Organisms that can survive well in conditions with changing salinity are well adapted to live in

The answer to this question is estuaries or mangrove swamps, which are found in estuary environments. Estuaries are the transition between fresh water rivers and the ocean, and the salinity of estuaries change dramatically depending on the relative dominance between the fresh water and marine environments. These are euryhaline species that can tolerate a wide range of salinities, and include mangrove swamp and certain fishes. 

Cells of an organism tell each other when predators are near.