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According to the enemy release hypothesis, one of the reasons for the successful establishment of non-indigenous species in a new habitat is the libera- tion from natural enemies, and among them the parasites. The rapid spread of the Red Sea/Indo-Pacific fish Fistu- laria commersonii in the Mediterranean Sea, which in just 7 years (2000–2007) invaded nearly all of the basin, gives an opportunity to study the dynamics of the host and its parasites in its recently invaded range. Informa- tion on the parasites of this fish in its original habitat is quite scarce. The present study describes the metazoan parasites of 40 specimens of F. commersonii (total length range 73–107 cm) caught in the Mediterranean Sea (Sar- dinia, Tunisia, Libya) from 2005 to 2015. The parasite fauna of this migrant in the recently invaded range is mainly a combination of generalist juvenile/larval spe- cies (probably acquired in the new habitat) with some of its adult natural parasites (probably co-introduced during migration). The results indicate that a non-indigenous species is not always released from its natural parasite and that its success is not simply associated with such liberation. Actually, the parasite fauna of F. commer- sonii increased along its migration path, acquiring new generalist species, but also conserving a subset of natu- ral parasites. These data suggest caution in the uncritical acceptance of the enemy release hypothesis, because the different phases of the invasion process and establish- ment of a non-indigenous species appear to be related to a combination of ecological, physiological and behav- ioural factors

the gills of 63 specimens of the Atlantic bluefin tuna Thunnus thynnus (linnaeus) (osteichthyes: scombridae) from three localities of the Mediterranean (sardinian, tyrrhenian and levantine seas) were examined for metazoan parasites. the parasite fauna of T. thynnus from the sea of sardinia included 11 species: five didymozoid trematodes, three capsalid and one hexostomid monogeneans, and one caligid and one pseudocycnid copepods. Four didymozoids were found in fish from the levantine sea and only one didymozoid was recorded in fish from the tyrrhenian sea. Dividing the hosts into four size-groups (small, medium-sized, large and extra large), the pairwise comparison of prevalence and mean abundance of the new and literary data) showed differences according to host size. the differences in the composition of the parasitic faunas and in the prevalence of parasites, observed between the small tunas from the tyrrhenian sea and the medium-sized tunas from the Adriatic sea, levantine sea and the NorthEast (NE) Atlantic ocean, indicated that these groups form discrete units. the parasite fauna of the large tunas from the sea of sardinia is the richest among the bluefin tuna populations of the Mediterranean and the NE Atlantic, due to the presence of species not found elsewhere in bluefin tunas, such as Caligus coryphaenae steenstrup et lütken, 1861, Capsala magronum (ishii, 1936) and C. paucispinosa (Mamaev, 1968). this fact and the prevalence of some parasites of this group (lower than those of medium-sized fish from the NE Atlantic and higher than the small and medium-sized tunas from the Mediterranean) suggest that the large-sized tuna group in the western Mediterranean is formed by Mediterranean resident tunas (poorly infected), and by tunas migrating from the Atlantic ocean (heavily infected).