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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Most marine bony fish survive seawater by constantly correcting the same imbalance: seawater is more concentrated than their body fluids, so water tends to leave their bodies while salts enter. They drink seawater, absorb its water in the intestine, and actively move excess ions—especially sodium and chloride—out through specialized cells in their gills. Their kidneys help conserve water and remove additional ions.
This is osmoregulation, not simply “putting up with” salt. The details differ among bony fish, sharks, freshwater species and euryhaline fish that move between salinities.
The problem seawater creates
Osmosis is the passive movement of water across a selectively permeable surface toward the more concentrated solution. For most marine teleosts (bony fish), seawater is hypertonic relative to the fluids inside their bodies.
- Water tends to diffuse out across the gills and other permeable surfaces.
- Dissolved ions from seawater tend to diffuse inward.
- Without compensation, the fish would dehydrate and its internal chemistry would become too salty.
The fish therefore has to replace water while eliminating salt. NOAA’s educational chapter Respiration and Osmoregulation summarizes the situation this way: “Saltwater fish live in a hyper-osmotic environment, and continuously lose water, which is replaced by drinking seawater.”
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How most marine bony fish regulate salt and water
1. They drink seawater
Drinking replaces water lost to the ocean, but it also brings more salt into the body. The water passes through the digestive tract, where the intestine absorbs water and handles ions before the remaining concentrated material is eliminated.
2. Gill cells actively export ions
The gills are not only respiratory organs. Specialized cells called ionocytes, also known as mitochondria-rich or chloride cells, use energy to transport excess ions from the blood to the seawater. This active secretion is central to removing sodium and chloride and maintaining the composition of body fluids.
Fish still use their gills for gas exchange; salt regulation is a separate function carried out by the same organ.
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3. The kidneys conserve water and excrete ions
Marine fish generally produce relatively small amounts of urine, helping limit water loss. Their kidneys contribute to ion excretion and water balance, but they do not “filter all the salt.” In marine teleosts, the gills are the major route for actively removing excess ions.
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Do saltwater fish drink seawater?
Most marine bony fish do. Drinking is necessary because their bodies continually lose water to the hypertonic environment. It is not a complete solution: the same seawater contains the ions the fish must later remove through gill ionocytes, the intestine and the kidneys.
How this differs from freshwater fish
Freshwater reverses the osmotic challenge. A freshwater fish’s body fluids are usually more concentrated than the surrounding water, so water enters the fish and ions tend to diffuse out.
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| Feature | Most marine bony fish | Typical freshwater fish |
|---|---|---|
| Relative concentration | Body fluids are less concentrated than seawater | Body fluids are more concentrated than freshwater |
| Passive water movement | Water tends to leave the body | Water tends to enter the body |
| Drinking | Usually drinks seawater | Usually drinks little |
| Gill function in ion balance | Actively excretes excess ions | Actively takes up ions |
| Urine | Relatively limited, helping conserve water | Large volumes of dilute urine |
| Main challenge | Prevent dehydration and salt loading | Prevent overhydration and replace lost salts |
Because the transport systems are tuned to a fish’s usual salinity, moving a freshwater fish directly into seawater—or a marine fish into freshwater—can overwhelm its ability to compensate.
Why some fish can move between fresh and salt water
Euryhaline fish tolerate a comparatively broad range of salinities. Salmon are a familiar example: different life stages can live in freshwater and seawater. Mollies and several other species also show this capacity.
These fish remodel their osmoregulatory systems as conditions change. Gill ionocytes alter their transport machinery, and the intestine and kidneys change how they handle water and ions. Tolerance is not unlimited or identical across a species. It depends on the species, life-history stage, acclimation history and the rate of salinity change.
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A 2024 review in Reviews in Aquaculture notes that salinity can affect metabolism, nutrition, reproduction and growth, and that the best conditions for survival and growth vary by species and life stage. A fish that can survive a transition may still grow or reproduce poorly under that condition.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How sharks solve the problem
Sharks are cartilaginous fish and do not regulate salt exactly like marine bony fish. They retain high concentrations of urea in their body fluids, while trimethylamine oxide (TMAO) helps protect proteins from the destabilizing effects of urea.
Retained urea brings the overall osmotic concentration of a shark’s body fluids close to that of seawater, reducing the tendency for water to leave. This does not mean shark blood has the same electrolyte composition as seawater. Sharks also use a specialized rectal gland to secrete excess salt.
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Are all marine fish regulators?
No. Fish lineages use several strategies. Most marine teleosts actively regulate their internal concentration, while some groups are closer to osmoconformers, allowing aspects of their internal osmotic state to track the environment. Hagfish are an example discussed in the USGS-hosted chapter on osmoregulation and acid–base balance. Marine elasmobranchs such as sharks retain urea, and coelacanths also have distinctive urea-retention physiology.
“Saltwater fish” is therefore a broad everyday label rather than one physiological category. The drinking-and-gill-excretion explanation applies most directly to marine bony fish.
Why salinity changes can be dangerous
Changing salinity changes the direction and strength of water and ion movements across the gills and digestive surfaces. A fish must adjust ion transport, drinking, intestinal processing and urine production; those adjustments take time and may differ by tissue and life stage.
- A sudden change can cause dehydration, excessive water uptake or dangerous shifts in blood-ion concentrations.
- Juveniles, eggs and larvae may have narrower tolerances than adults.
- Survival at a salinity does not guarantee normal feeding, growth or reproduction there.
- Species described as euryhaline still have limits and may require acclimation.
In natural estuaries, gradual changes and behavioral access to different water layers can help some species adjust. In captivity, the appropriate salinity and any transition schedule are species-specific; adding salt is not a universal remedy.
How fish get rid of salt: the short answer
- Seawater enters through drinking.
- The intestine absorbs water and processes the ingested ions.
- Gill ionocytes actively secrete excess ions, especially sodium and chloride, into the surrounding water.
- The kidneys conserve water and contribute to ion excretion in the urine.
Together, these processes keep the internal fluid concentration within the range the fish’s cells require, even while the surrounding seawater continuously pushes water and salt in the wrong directions.
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