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.

银边的Odontesthes argentinensis通常被报道为一种淡紫色的物种，能够将河口用作苗圃和/或觅食地。然而，在阿根廷北部海马基基塔海岸泻湖系统（MCH）中，尽管没有地理障碍，但该物种的河口和邻近海洋种群之间已经观察到了巨大的遗传差异。除繁殖隔离之外，这使该种群成为海洋到淡水初期物种形成事件的重要候选者。在这里，我们结合了鱼类综合分析和寄生虫生态学的证据，以评估是否：1）阿根廷O. argentinensis的寄生虫组合的结构和组成是人口之间隔离的指标；2）阿根廷O.对泻湖及其淡水支流的不同利用反映在它们的寄生虫群落中，因此可以表明河口种群对淡水生境定殖的初期过程。在MCH中发现了23种寄生该寄主的物种，这些物种的组合证明了与邻国海水有关的质量和数量上的差异。因此，本研究证明了寄生虫作为种群差异指标的有用性，揭示了先前被更传统的方法（遗传学，形态学，生殖生物学）认可的相同的两个进化单位。因此，结果支持了正在进行的针对O. argentinensis在MCH。这些来自河口和淡水生境的鱼类中的寄生虫组合的同质性突显了该物种在新环境中定殖的能力。作为宿主进化的选择性媒介，寄生虫可以引发，促进和促进由其他因素触发的宿主种群某些性状的适应性分化。因此，这里在阿根廷和阿根廷的海洋和河口种群的寄生虫组合中观察到的高度分化证明了它们作为宿主种群结构的指标及其目前在妇幼卫生中正在进行的初期物种形成过程的效用。此外，每个环境中不同的寄生虫动物区系也可能在进化过程中扮演这个角色。