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Persons lost at sea without any fresh water to drink are at risk of severe dehydration because the human body cannot adapt to drinking seawater, which is hypertonic (having higher osmotic pressure) in comparison to body fluids.
Stenohaline organisms, such as goldfish, can tolerate only a relatively-narrow range of salinity.
About 90 percent of bony fish species can live in either freshwater or seawater, but not both.
These fish are incapable of osmotic regulation in the alternate habitat.
However, a few species, known as euryhaline organisms, spend part of their lifecycle in fresh water and part in seawater.
These organisms, such as the salmon, are tolerant of a relatively-wide range of salinity.
They evolved osmoregulatory mechanisms to survive in a variety of aquatic environments.
In relatively hypotonic (low osmotic pressure) fresh water, their skin absorbs water (see [a] in ).
The fish do not drink much water and balance electrolytes by passing dilute urine while actively taking up salts through the gills.
When they move to a hypertonic marine environment, the salmon lose water, excreting the excess salts through their gills and urine (see [b] in ).
Most marine invertebrates, on the other hand, may be isotonic with sea water (osmoconformers).
Their body fluid concentrations conform to changes in seawater concentration.
The blood composition of cartilaginous fishes, such as sharks and rays, is similar to that of bony fishes.
However, the blood of sharks contains urea and trimethylamine oxide (TMAO).
The shark's blood electrolyte composition is not similar to that of seawater, but maintains isotonicity with seawater by storing urea at high concentrations.
Sharks are "ureotelic" animals that secrete urea to maintain osmotic balance.
TMAO stabilizes proteins in the presence of high urea levels, preventing the disruption of peptide bonds that would otherwise occur at such high levels of urea.
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skin absorbs water in a hypertonic environment, concentrated urine is passed in a hypotonic environment, blood releases urea to maintain an isotonic environment, and gills take up salts in a hypotonic environment
a fish that spends some of its lifecycle in fresh water and part in seawater, a stenohaline organism that can tolerate only a relatively-narrow range of salinity, a ureotelic animal that secretes urea to maintain osmotic balance, and a marine invertebrate that is isotonic with seawater