Skip to main content
[ "article:topic", "authorname:boundless", "showtoc:no" ]
Biology LibreTexts

41.1D: Osmoregulators and Osmoconformers

  • Page ID
    14066
  • Aquatic organisms with various salt tolerances adapt to their environments through osmoregulation and osmoconformation.

    LEARNING OBJECTIVES

    Compare the ability of stenohaline and euryhaline organisms to adapt to external fluctuations in salinity

    KEY TAKEAWAYS

    Key Points

    • Stenohaline organisms can tolerate only a relatively-narrow range of salinity.
    • Euryhaline organisms are tolerant of a relatively-wide range of salinity.
    • Osmoconformers are organisms that remain isotonic with seawater by conforming their body fluid concentrations to changes in seawater concentration.

    Key Terms

    • euryhaline: able to tolerate various saltwater concentrations
    • osmoconformer: a marine organism (usually an invertebrate) that maintains its internal salinity such that it is always equal to the surrounding seawater
    • stenohaline: tolerant of only a narrow range of saltwater concentrations

    Osmoregulators and osmoconformers

    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 ).

    image

    Salmon physiology responds to freshwater and seawater to maintain osmotic balance: Fish are osmoregulators, but must use different mechanisms to survive in (a) freshwater or (b) saltwater environments.

    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.

    LICENSES AND ATTRIBUTIONS

    CC LICENSED CONTENT, SHARED PREVIOUSLY

    CC LICENSED CONTENT, SPECIFIC ATTRIBUTION