Abstract
Myxobolus is one of the most speciose genera of myxozoans, most species are found infecting freshwater fish. Although the absence of caudal appendages is one of the main taxonomic criteria to differentiate this genus from other species, in some cases anomalous forms of Myxobolus are observed, with the presence of caudal appendages. The Brazilian Amazon contains a diverse fish population, which includes a large number of Myxobolus species, some of which are pathogenic. Pellona castelnaeana (Valenciennes, 1847) is a species with huge migratory flows that contributes for the population's diet food. This study reports the observation of Myxobolus, including aberrant forms, on the gills of P. castelnaeana, a freshwater fish. Gill arch cysts with myxospores were found in 90% of the specimens analyzed, with 5% having elongated caudal appendages ranging in size from 4 to 16 µm. This is the first report of Myxobolus Abnormal forms in Clupeiformes, adding to our understanding of the genus Myxobolus phenotypic plasticity in South American fish.