A new species of Myxobolus (Myxosporea: Myxobolidae) infecting the gills of Pellona castelnaeana (Clupeiformes: Pristigasteridae) in the Brazilian Amazon region.

The genus Myxobolus comprises cnidarians belonging to the subphylum Myxozoa. These parasites have great pathogenic potential in animals, mainly in teleost fish worldwide, where studies have demonstrated great diversity, but with a high possibility of novelty in the face of the existing ichthyofauna. In this context, the present work characterizes morpho-molecularly Myxobolus cajueiroi n. sp. infecting the gills of the estuarine species of great socioeconomic importance Pellona castelnaeana from the district of Mosqueiro, Belém, Pará, Brazilian Amazon. Thirty specimens captured in the Cajueiro River between April 2024 and May 2025 were analyzed. Whitish cysts were observed in the gill arches in all specimens (prevalence of 100%). In the inner region of the cysts, ovoid myxospores measuring 10.6 ± 0.5 (10.3-11.3) μm in length by 8.0 ± 0.4 (7.6-8.4) μm in width, and 5.7 ± 0.4 (5.3-6.1) μm in thickness were observed, with two symmetrical pear-shaped polar capsules, each measuring 4.0 ± 0.3 (3.8-4.6) μm in length and 2.3 ± 0.1 (2.3-2.3) μm in width. Histopathology revealed significant lesions, such as hyperplasia and the formation of fibrous capsules around the cysts. Molecular identification was performed by amplification and sequencing of small subunit ribosomal RNA gene, obtaining a nucleotide sequence of 1935 base pairs. These results allowed the description of Myxobolus cajueiroi n. sp. and reinforce the relevance of integrative studies in characterizing parasites in commercially important fish, in addition to providing new data on parasitic diversity in the Amazonian ichthyofauna.