Hitching a ride across the continental divide of Mexico: the twospot livebearer Pseudoxiphophorus bimaculatus carried its Gyrodactylus spp. when translocated from the Atlantic to the Pacific Ocean versant.

We studied flatworm parasites of the genus Gyrodactylus associated with twospot livebearer Pseudoxiphophorus bimaculatus both in this host's native distribution in the Gulf of Mexico (Atlantic Ocean) versant, as well as in its invasive range across the continental divide, in the Pacific Ocean watershed. We surveyed 12 river basins, six on each slope of the continent, and recorded -apparently- overdispersed parasite distributions, both in terms of localities within basins and on fish hosts. Six gyrodactylid species were previously known to infect this poeciliid host, and we add five taxa to the list, three known and two undescribed species. From the 11 parasite species we characterized molecularly (cox 2 mDNA and ITS rDNA sequences), 9-10 species (depending on the markers used for species delimitation; cox 2 + ITS, nine species; cox 2, 10 species) were recorded in river basins draining into the Atlantic, and only four in rivers flowing into the Pacific. Only one species, Gyrodactylus takoke, was recorded frequently on both sides of the country, and cox 2 haplotype networks suggest that parasites co-translocated with their fish hosts originated from the central Gulf of Mexico slope and successfully invaded two river basins draining into the Pacific. We hypothesize that following the translocation of P. bimaculatus across the continental divide of Mexico, different types of host-parasite interactions (enemy release, spill-over and spill-back) may involve particular parasite species, some of which are known to be pathogenic.