Morphology and morphometry of Pseudoproleptus sp. (Nematoda: Cystidicolidae) parasitic on amazonian fish.

Nematodes of the family Cystidicolidae Skryabin, 1946 represent a complex group of parasites of aquatic organisms. They are present in freshwater, estuarine, and marine environments, with fish as intermediate, paratenic, and definitive hosts. This study adds relevant data on the morphology, morphometry, and parasitic ecology of third-stage larvae of Pseudoproleptus Khera, 1953 in fish consumed by riverside populations in the Amazon. A total of 92 fish specimens were acquired between March 2024 and January 2025 from artisanal fishing, distributed among five species: Astronotus ocellatus; Cichla ocellaris; Cynoscion microlepidotus; Gymnotus carapo; and Hoplerythrinus unitaeniatus. Nematodes were recovered free and encysted in the visceral peritoneum, coelomic cavity, and lumen. The parasites collected had morphological characteristics compatible with the genus Pseudoproleptus. This is the first record of Pseudoproleptus infection in Gymnotus carapo and has provided new data on the biodiversity of larvae that parasitize fish in the Marajó region, Pará, Brazilian Amazon.