Fish kidneys solve opposite osmotic problems depending on salinity. In freshwater, water continually enters the body while dissolved salts tend to leave, so a teleost kidney filters a large volume, recovers valuable ions and produces plentiful dilute urine. In seawater, water tends to leave and salts enter, so the kidney sharply reduces filtration, conserves water and removes excess divalent ions such as magnesium and sulfate. Gills and the intestine share this work, making the kidney part of an integrated osmoregulatory system rather than the fish’s only salt-and-water organ.
The basic freshwater–seawater comparison
| Feature | Freshwater teleost | Seawater teleost |
|---|---|---|
| Main osmotic challenge | Water enters the fish; ions tend to diffuse out | Water leaves the fish; salts tend to enter |
| Kidney filtration | Generally high glomerular filtration | Greatly reduced relative to freshwater |
| Urine | High-volume and dilute (hypo-osmotic) | Low-volume and near-isosmotic to blood plasma |
| Tubular handling | Reclaims needed ions while allowing excess water to leave | Reabsorbs water and secretes divalent ions, especially magnesium and sulfate |
| Other organs | Gills and other epithelia help recover ions | Drinking, intestinal absorption and gill salt secretion work alongside the kidney |
This broad teleost pattern is summarized in reviews of renal transport and osmoregulation by Takvam and colleagues (Frontiers in Physiology, 2021; PubMed record).
What the kidney does in freshwater
It removes the water that enters by osmosis
A freshwater fish’s body fluids contain more dissolved substances than the surrounding water. Water therefore moves inward across permeable surfaces, including the gills. The kidney responds with a high filtration rate and a large urine output. The urine is dilute compared with the fish’s blood, so the fish can eliminate water without throwing away all of its salts.
It saves ions instead of wasting them
Sodium, chloride and other ions tend to diffuse from the fish into the dilute environment. Renal tubules actively reabsorb most of the ions that were filtered, returning them to the body while water continues toward the bladder and out of the fish. Gills and other epithelia also take up ions from freshwater, so kidney recovery is one component of the overall strategy.
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The 2021 review describes the freshwater pattern as “large volumes of water through high glomerular filtration rates (GFR) and low tubular reabsorption rates, while actively reabsorbing most ions at high rates” (Takvam et al., 2021).
What the kidney does in seawater
It limits water loss
Seawater is more concentrated than a teleost’s body fluids. Water consequently tends to leave the fish, while salts move inward. The fish drinks seawater and absorbs water and ions through the intestine, but that intake also creates a salt-excretion problem. The kidney reduces glomerular filtration and urine flow, limiting the amount of water lost as urine.
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It removes ions that the gills do not handle as the main route
Marine teleost renal tubules reabsorb as much water as practical and secrete a fluid containing divalent ions, notably magnesium and sulfate; chloride is also present in the tubular secretion. This is why it is misleading to say that a marine fish simply makes highly concentrated urine. The characteristic urine flow is low and close to the osmotic concentration of plasma, while the kidney’s distinctive contribution is divalent-ion excretion with water conservation.
As the review puts it, “In SW, GFR is greatly reduced, and the tubules reabsorb as much water as possible, while actively secreting divalent ions” (Takvam et al., 2021).
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How renal tubules change the final urine
Filtration creates an initial fluid, but epithelial cells lining different nephron regions determine what remains in the urine. Transport proteins move ions and water between the tubular fluid, cells and surrounding blood. In freshwater, the balance favors high water excretion and strong ion reclamation. In seawater, filtration falls, water reabsorption increases and secretion of divalent ions becomes prominent. The exact transporter locations and relative activities vary with nephron region, salinity and species; the 2021 review and its indexed record provide the comparative framework (full review; PubMed).
The kidney is only one part of osmoregulation
Gills
Gills are major sites of ion transport. In freshwater they help acquire ions lost to the environment. In seawater they help excrete excess salts, working with the kidney rather than being replaced by it.
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Intestine and drinking
Marine teleosts drink seawater. The intestine absorbs water, and specialized transport processes move ions so that usable water can enter the body. The kidney then handles a low-volume, ion-rich waste stream while conserving water.
Coordination
Changes in filtration, tubular transport, gill transport and intestinal function are coordinated as environmental salinity changes. Looking at urine alone therefore cannot explain the whole salt-and-water budget of a fish.
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What happens when salinity changes?
Species that tolerate broad salinity swings are called euryhaline. They can alter renal filtration and tubular transport as conditions shift between freshwater and seawater. The 2021 review gives an approximate estimate that only 3–5% of teleost species are euryhaline, so most fish tolerate a narrower salinity range. A fish’s ability to adjust is species-specific; a gradual change that one species handles may be dangerous for another.
At the molecular level, the best-integrated evidence covers only some groups, including salmonids, eels, tilapias and fugu. Mechanisms established in those fish should not automatically be applied to every teleost, or to sharks and rays, without species-specific evidence.
What this means for aquarium fish
- A freshwater fish normally needs to excrete abundant dilute urine and retain ions; a marine fish normally needs to conserve water and dispose of excess salts.
- Moving a fish between salinities changes demands on the gills, intestine and kidney together, not just the urine volume.
- Salinity tolerance depends on species and life stage. Do not infer that a fish can safely acclimate to any salinity because another euryhaline species can.
These are physiological patterns, not a substitute for species-specific husbandry guidance. Aquarium decisions should follow the documented salinity range and acclimation requirements for the particular fish.
Key points to remember
- Freshwater teleosts gain water and lose ions, so their kidneys usually produce lots of dilute urine while reclaiming ions.
- Seawater teleosts lose water and gain salts, so their kidneys reduce filtration, conserve water and secrete magnesium and sulfate.
- Urine from a marine teleost is generally low-volume and near-isosmotic to plasma, not simply maximally concentrated.
- Gills and intestine perform major parts of ion and water regulation alongside the kidney.
- Euryhaline species can remodel these processes across salinities, but exact limits and transport mechanisms vary by species.
Further reading
For specialist historical background, Cleveland P. Hickman Jr. and Benjamin F. Trump’s chapter “The Kidney” appears in Fish Physiology: Excretion, Ionic Regulation, and Metabolism (Elsevier, 1969). It is foundational rather than a current review of transporter biology; check the edition and availability in the Elsevier catalog record.
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