Biodiversity ultimately is genetic diversity. Genetic diversity within species is eroded before negative trends in biodiversity become evident as loss of species or habitats. Hence, monitoring biodiversity at the genetic level may indicate what will happen at higher levels of organisation if the trend is allowed to continue. There is a pervasive belief that marine ecosystems are less vulnerable to biodiversity loss than terrestrial ones, due to marine species' high dispersal ability and connectivity, large geographic ranges, low genetic differentiation among populations and high genetic variation within populations. Many studies offer compelling evidence that it is not so: loss of genetic variation due to natural and anthropogenic factors has been detected even in marine species with potentially high dispersal. In this context the genetic pattern of the European barnacle Chthamalus montagui, a species with high dispersal capability, was investigated from three different perspectives using polymorphic microsatellite loci as molecular markers. The effect of structures created to protect coastal areas in the Adriatic Sea, was investigated to test the hypothesis that artificial substrates can act as "corridors" facilitating gene flow among previously isolated populations. The genetic pattern of central populations was compared to that of peripheral/marginal populations over the range of C. montagui in the UK, to test the hypothesis that marginal and peripheral populations tend to be less genetically variable than central ones. For both studies results were consistent with the formulated hypotheses at the 3 analysed loci. Finally, a broader survey of the NE Atlantic and Mediterranean range of this barnacle was carried out to assess spatial scales of genetic variation. A clear differentiation between Atlantic and Mediterranean samples was detected; however, the major source of genetic variation was within sites at a very small spatial scale. The information gained generates insights for marine genetic management and conservation planning.

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