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Most marine bony fish replace water lost to seawater by drinking, then absorb water in the intestine and send excess salt out mainly through specialized gill cells. Their kidneys conserve water and remove other ions. Sharks and rays use a different strategy: they retain urea and other osmolytes and excrete salt through a rectal gland.
Why seawater creates an osmoregulation problem
Seawater is more concentrated than the internal fluids of most marine bony fish, or teleosts. Because the gills are thin and permeable for gas exchange, water passively leaves the fish across the gill surface while sodium and chloride ions (NaCl) diffuse inward. The American Physiological Society’s 2023 review describes this as osmotic water loss and diffusional NaCl gain.
Osmoregulation is the set of processes that keeps the fish’s water and ion balance within workable limits despite those opposing flows. In teleosts, it depends on coordinated drinking, intestinal absorption, gill salt secretion and kidney function—not on a single organ.
How marine teleosts replace water and remove salt
They drink seawater and recover water in the gut
Drinking seawater replaces some of the water that leaves across the gills. The esophagus and intestine absorb water and NaCl; magnesium and sulfate become concentrated in the gut contents and are later removed mainly in urine. As salt and water are absorbed, the intestinal fluid becomes less concentrated.
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A broad reported drinking range for marine fish is about 2–5 mL per kilogram of body mass per hour, according to the American Physiological Society’s 2023 review. The rate varies with factors including temperature, body size, metabolic rate and lifestyle, so it is not a fixed rate for every species or individual.
In experiments later discussed in a 2010 Frontiers historical review, Homer Smith found swallowed seawater in the intestines of eels and sculpins. The review reports that about 90% of ingested fluid in Smith’s experiments was excreted extrarenally—that is, by routes other than the kidneys. This is a historical experimental result, not a universal percentage for marine fish.
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Gill ionocytes actively secrete salt
Marine teleosts send most excess NaCl out through mitochondria-rich gill cells, also called chloride cells or ionocytes. The cells use energy-dependent transport to move ions from the blood side toward the seawater side:
- Na+/K+-ATPase on the cell’s blood-facing side establishes an ion gradient using energy.
- NKCC uses that gradient to bring sodium (Na+), potassium (K+) and chloride (Cl−) into the cell.
- CFTR-family chloride channels on the seawater-facing side allow chloride to leave the cell. Sodium follows through spaces between cells, maintaining electrical balance.
This active transport can move salt against the concentration gradient. In a historical account cited by the Frontiers review, physiologist Ancel Keys described the experiments as demonstrating “that a concentrated chloride solution is secreted by the gills in opposition to a large concentration gradient.”
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The kidneys conserve water and remove other ions
Marine teleosts produce a low volume of urine, helping limit water loss. Their urine is generally isotonic or slightly hypotonic to their plasma; unlike a mammalian kidney, a teleost kidney does not produce strongly hyperconcentrated urine. The kidney preferentially removes divalent ions such as magnesium and sulfate, while the gills handle most excess NaCl.
Homer Smith’s observation, quoted in the 2010 Frontiers historical review, was that “the osmotic pressure and inorganic composition of normal urine is not significantly dependent on the pressure or absence of glomeruli.” The point is that kidney structure and urine formation in fish should not be assumed to work just like those in mammals.
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How sharks and rays use a different strategy
Sharks and rays are marine elasmobranchs, not teleosts. Rather than keeping their internal fluids much less concentrated than seawater, they retain high concentrations of urea, stabilized by trimethylamine oxide (TMAO). This makes their body fluids approximately isotonic or slightly hypertonic to seawater and changes the water-balance problem they face.
Elasmobranchs also use a rectal gland to secrete NaCl. Their kidneys manage urea and water differently from teleost kidneys. The available reviews do not give a comparable urine-volume or urine-concentration value for elasmobranchs, so those measures should not be inferred from the teleost figures.
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- 5-in-1 Master Test Kit: Monitor all 5 critical water parameters – pH, nitrite, nitrate, ammonia, and phosphate – in one comprehensive aquarium test kit. The rigorous analysis helps you maintain an ideal haven for your aquatic friends
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| Feature | Marine teleosts (bony fish) | Marine elasmobranchs (sharks and rays) |
|---|---|---|
| Internal fluids compared with seawater | Hyposmotic: less concentrated than seawater (American Physiological Society, 2023 review). | Approximately isotonic or slightly hypertonic, largely through retained urea and TMAO (cited fish-physiology reviews). |
| Drinking seawater | Drink seawater to replace osmotic water loss; reported rates are about 2–5 mL kg−1 h−1 as a broad range in the American Physiological Society’s 2023 review. | Not stated as a comparable routine rate in the cited reviews. |
| Main salt-excretion route | Gill ionocytes secrete most excess NaCl; the kidney removes divalent ions such as magnesium and sulfate. | Rectal gland secretes NaCl. |
| Urine | Low volume; generally isotonic or slightly hypotonic to plasma. | Urine volume and concentration: not stated in the cited reviews; kidneys handle urea and water differently from teleost kidneys. |
| Major osmolytes | No urea/TMAO strategy is identified as the defining mechanism in the cited teleost account. | Urea, stabilized by TMAO. |
How salinity tolerance and hormones affect the response
Fish species differ in transporter activity, kidney structure, drinking rate and tolerance of changing salinity, so the teleost sequence is a general pattern rather than an identical recipe for every marine fish. During seawater acclimation, cortisol and growth hormone promote seawater-type ionocytes and greater salt-transport capacity. Prolactin favors freshwater-type ion uptake. The balance and effects of these hormones vary by species.
A 2011 review by McCormick estimates that 95% of teleost species are stenohaline, meaning they tolerate only a relatively narrow salinity range, while 5% are euryhaline and tolerate a wider range. Treat this as the chapter’s estimate, not a census of every fish species: only a small fraction of teleost diversity has been studied in detail.
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