osmoregulators

Osmoregulators are organisms that maintain stable internal osmolarity regardless of external salinity. They actively regulate water and solute movements through specialized organs and cellular transport processes, in contrast to osmoconformers whose internal fluids track the environment.

In freshwater fish, the body fluids are hyperosmotic to the dilute surrounding water, so they gain water and lose salts. They excrete large volumes of dilute urine and take up ions across gill epithelia via ion pumps. In seawater fish, the situation is reversed: they lose water to the environment and drink seawater; they excrete the excess salts through chloride cells in the gills and produce small amounts of concentrated urine.

Terrestrial vertebrates conserve water with impermeable skin and kidneys capable of concentrating urine; mammals regulate salt and water balance through nephron segments and hormones such as vasopressin.

Some marine cartilaginous fishes, such as sharks, maintain blood osmolarity close to seawater by retaining urea and trimethylamine N-oxide (TMAO), and excreting excess salts via rectal glands.

Osmoregulation is energetically costly but critical for survival across habitats with fluctuating salinity or water availability.