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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).

The gills of 31 skipjack tuna Katsuwonus pelamis (L.) caught in the Alboran Sea (western Mediterranean Sea) were examined for metazoan parasites, and the gills of 4 specimens from the Balearic Sea (also western Mediterranean Sea) were analysed for comparative purposes. Nine species of parasites were found, including 8 didymozoid trematodes (Atalostrophion cf. biovarium, Didymocylindrus filiformis, Didymocylindrus simplex, Didymocystis reniformis, Didymoproblema fusiforme, Didymozoon longicolle, Koellikeria sp. and Lobatozoum multisacculatum) and 1 caligid copepod (Caligus bonito). Koellikeria sp. and L. multisacculatum were not recorded in the Balearic Sea. Most of the parasites (79.2% of all specimens) were didymozoids. Didymozoon longicolle was the dominant species; A. cf. biovarium, D. simplex, D. fusiforme and L. multisacculatum are reported from the Mediterranean Sea for the first time. No correlation was found between the intensity of infection of any parasite species and host size or sex. Most of the parasites, particularly didymozoids, showed a high site-specificity. Significant differences were found between the parasite assemblages of K. pelamis from the Alboran Sea and from the Atlantic Ocean. D. fusiforme, D. longicolle and L. multisacculatum are suggested as potential tags to follow skipjack tuna migrations between the Atlantic Ocean and Mediterranean Sea.

The head region of 72 bullet tuna Auxis rochei from the western Mediterranean Sea (south-east Spain and the Strait of Gibraltar) was examined for parasites. Seven metazoan species were found in the fish from south-east Spain: three monogeneans, two trematodes and two copepods, whereas only three species were isolated in the fish from the Strait of Gibraltar. A comparison of the levels of infection of the parasites according to fish size in south-east Spain showed that the prevalence of Didymozoon auxis and the mean abundance of Allopseudaxine macrova were higher in the larger hosts (range of fork length = 38-44 cm) than in the smaller ones (33-37 cm). A comparison of the parasite infections according to geographical region showed that the mean abundances of Nematobothriinae gen. sp. and Caligus bonito were higher in fish from south-east Spain than in those from the Strait of Gibraltar. A comparison of the parasite fauna of A. rochei from the Mediterranean Sea with the published data on Auxis spp. from the Atlantic, Indian and Pacific Oceans revealed the closest similarity between the Mediterranean A. rochei and the Atlantic A. thazard.

Blood flukes of the genus Cardicola Short, 1953 are considered the most potentially pathogenic parasites in bluefin tuna cultures. Morphological study and genetic analyses of the ribosomal internal transcribed spacer ITS-2 and the mitochondrial cytochrome c oxidase 1 (cox1) gene fragments revealed the occurrence of four aporocotylid species (C. forsteri Cribb, Daintith & Munday, 2000, C. orientalis Ogawa, Tanaka, Sugihara & Takami, 2010, C. opisthorchis Ogawa, Ishimaru, Shirakashi, Takami & Grabner, 2011 and Cardicola sp.) in 421 Thunnus thynnus (L.) from the Western Mediterranean (274 fished from the wild and 147 from sea-cages). Cardicola opisthorchis was the most abundant species, with higher prevalence in the cage-reared fish than in those fished in the wild (21 vs 6%, p < 0.05). Adults of three species were recovered: C. forsteri from both gills and heart, C. opisthorchis from heart and C. orientalis from gills. The secondary gill lamellae were profusely infected by eggs of C. orientalis. A fourth species was found in four tunas, based on the molecular analyses of eggs apparently indistinguishable in size and shape from the eggs of C. orientalis. The findings provided evidence that infections with Cardicola spp. differed in relation to locality, host origin (wild vs cage-reared) and site of infection. It is necessary to estimate the possible different pathogenic effects of each species of Cardicola in order to take appropriate control measures.

Between 2008 and 2011, the head of 150 Euthynnus alletteratus (Osteichthyes: Scombridae) caught inshore off the southeastern Iberian coast (western Mediterranean Sea) were examined for parasites. Two monogeneans, four didymozoid trematodes and four copepods were found. Parasite abundance showed a positive relationship with the annual sea surface temperature, except for Pseudocycnus appendiculatus, but negative with the sea depth (Capsala manteri, Neonematobothrium cf. kawakawa and Caligus bonito). Prevalences and mean abundances differed significantly among sampling areas, except for C. manteri, Oesophagocystis sp. 2 and Ceratocolax euthynni, and sampling years (Melanocystis cf. kawakawa, N.cf. kawakawa, P. appendiculatus and Unicolax collateralis). Results indicate that the parasite abundances of E. alletteratus in the western Mediterranean Sea depend mainly on regional environmental variables, which can show interannual variations. The presence of pelagic parasites, i.e. didymozoids and P. appendiculatus, could indicate that E. alletteratus migrates between inshore and offshore pelagic domains. The different parasite faunas reported in E. alletteratus populations from the western Atlantic Ocean and the Mediterranean Sea appear to point out the geographical host isolation. These results suggest that E. alletteratus inhabiting the western Mediterranean Sea performs inshore-offshore small-scale migrations, and not transoceanic migrations between the western Atlantic Ocean and Mediterranean Sea.