane channels to maintain homeostasis, a fundamental principle of physiology.