The silverside Odontesthes argentinensis is commonly reported as a euryhaline species, capable of using estuaries as nurseries and/or feeding grounds. However, in Mar Chiquita Coastal Lagoon system (MCH), northern Argentine Sea, substantial genetic differences have been observed between estuarine and adjacent marine populations of this species, despite no geographic barriers separate them. This, in addition to their reproductive isolation, rendered this population a considerable candidate for a marine to freshwater incipient speciation event. Here we combined evidence from fish meristic analysis and parasite ecology to assess if, 1) the structure and composition of parasite assemblages of O. argentinensis are indicators of isolation between populations; 2) a differential use of the lagoon and its freshwater tributaries by O. argentinensis is reflected in their parasite communities, so that it could indicate incipient processes of colonization of freshwater habitats by the estuarine population. Twenty-three species were found parasitizing this host in MCH, whose assemblages evidenced qualitative and quantitative differences regarding those of the neighboring marine waters. Thus, the present study demonstrates the usefulness of parasites as indicators of population differences, revealing the same two evolutionary units previously recognized by more traditional methods (genetics, morphometry, reproductive biology). Consequently, the results support the ongoing speciation process proposed for O. argentinensis in MCH. The homogeneity of the parasite assemblages in these fish from estuarine and freshwater habitatshighlights the capability of this species to colonize new environments. As selective agents of host evolution, parasites can initiate, facilitate and promote adaptive differentiation, triggered by other factors, of certain traits in host populations. Therefore, the high degree of differentiation here observed in the parasite assemblages between marine and estuarine populations of O. argentinensis proves their utility as indicators of host population structure and its ongoing incipient speciation process in MCH at the present. Furthermore, differential parasite faunas in each environment could also play a role in this process in evolutionary time.
