Morphological and molecular characterization of Eustrongylides spp. (Nematoda, Dioctophymatidae) parasitizing Synbranchus marmoratus

This study reports the morphological and molecular characterisation of nematode larvae of the genus Eustrongylides parasitising Synbranchus marmoratus (Synbranchidae), a freshwater fish species widely used as live bait in the Pantanal region of Mato Grosso do Sul, Brazil. A total of 40 fish specimens were obtained from a local bait trade in Campo Grande municipality. After dissection and examination, nematodes were collected, fixed and processed for morphological and scanning electron microscopy. Morphological features identified the larvae as Eustrongylides spp. characterised by a finely striated cuticle, rounded extremities, and cephalic papillae arranged in two concentric circles. Molecular analysis of the 18S rRNA gene (871 bp) revealed 99.77% identity with Eustrongylides sp. sequences available in GenBank. To the best of our knowledge, this represents the first molecular record of Eustrongylides from the Neotropical region. However, due to the conserved nature of 18S rRNA and the limited availability of well-characterised reference sequences, a definitive species-level identification was not possible, and the specimens are therefore conservatively identified as Eustrongylides spp. The behaviour and habitat of S. marmoratus, including its tolerance to hypoxic environments and exposure to piscivorous birds, likely facilitate its role as an intermediate host. These findings highlight the need for further research on the diversity and life cycle of Eustrongylides, especially given the scarcity of integrative taxonomic studies combining morphology and molecular data in this group.