Euryhaline organisms are able to adapt to a wide range ofsalinities. An example of a euryhalinefish is the short-finned molly,Poecilia sphenops, which can live infresh water,brackish water, orsalt water.

The green crab (Carcinus maenas) is an example of a euryhaline invertebrate that can live in salt and brackish water. Euryhaline organisms are commonly found in habitats such asestuaries andtide pools where the salinity changes regularly. However, some organisms are euryhaline because theirlife cycle involves migration between freshwater and marine environments, as is the case withsalmon andeels.

The opposite of euryhaline organisms arestenohaline ones, which can only survive within a narrow range of salinities. Most freshwater organisms are stenohaline, and will die in seawater, and similarly most marine organisms are stenohaline, and cannot live in fresh water.

Osmoregulation

Movement of water and ions in asaltwater fish
(yellow jack)

Movement of water and ions in afreshwater fish
(brown trout)

Osmoregulation is the active process by which an organism maintains its level of water content. Theosmotic pressure in the body ishomeostatically regulated in such a manner that it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis.

Two major types of osmoregulation are osmoconformers and osmoregulators.Osmoconformers match their body osmolarity to their environment actively or passively. Most marine invertebrates are osmoconformers, although their ionic composition may be different from that of seawater.

Osmoregulators tightly regulate their body osmolarity, which always stays constant, and are more common in the animal kingdom. Osmoregulators actively control salt concentrations despite the salt concentrations in the environment. An example is freshwater fish. The gillsactively uptake salt from the environment by the use of mitochondria-rich cells. Water will diffuse into the fish, so it excretes a veryhypotonic (dilute) urine to expel all the excess water. A marinefish has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water (to the more negative surroundings) and gain salt. It actively excretessalt out from thegills. Most fish arestenohaline, which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than they are adapted to. However, some fish show a tremendous ability to effectively osmoregulate across a broad range of salinities; fish with this ability are known aseuryhaline species, e.g.,salmon. Salmon has been observed to inhabit two utterly disparate environments — marine and fresh water — and it is inherent to adapt to both by bringing in behavioral and physiological modifications.

Some marine fish, like sharks, have adopted a different, efficient mechanism to conserve water, i.e., osmoregulation. They retain urea in their blood in relatively higher concentration. Urea is damaging to living tissue so, to cope with this problem, some fish retaintrimethylamine oxide. This provides a better solution to urea's toxicity. Sharks, having slightly higher solute concentration (i.e., above 1000 mOsm which is sea solute concentration), do not drink water like marine fish.

• Some euryhaline fish
• 
  Short-finned molly
• 
  Round goby
• 
  Atlantic stingray
• 
  Bat ray
• 
  Longnose stingray^[1]
• 
  Big-scale sand smelt
• 
  Moonyfishes
• 
  Pink salmon
• 
  Barramundi
• 
  Green sawfish
• 
  Spanish toothcarp
• 
  Atlantic threadfin
• 
  Desert pupfish
• 
  Mayan cichlid
• 
  Crevalle jacks

The level ofsalinity inintertidal zones can also be quite variable. Low salinities can be caused by rainwater or river inputs of freshwater. Estuarine species must be especially euryhaline, or able to tolerate a wide range of salinities. High salinities occur in locations with high evaporation rates, such as insalt marshes and high intertidal pools. Shading by plants, especially in the salt marsh, can slow evaporation and thus ameliorate salinity stress. In addition, salt marsh plants tolerate high salinities by several physiological mechanisms, including excreting salt through salt glands and preventing salt uptake into the roots.

Despite having a regular freshwater presence, the Atlantic stingray is physiologically euryhaline and no population has evolved the specializedosmoregulatory mechanisms found in theriver stingrays of the familyPotamotrygonidae. This may be due to the relatively recent date of freshwater colonization (under one million years), and/or possibly incomplete genetic isolation of the freshwater populations, as they remain capable of surviving insalt water. Freshwater Atlantic stingrays have only 30-50% the concentration ofurea and otherosmolytes in their blood compared to marine populations. However, theosmotic pressure between their internal fluids and external environment still causes water todiffuse into their bodies, and they must produce large quantities of diluteurine (at 10 times the rate of marine individuals) to compensate.^[2]

Partial list

• 
  Cobia
• 
  Flathead mullet
• 
  Bull shark

• other euryhaline organisms
• 
  the seagrassHalodule uninervis
• 
  Green sea urchin
• 
  White-spotted jellyfish
• 
  Lagoon cockle
• 
  New Zealand mud snail
• 
  Amphipods of the familyGammaridae
• 
  Irrawaddy dolphin
• 
  Asian shore crab
• 
  Shore crab
• 
  Crab-eating frog
• 
  Diamondback terrapin

• Fish migration
• Osmoregulation
• Stenohaline
• Osmoconformer

• Aquatic ecology

• Articles with short description
• Short description is different from Wikidata