Morphological and molecular characterization of a novel Spirurina species (Nematoda: Spirurida) from Brachyplatystoma platynema (Siluriformes: Pimelodidae) in the southern Peruvian Amazon.

Larval nematodes are frequently overlooked in parasitological surveys of Neotropical freshwater fishes due to their limited morphological differentiation and the scarcity of molecular references. As a result, their diversity and life cycles remain poorly understood. In this study, we provide the first record and integrative characterization of a spirurine nematode larva infecting the slobbering catfish Brachyplatystoma platynema Boulenger, 1898 (Siluriformes: Pimelodidae) from the southern Peruvian Amazon. A total of 135 larvae were recovered. Molecular analyses based on the 18S ribosomal RNA gene yielded three sequences forming a highly supported monophyletic lineage within the suborder Spirurina. This group was recovered as a sister to members of Habronematoidea (Cyrnea) and an unidentified Spirurina lineage. Genetic distances and phylogenetic placements indicate that the material examined does not correspond to any previously characterized species, suggesting the presence of an undescribed taxon. The detection of heavily encysted larvae in B. platynema suggests that this species may function as an intermediate or paratenic host in local transmission cycles, likely bridging aquatic prey and piscivorous birds or mammals. This study expands the known parasite fauna of B. platynema, previously lacking any parasite records, and underscores the importance of integrative approaches for uncovering hidden diversity of larval spirurines in Amazonian fishes.