Biodiversity and adaptive mechanisms in brackish water fauna

A comparative analysis of estuaries, lagoons and coastal ponds focusing on population differentiation, and community structure is necessary to correct ly address the issue of brackish water biology. Although the different biot opes 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 stenoha line species typical of marine habitats, as well as some recently introduce d species, are also found. These species have given rise to euryhaline popu lations, reaching their maximum development in an optimal site. This situat ion occurs between basins with essentially similar ecological features and probably depends both on the different degree of adaptability of many speci es to a specific environmental parameter and the type of biocoenoses adjace nt to the brackish basin. These populations possess genotypes allowing adap tation to brackish waters, which have resulted in the differentiation, thro ugh selection, of individuals capable of fine-grained perception of environ mental unpredictability. Experimental works demonstrated the existence of g enetically differentiated populations, or, ultimately, sibling species comp lexes, in several brackish species with broad geographical distribution and belonging to a wide range of taxonomic groups. The conceptions regarding t he uniformity of brackish elements and the doubts concerning the existence of a specific brackish fauna come from the fact that attention generally fo cuses on species in the traditional meaning of the term, that is to sag at the macrosystematic level. Comparative analyses of very fine morpho-physiol ogical changes and genetic analyses result in a rather different picture, l eading to the conclusion that in brackish waters a given species of marine origin often consists of many different forms at various levels of differen tiation. (C) 2000 Elsevier Science Ltd. All rights reserved.