The construction of artificial reefs in a sandy beach environment creates virgin habitat for rocky shore organisms. Earlier isolated patches of natural habitat may become connected via a chain of "stepping-stones" such that gene flow is pemitted and also the dynamics of some or all species is shifted from the original situation. Such a situation may promote the invasion of introduced or alien species. Depending on the actual positions of new artificial substrate the dispersal speeds, i.e., km./generation or similar, may differ. In W.P 3.5 we are assessing the large-scale effects of breakwaters on the distribution and abundance of species. For tjhis report we have used Patelia caerulea as a model organism. P. caerulea is a long lived prosobranch gastropod living in the intertictal (Della Santina et al. 1993). This marine snail grazes on the rock surface consuming the microfilm as well as recruits of barnacles and macroalgae. It bas the potential of stucturing its environment. Spatial ecology and spatial population dynamics are wide fields of research. However, it is still unknown what the general effect on originally isolated natural communities is when gene flow is established. This report tries to answer some questions about the effect of the positioning of Low Crested Structures on the regional population dynamics of P. caerulea in the Ravenna region.