Genetic structure relates to the relationship between local and distant populations. Gobies have limited dispersal capabilities while their parasite fauna is thightly linked to the host. On their turn, parasites influence the general fitness of the host. This study determines if parasites can function as genetic markers for the life history of their host. This study evaluates parasites Pomatoschistus (gobies) has been identified as study model, (2) the parasite species will be unambiguously identified with genetic markers and (3) the host and parasite will be screened genetically in space and time.