Osmoregulation is the regulation of water and salt concentrations in an organism.
Enantiostasis is the maintenance of metabolic and physiological functions in response to variations in the environment. Estuarine organisms have to cope with external variations in salt concentrations while at the same time maintaining an internal homeostasis.
So pretty much the estuarine organisms have to maintain their water and salt concentrations levels in different environmental conditions. When the water is salty, they have to drink more water and excrete more salts. When the water is fresh, they produce dilute urine and absorb more salt. Salt absorbtion and excretion is through the gills.
