engleski

Molecular evidence for cosmopolitan distribution of platyhelminth parasites of tunas (Thunnus spp.)

We analysed molecular variation in three taxa of platyhelminths (digeneans, polyopisthocotyleans and monopisthocotyleans) infecting four species of tuna (Thunnus spp.): T. albacares from Geraldton, Western Australia, T. maccoyii from Port Lincoln and Cabbage Patch, South Australia, T. orientalis from Islas Coronados, Pacific Mexico, and T. thynnus from the Mediterranean (Puerto de Mazarron, Spain ; and Bay of Maslinova, Croatia). We found, through comparisons of ITS2 and partial 28S rDNA, two species of Cardicola (Digenea: Sanguinicolidae) from multiple hosts and localities: C. forsteri from T. maccoyii (South Australia) and T. thynnus (Spain) and C. sp. from T. orientalis (Mexico) and T. thynnus (Spain) ; and Hexostoma thynni (Polyopisthocotylea: Hexostomatidae) from T. albacares (Western Australia), T. maccoyii (South Australia) and T. thynnus (Croatia). Partial 28S rDNA indicates that Capsala sp. (Monopisthocotylea: Capsalidae) occurs on both T. maccoyii (South Australia) and T. orientalis (Mexico). Bayesian inference analysis suggests the two Cardicola spp. from Thunnus spp. to be sister taxa, to the exclusion of other Cardicola spp., while Hexostoma thynni appears to be distinct from other polyopisthocotyleans analysed to date, and most closely related to representatives of 2 taxa possessing multiple clamps (Heteraxinidae and Microcotylidae) rather than to 3 other taxa also bearing four pairs of clamps (Diclidophoridae, Mazocraeidae, Allodiscocotylidae). This is, to the best of our knowledge, the first report of conspecific platyhelminth parasites in a group of teleosts with a wide-ranging geographical distribution that has been confirmed through molecular approaches. We conclude that the only feasible hypothesis for the cosmopolitan distribution of Hexostoma thynni and Capsala sp. is the migrations of the host tunas themselves. Host migration also seems likely to be responsible for the widespread occurrence of Cardicola spp., although it is also possible that these were translocated recently as a result of the spread of infected intermediate hosts (bivalves or polychaetes) by international shipping, either as biofouling attached to hulls, or in ballast waters.

Thunnus; Digenea; Monogenea; ITS2; 28S; rDNA; Biogeography; Dispersal

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