Molecular and morphological identification of Cardicola (Trematoda: Aporocotylidae) eggs in hatchery-reared and migratory Atlantic bluefin tuna (Thunnus thynnus L.)

Abstract

A microscopic and a molecular approach was used to investigate blood fluke infection in the very first specimens of Atlantic bluefin tuna (Thunnus thynnus, Scombridae) that were born in captivity and that were never moved from their specific cage culture sites in the Mediterranean. Data were compared with infection in wild, migratory bluefin tuna which were captured and fattened along the Spanish Mediterranean coast. Wild and cultured pop ulations sampled in the present study harbored the same Cardicola spp. richness and showed statistically insig nificant differences in overall parasite prevalence. Three morphotypes, crescent-shaped, fusiform and oval eggs, were observed in both non-migrant and migrant tuna. According to partial ITS2 and 28S rDNA sequences they belong to Cardicola opisthorchis, Cardicola forsteri, and to Cardicola sp., a recently described species, phyloge netically closely related to Cardicola orientalis. All three species were simultaneously present in some specimens. No significant difference in overall parasite prevalence was observed, though the prevalence of eggs of individual species, i.e. C. opisthorchis and C. forsteri, was significantly higher in fattened bluefin tuna. Our results strongly support a Mediterranean origin of the three Cardicola species encountered in cultured specimens and suggest that the intermediate hosts of these Cardicola species occur along the Mediterranean coast. Thus, infection with these parasites is not exclusively a result of infections happening along the migratory routes of wild bluefin tuna, in the Atlantic Ocean, as previously suggested. Once transferred to the sea for fattening, Atlantic bluefin tuna are at risk of infection with up to three different species of the genus Cardicola in along the Mediterranean coast in southeast Spain. On the contrary, C. orientalis infections might be associated to long-distance migration and an overlapping Thunnus maccoyii and Thunnus orientalis distribution. Statement of relevance This is the very first parasitological study conducted in T. thynnus born in captivity and cultured local in the Mediterranean. The results show that once transferred to the sea for fattening, they are at risk of infection with up to three different species of the genus Cardicola. These results strongly imply a Mediterranean distribution of these species. © 2015 Elsevier B.V. All rights reserved.