Morphological and molecular characterization of nematodes in a freshwater fish community from La Mojana, Sucre, Colombia

Abstract Nematodes constitute one of the most diverse and ecologically significant parasite groups in fishes, yet their diversity and host associations remain poorly documented in tropical freshwater systems. In this study, a community-wide survey of 10 fish species from La Mojana wetland was conducted to assess nematode diversity and host associations. The highest infection indices were recorded in Pseudoplatystoma magdaleniatum (Pimelodidae), Sorubim cuspicaudus (Sciaenidae) and Sternopygus macrurus (Sternopygidae). Morphological examination revealed 2 distinct nematode morphotypes corresponding to larval Contracaecum (Anisakidae) and adult camallanids. Molecular analyses based on ITS2, 18S rRNA, 28S rRNA and mitochondrial rrnS markers resolved well-supported lineages corresponding to Contracaecum jorgei Sardella, Mancini, Salinas, Simões & Luque, 2020 and Spirocamallanus sp. The multilocus approach demonstrated the coexistence of phylogenetically distinct nematode groups within the same fish community. Patterns of parasite association revealed a broader host distribution for Contracaecum , whereas Spirocamallanus showed more restricted occurrence. Coinfections reflected the structural complexity of parasite communities in this tropical system. Because fish represent a primary protein source for local communities in La Mojana, the detection of anisakids raises considerations for food safety and public health. This study provides the first multilocus characterization of nematode diversity and host associations in the region, contributing to the understanding of parasite ecology in tropical freshwater ecosystems.