Practice question

Euryhaline organisms tolerate a wide range of environmental salinities. They maintain internal water and ion balance by adjusting osmoregulatory processes as external salt concentration changes. Estuarine fishes such as salmon during migration alter gill ion transporters, kidney function, drinking behaviour, and hormonal regulation when moving between fresh water and seawater. In fresh water, water enters osmotically and salts tend to be lost, so animals excrete dilute urine and actively absorb ions. In seawater, they risk dehydration and salt gain, so many drink water and actively secrete excess ions through gills or specialized glands. Stenohaline species have a narrow salinity tolerance and are more vulnerable to abrupt dilution or concentration. Euryhalinity does not imply endothermy and does not restrict an organism to freshwater; it specifically concerns salt concentration. The capacity may vary by life stage, season, or acclimatization history, and adjustment requires time, so even a euryhaline organism can suffer osmotic shock after sudden transfer. This broad tolerance enables use of estuaries, tidal pools, and migratory routes where salinity changes predictably or unpredictably.