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We investigated the factors providing structure to the helminth communities of 182 loggerhead sea turtles, Caretta caretta, collected in 6 localities from Central and Western Mediterranean. Fifteen helminth taxa (10 digeneans, 4 nematodes and 1 acanthocephalan) were identified, of which 12 were specialist to marine turtles; very low numbers of immature individuals of 3 species typical from fish or cetaceans were also found. These observations confirm the hypothesis that phylogenetic factors restrict community composition to helminth species specific to marine turtles. There were significant community dissimilarities between turtles from different localities, the overall pattern being compatible with the hypothesis that parasite communities reflect the ontogenetic shift that juvenile loggerheads undergo from oceanic to neritic habitats. The smallest turtles at the putative oceanic, pelagic-feeding stage harboured only the 2 digenean species that were regionally the most frequent, i.e. Enodiotrema megachondrus and Calycodes anthos; the largest turtles at the putative neritic, bottom-feeding stage harboured 11 helminth taxa, including 3 nematode species that were rare or absent in turtles that fed partially on pelagic prey. Mean species richness per host was low (range: 1.60-1.89) and did not differ between localities. Variance ratio tests indicated independent colonization of each helminth species. Both features are expected in ectothermic and vagrant hosts living in the marine environment.

The blue runner Caranx crysos is a thermophilic species poorly investigated in the Mediterranean Sea. Although considered a typical inhabitant of the southern regions, it has recently established in northwest Sicily and, according to the data presented in this study, is now regularly caught in the central-southern Tyrrhenian Sea up to the Gulf of Naples. Some occurrences of the species were also documented in the Gulf of Taranto (Ionian Sea) and in more northerly localities of the Tyrrhenian Sea, such as Gaeta and Civitavecchia. These recent distributional records are in contrast with historical literature and museum collections which depict this species as very rare in the Italian seas and only sporadically occurring above the Strait of Sicily. In this paper, literature and museum collections were thoroughly investigated in order to obtain a satisfactory understanding of the species distribution in the Mediterranean. In addition, the hypothesis of a recent displacement of the northern limit of this species distributional range in the Mediterranean is discussed.

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