Abnormal forms of sp. (Myxosporea: Bivalvulida) spores with caudal appendages in (Teleostei: Clupeiformes) from Marajó island, Brazil.

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 are observed, with the presence of caudal appendages. The Brazilian Amazon contains a diverse fish population, which includes a large number of species, some of which are pathogenic. (Valenciennes, 1847) is a species with huge migratory flows that contributes for the population's diet food. This study reports the observation of , including aberrant forms, on the gills of , 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 Abnormal forms in Clupeiformes, adding to our understanding of the genus phenotypic plasticity in South American fish.