• brackish waters
• phylogeography
• Population genetics
• transitional waters

In the present study, the genetic diversity of brackish water environments was studied analysing three model species with different ecological traits by mean of mitochondrial genes sequences. In the first model species, Hediste diversicolor (O.F. Müller, 1776) (Annelida, Polychaeta), a portion of the COI gene (544 bp) was amplifyed in 181 individuals from nine population, two from the Atlantic ocean, and seven from Western Mediterranean. In Mytilaster minimus (Poli, 1795) (Mollusca, Bivalvia), the second analysed species, a fragment of the 16S gene (445 bp) was amplifyed in 25 individuals from five populations in the Mediterranean Sea, two sampled in marine sites, three sampled in brackish-water sites. The third model species is Xenostrobus securis (Lam., 1819) (Mollusca, Bivalvia), original of southern Australia and New Zealand estuaries. In the latter species, a fragment of the COI gene (564 bp) was amplifyed in 39 individuals from five recently settled populations in the Mediterranean Sea.
Analysis of the data collected from the three species yielded different phylogeographycal patterns. H. diversicolor showed a deep genetic structure, and a hierarchical organisation of genetic diversity on different spatial scales. These features were related to the effect of a major phylogeographical break between Atlantic and Meediterranean, and, inside each basin, to habitat fragmentation and scarce levels of gene flow given the lack of a dispersal stage. M. minimus revealed the presence of three partly sympatric lineages, and a strong genetic divergence between populations from marine- and brackish-water habitats, maybe due to the effects of different selective pressure in the two environmental typology. X. securis was characterised by high levels of within-population genetic diversity, presence of sympatric divergent lineages, and total absence of genetic structure among the sampled populations. The absence of genetic divergence among populations is to relate to their recent settling, no more than 20 years ago.
Levels of genetic divergence encountered in this study amond different populations and among different habitats evidenced the heterogeneity of brackish water environments and the effects that thiss heterogeneity exerts on organism. As a consequence, the managment of such biotopes needs to be finely tuned from case to case and cannot rely on general assumpionts.