A trout in a river and a tuna in the ocean are built the same way. Both keep their blood at roughly a third of the salt in seawater, give or take a little.
That single fact creates two opposite problems, because water follows salt wherever it goes.
In the river, the fish is the salty thing, so water walks into it all day. In the ocean, the sea is the salty thing, so water walks out of it all day.
The earlier version of this post got the freshwater half right. Its marine half repeated an error that is in a lot of textbooks. And it never noticed the best part: the two enemies are mirror images, and one kind of cell solves both, in opposite directions.
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| OpenStax. Osmosis across a membrane: water moves toward the saltier side. Every problem in this article follows from that one rule. CC BY 4.0, via Wikimedia Commons. |
One Problem, Two Directions
Osmosis is water moving through a membrane toward whichever side holds more dissolved stuff. The membrane does not care which side is the fish.
Fish blood measures about 260 to 380 milliosmoles per kilogram. Seawater sits near 1,000. Freshwater fish blood, measured in carp, is at the low end, around 260.
So a freshwater fish is salty compared with its pond, and a marine fish is dilute compared with its ocean. The water moves the same way in both cases: toward the salt.
The stakes are the same too. Cells swell and burst, or shrink and stop working. Every fish in this article is holding a line against a rule it cannot change.
| The daily problem | Freshwater fish | Marine fish |
| Water moves | In, through the skin and gills | Out, through the skin and gills |
| Salt moves | Out, by diffusion | In, with swallowed seawater |
| Urine | Large volume, very dilute | Small volume, about the saltiness of blood |
| Drinking | Rarely, if ever | Constantly |
| Gill cell job | Pull ions out of the water | Pump ions out of the body |
| Kidney job | Keep the ions, dump the water | Keep the water, dump the magnesium and sulfate |
The same job, run in reverse. Both columns exist to stop one number: the saltiness of the blood.
Freshwater Fish: Water Arrives All Day
A freshwater fish is drowning slowly without going anywhere. Water presses in through the skin and across the gill surfaces, and ions leak out the same way.
The answer on the water side is the kidney. It filters blood fast, reabsorbs little of what it filters, and sends the surplus out as dilute urine. A fish can pass up to a third of its body weight in urine in a single day.
The answer on the salt side is the gill. Specialized cells pull sodium and chloride back in from the water, against the gradient, using food and pumps to pay for it.
Because water keeps arriving, there is nothing to gain from drinking it. Freshwater fish drink rarely or not at all.
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| Raver, Duane; modified by Biezl translation improved by User:smartse. What a freshwater fish manages each day: water flows in through the body surface and the gills, while ions leak away. The kidney dumps the water and the gills claw the ions back. Public domain, via Wikimedia Commons. |
Marine Fish: Water Leaves, Salt Arrives
Turn the picture around and every arrow flips. The ocean pulls water out through the skin and gills. Salt arrives with every swallow of food and sea.
Marine fish drink seawater, and this was proven in the 1930s with a dye. Homer Smith fed a marker to sculpin and eels. He found it in the gut but not in the surrounding water, so it had gone in through the mouth.
The intestine then does something clever. It absorbs the water and the sodium chloride, and passes the rest down. Smith calculated that about 90 percent of the swallowed fluid leaves the fish without ever reaching the blood.
The salt that does reach the blood leaves through the gills. That is the main exit for sodium chloride in a marine fish, and it is handled by the same kind of cell that loads salt in a freshwater one. The next section is about that cell.
What the marine kidney actually does
Here is where the earlier version went wrong. It said marine fish produce "small amounts of concentrated urine." The volume part is right. The concentration part is not.
Measured against the blood, marine fish urine is not concentrated at all. Smith found it invariably about equal to the blood, or even more dilute. Modern reviews describe it the same way: almost the same saltiness as plasma, carrying magnesium and sulfate above everything else.
One fisheries explainer still calls that urine highly salty, and in a narrow sense it is: it is far saltier than the watery urine of a freshwater fish. But it never beats the blood itself. A fish cannot drink the ocean and then concentrate it, because the kidney has no machine for that.
What the marine kidney does instead is save water and eject the wrong ions. It filters slowly and reabsorbs most of what it filters. Its effort goes to magnesium and sulfate, the two salts that arrive with food and are hardest to get rid of.
Some marine fish go further and switch the filter off. Goosefish, toadfish, pipefish and seahorses have no filtering units in the kidney at all. Their urine is made entirely by secretion, which tells you where the organ's priorities ended up.
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| Kare Kare modified by Biezl translation improved by smartse. The mirror image in seawater: water leaves the body through the surfaces, and salt moves in. The fish drinks to replace the water and pumps the salt out through its gills. CC BY-SA 3.0, via Wikimedia Commons. |
The Same Cell, Reversed
The cells doing this work are called ionocytes. Until recently they were called chloride cells, and older papers still use that name. They are packed with mitochondria, because moving salt against a gradient costs energy.
They sit mainly in the gills, and also in the kidney and gut. One review puts it plainly: these cells perform ion uptake or ion excretion, depending on what the animal needs.
In seawater the recipe is now known in detail. Two pumps sit inside the cell, sodium-potassium ATPase and a cotransporter that loads sodium, potassium and chloride together. A chloride channel sits on the outer face, and chloride pours out through it.
That design was first worked out in a shark's rectal gland, then found in the gills of marine fish. Vertebrates reuse it wherever they need to throw salt out, from bird salt glands to frog skin.
In fresh water the same cell runs the other way. It grabs sodium from the water, sometimes with a proton pump helping on the outer membrane when the water is very dilute, and hands it inward to the blood.
This is also why the gill is more than a breathing organ. Its filaments exchange ions, water, acids and ammonia as well as gases. The fish uses one surface to breathe, to feed itself salt or strip salt out, and to dump nitrogen waste.
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| Користувач:Shao \ Anaxibia. Structure of a fish gill. The filaments carry the ionocytes: the same cells that load salt in fresh water unload it in seawater. CC BY 3.0, via Wikimedia Commons. |
Euryhaline fish change direction
Most fish never switch. They are stenohaline: built for one range of saltiness and unable to tolerate another. Only about 3 to 5 percent of bony fish species are euryhaline, able to work in both kinds of water.
Salmon are the famous case. They hatch in fresh water, grow there, and go to sea as smolts. Eels run the same trip in reverse, born in the ocean and coming inland to grow.
Switching is not a matter of holding a setting. The whole system is rebuilt. The kidney changes shape: one segment exists only in freshwater nephrons, and another is sometimes missing in marine ones. Filtration is turned up in fresh water and down in the sea. Hormones such as prolactin, cortisol and growth hormone drive the change.
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| William Greene. A sea trout from Galway Bay. Brown trout that go to sea are euryhaline: they run the freshwater program and the seawater program at different times of life. CC BY 3.0, via Wikimedia Commons. |
The switch is still being worked out in the lab. A 2025 study of the sea lamprey found something odd. This anadromous fish runs the same migration as salmon, yet its gills secrete chloride through a channel that no other marine fish uses. The textbook recipe is not the only recipe.
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| Fernando Losada Rodríguez. A sea lamprey in an aquarium. Lampreys are anadromous like salmon, and a 2025 study found they secrete salt through a chloride channel no other marine fish uses. CC BY-SA 4.0, via Wikimedia Commons. |
Sharks Pick a Third Way
Sharks, skates and rays answered the same question differently. Instead of keeping their blood dilute to the sea, they match it.
Their blood is slightly saltier than seawater, mostly because the kidney reabsorbs urea instead of throwing it away. With saltier blood than the water around it, a shark gains water by osmosis and loses none.
Urea is hard on proteins. Sharks also hold trimethylamine oxide, which keeps their own molecules folded while bathed in it.
The salt that comes with food still has to go, so sharks carry a dedicated gland for it. The rectal gland was solved in 1960, and it was that organ's sodium chloride pump that gave researchers the cellular design described in the previous section.
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| Knepp Timothy, U.S. Fish and Wildlife Service. A shark. Sharks take the third route: they keep their blood salty as seawater by holding urea, so water flows in rather than out. Public domain, via Wikimedia Commons. |
Three answers, then. Freshwater fish dilute the problem away and keep every ion. Marine fish swallow the problem and pump it back out. Sharks join the problem and build a pump to live with it.
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means |
| Osmosis | Water moving through a membrane toward whichever side has more dissolved material in it. |
| Osmoregulation | Actively controlling the water and salt in your body fluids, whatever the water around you is doing. |
| Ionocyte | A cell built to move salt. Called chloride cells in older papers, and packed with mitochondria for the work. |
| Euryhaline | Able to live across a wide range of saltiness. Salmon and eels are the familiar examples. |
| Stenohaline | Locked into a narrow range of saltiness. Most fish are this, and die in the wrong water. |
| Milliosmole (mOsm) | A measure of how much dissolves in a fluid. Blood runs near 300, seawater near 1,000. |
| Glomerulus | The kidney's filter. Some marine fish have lost theirs entirely. |
| Anadromous | Born in fresh water, growing up at sea, returning to spawn. Salmon and lampreys. |
| Catadromous | Born at sea, growing up in fresh water. Eels do the reverse commute. |
| Rectal gland | The shark's salt-disposal organ, and the place the modern salt-secretion model was first worked out. |
| Trimethylamine oxide | The stabilizer sharks carry alongside urea, which would otherwise damage their proteins. |
Terms used above, in roughly the order they first appear.
Key Takeaways
- Both problems are osmosis: in fresh water a fish gains water and leaks salt, and in seawater it loses water and takes salt in.
- Freshwater fish solve it with dilution: up to a third of the body weight in dilute urine a day, no drinking, and gill cells that claw ions back from the water.
- Marine fish solve it by drinking and pumping: seawater goes in, the gut takes the water, and sodium chloride leaves through the gills.
- Marine urine is never concentrated above the blood: it runs about isosmotic with plasma, rich in magnesium and sulfate, so the saving comes from making very little of it.
- The ionocyte does both jobs: the same cell loads salt in fresh water and unloads it in seawater, which is why the gill is a breathing organ and a kidney at once.
- Only 3 to 5 percent of bony fish switch worlds, and they rebuild gills, kidneys and hormones to do it; sharks refuse the choice and match the sea with urea instead.
Frequently Asked Questions
Do saltwater fish drink water?
Yes, constantly. The experiment that settled it used a dye in the 1930s: the marker appeared in the gut, so it had come in through the mouth.
The swallowed seawater is stripped for water and sodium chloride, and most of the fluid never reaches the blood.
Why can't a marine fish just pee more?
Because it cannot make urine stronger than its own blood. That limit has been measured since the 1930s and never beaten.
Making more urine would only lose more fresh water, which is the one thing the fish cannot spare. It makes very little urine instead, and sends the salt out through the gills.
Is a euryhaline fish just one that visits both waters?
Visiting is the easy part. The hard part is rebuilding the plumbing for each one: filtration up or down, segments added or dropped, hormones driving the change.
Most fish cannot do it at all. Only about 3 to 5 percent of bony fish manage the switch, and salmon and eels do it on a schedule.
Do sharks drink seawater?
No. A shark's blood is slightly saltier than the sea, so water moves into it rather than out, and it never needs to drink.
The urea that makes this work is handled by the kidney, and the excess salt from food leaves through a dedicated rectal gland.
Key references
• Osmoregulation: regulators, conformers, and the basic directions of water and salt movement
• Osmosis: the rule the whole problem follows from
• Fish gill: the filaments, and their jobs beyond breathing
• Euryhaline: fish that tolerate a wide range of salinities
• Evans 2010, a history of fish osmoregulation, Front Physiol 1:13 -- Homer Smith's drinking experiment, urine that never beats plasma, glomerular and aglomerular kidneys, the shark urea strategy
• Takvam et al. 2021, ion transport in the teleost kidney, Front Physiol 12:664588 -- dilute high-volume urine in fresh water, low-volume almost-isosmotic urine in seawater, 3-5 percent of teleosts euryhaline
• Lee, Charmantier and Lorin-Nebel 2022, sodium uptake from fresh water, Front Physiol 13:1006113 -- ionocytes, formerly chloride cells, doing uptake or excretion, and the blood salt figures for carp and sea bass
• Shaughnessy et al. 2025, a Cftr-independent ionocyte in sea lamprey gills, J Exp Biol 228:jeb250110 -- the Nka-Nkcc1-Cftr salt secreter and the lamprey that uses something else
• FISHBIO 2025, fish osmoregulation explained -- freshwater urine up to a third of body weight per day, and the "very high salt concentration" claim this article weighs against the peer-reviewed reviews
Figures are from Wikimedia Commons, with the author and license named in each caption. Measurements are from the cited papers.
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