Freshwater animals have the reverse osmotic problem of saltwater animals.
The cells and internal fluids of freshwater animals are hypertonic in relation to the environmentso they are always absorbing water by osmosis.
Freshwater fish must eliminate large amounts of water in their urine and thus lose important salts. This saline loss is offset by Active absorption of salts through the epithelium lining the gills.
Stenoaline and Eurialine animals
Aquatic animals, whether freshwater or saltwater, generally cannot withstand pronounced variations in the salinity of their environment. They are called stenoalines (from Greek stenonarrow and halos, salt) because they survive only within a narrow limit of water salinity.
There are animals, however, able to survive well in aquatic environments where salinity varies greatly. They are called eurialinos (from Greek eury, broad) because they support a wide range of salinity variation. Shellfish, crustaceans and fish who live in river estuaries, where salinity varies by tides, are examples of Eurialine animals.
Osmoregulation in the terrestrial environment
In the terrestrial environment animals have to ingest water by drinking it or eating food containing water. They also have to prevent desiccant water loss by developing impermeable layers such as the shell of the terrestrial mollusks, the exoskeleton of insects or the keratin layer of the terrestrial vertebrate epidermis.
Water loss was the main factor limiting the colonization of the dry land environment, you have seen that only a small number of current animal phyla have terrestrial representatives. Among these, arthropods and vertebrates developed the most efficient mechanisms for obtaining and saving water and mineral salts.
For terrestrial vertebrates, osmoregulation consists of ingesting sufficient water and salts to prevent these substances from missing or accumulating in the blood. The kidneys They are the main organs in charge of keeping blood in the proper tonicity by eliminating excess water, leaves and other osmotically active substances in the urine.