6. CONCLUSIONS
The present study provided deep insights into the genetic levels of biodiversity of brackish-water habitats. One of the three experimental models considered, Hediste diversicolor, showed a remarkable population genetic structuring and a hierarchical organisation of genetic diversity depending on spatial scales. High level of genetic divergence were found between Atlantic Ocean and Mediterranean Sea populations, corroborating the effectiveness of the Strait of Gibraltar as a major biogeographical break (Patarnello et al., 2007; Virgilio et al., 2009). Within each basin, a degree of genetic substructuring was found at a lesser extent: observed levels of genetic subdivision could be related mainly to habitat fragmentation and restricted gene flow, due to the lack of dispersal stages and the sedentary life-style of adults (Scaps, 2002). The influence of mesoscale and local hydrographical regimes could have played a role in promoting the differentiation among populations from different areas (Send et al., 1999). Additional effects deriving from demographic events might have favoured further differentiation, but the use of other markers is needed to shed light on this issue. In these sense, the application of inter-simple sequence repeat (ISSRs) markers could be of help. In fact, there are examples in which ISSRs, being a technique of fingerprinting, detected genetic divergence at very small scale (Casu et al., 2005; Casu et al., 2008).
The second experimental model used in the present work, Mytilaster minimus, provided evidence of remarkable habitat-related genetic divergence between populations from marine environments and those from brackish-water environments.
The different selective pressures acting in the different habitats may account for this result, as proposed by Cognetti (1994) and Cognetti & Maltagliati (2000) on the basis of observations on the patterns of colonization, adaptive strategies and genetic diversity of several faunal elements from brackish-water environments. Even though sampling error due to the relatively small sample sizes may have partially affected these results, yet they corroborate the outcome of previous studies conducted on this species with allozyme electrophoresis (Camilli, 2000; Camilli et al., 2001) and are worthy of deeper investigation. The use of increased sample sizes and appropriate sampling design will be needed to properly examine the divergence detected among the haplogroups detected within each habitat-type. The use of genome-wide molecular markers could promote M. minimus as a model species for the study of the
signature of selection.
The analysis of the third experimental model, Xenostrobus securis, revealed absence of genetic structuring in Italian waters. It is quite intuitive that, given the recent time of invasion, populations had no sufficient time to diverge significantly.
Considering its invasive nature and recent settling, unexpectedly high levels of genetic polymorphism were detected for the recently established populations of X.
securis. This characteristic may favour the spreading of the species, assuring high potential for plasticity and adaptation. The use of molecular markers for the analysis of samples from Australia and New Zealand, where X. securis is native, and from Japan, France and Spain, where the species is invasive, will shed light on the history of colonization routes chracterising the global spread of this pest.
Results of this study revealed significant levels of genetic divergence both among different brackish-water sites and among brackish-water sites and the sea.
Such a diversity results partly from the features of brackish-water environments as a whole, partly from the ecological heterogeneity among different categories of environments, and, ultimately, from the peculiarity and uniqueness of each single brackish-water site. As a consequence, it seems not possible to outline general rules or principles for a universal management of brackish-water environments.
