ABSTRACT
Living organisms occupy a wide range of aquatic habitats that differ greatly in water and solute chemistry. The primacy of salinity as a determinant of species richness in aquatic habitats is exemplified by Remane’s curve. The number of species in marine habitats and in freshwaters is large. Fewer species are found in brackish waters, and even fewer in hypersaline environments. Gastropods have invaded terrestrial habitats and are widely distributed. Some snails are amphibious, moving between aquatic and terrestrial habitats. Additionally, a few species of bivalves that occur in high intertidal habitats may be out of the water for longer periods of time than they are submerged. In all animals, the cytoplasm and the extracellular fluid are in osmotic equilibrium; however, the ionic composition of the two media are dissimilar in that the cells of all animals have relatively high concentrations of K+ and low concentrations of Na+.
