This study used light and electron microscopy to describe a myxosporean, polysporic, histozoic plasmodium infecting the gill filaments of the freshwater teleost, Semaprochilodus insignis, specimens of which were collected from the Trombetas River (Central Amazonian Region, Brazil). Ultrastructural analyses of the fish-infecting spores identified the parasite as Myxobolus insignis, an organism that occurs within whitish unequal-sized plasmodia located in the intralamellar epithelium of the gill. Based on the observed morphological and ultrastructural features of the plasmodia in this study three stages in the plasmodial evolution were distinguished, related to the sporogonic stages of Myxobolus insignis. The plasmodium walls were also found to constitute a number of layers of fibroblasts, surrounded by collagen fibres, which displayed different morphological arrangements according to the different phases of evolution. This represents the first time such ultrastructural features have been described in detail for Myxobolus insignis plasmodia and offers potentially significant points of comparison with plasmodia from other species of myxosporea.
