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Abstract This study presents the first helminthological investigation of Piabina argentea from the Pardo River Basin, Brazil, revealing a novel parasite-host system involving Hysterothylacium sp. larvae and a previously uncharacterized South American freshwater raphidascaridid lineage. Molecular phylogenetic analysis based on 18S, ITS1, 5.8S, ITS2, and COXII, demonstrated the non-monophyly of Hysterothylacium , supporting multiple independent marine-to-freshwater incursions. This underscores the genus’ taxonomic complexity, as morphologically similar larvae can be molecularly distinct, highlighting the urgent need to sequence the type species, Hysterothylacium brachyurum , a freshwater lineage for which genetic data are currently unavailable. Although fishes have been collected in two localities, host size and water quality did not explain the exclusive presence of Hysterothylacium sp. in hosts from the Hortelã River. Fish size did not differ significantly, and water quality indices were remarkably similar between the two sampling sites. Instead, distinct landscape features provided critical insights: the Hortelã River site exhibited significantly greater soil cover by native vegetation (37.9%) and less area for agricultural activity (42%) compared to the Pardo River locality (18.7% native, 74% agricultural). The GLMs revealed a nuanced infection pattern: while larger fish generally harbored more larvae, lighter individuals within larger size classes carried higher parasite infection, suggesting impaired host condition. Given the importance of allochthonous food sources for P. argentea and its tendency towards herbivory in larger individuals, we hypothesize that increased allochthonous inputs in vegetated areas benefit microcrustacean intermediate hosts, leading to incidental P. argentea infection through herbivory. This research emphasizes the intricate interplay of host-parasite dynamics and local environmental context.
Canal et al. (Tue,) studied this question.