A comparative analysis of estuaries, lagoons and coastal ponds focusing on population differentiation, and community structure is necessary to correctly address the issue of brackish water biology. Although the different biotopes all present similar features of environmental unpredictability and the common presence of the hypohalobic contingent (ar artenminimum), they each have their own characteristics, due to the evolution of peculiar balances in their relation to the sea on the one hand and inland waters on the other. In addition to euryhaline species, locally adapted populations of stenohaline species typical of marine habitats, as well as some recently introduced species, are also found. These species have given rise to euryhaline populations, reaching their maximum development in an optimal site. This situation occurs between basins with essentially similar ecological features and probably depends both on the different degree of adaptability of many species to a specific environmental parameter and the type of biocoenoses adjacent to the brackish basin. These populations possess genotypes allowing adaptation to brackish waters, which have resulted in the differentiation, through selection, of individuals capable of fine-grained perception of environmental unpredictability. Experimental works demonstrated the existence of genetically differentiated populations, or, ultimately, sibling species complexes, in several brackish species with broad geographical distribution and belonging to a wide range of taxonomic groups. The conceptions regarding the uniformity of brackish elements and the doubts concerning the existence of a specific brackish fauna come from the fact that attention generally focuses on species in the traditional meaning of the term, that is to sag at the macrosystematic level. Comparative analyses of very fine morpho-physiological changes and genetic analyses result in a rather different picture, leading to the conclusion that in brackish waters a given species of marine origin often consists of many different forms at various levels of differentiation.
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