Agradecimentos: We thank Nadayca Mateussi for help identifying the fish host and Juliana Naldoni for assisting in Transmission Eletron Microscopy methodology. The authors thank the fishermen Fernando Dias de Souza, Francisco dos Santos Pinto and Arlindo Teixeira Guimarães for their local knowledge of fish and providing study material from the Amazon and Tapajós rivers. This study was partially supported by regular project from São Paulo Research Foundation (FAPESP) (grants # 2018/24980-8 and 2019/17427-3) and by the Coordination for the Improvement of Higher Education Personnel (CAPES), Brazil, Finance Code 001. M.I. Muller was supported by postdoctoral and BEPE scholarships from the São Paulo Research Foundation (FAPESP) (grants # 2017/16546–3 and 2021/06692-8, respectively). E.A. Adriano received a research productivity grant from the Brazilian Fostering Agency CNPq (grant # 304687/2020-0) Ver menos

Abstract: We used a combination of morphological, molecular and biological data to characterize a novel Henneguya (Myxozoa) species infecting the Amazonian prochilodontid Semaprochilodus insignis or "kissing prochilodus", a popular food fish and aquarium species in the Brazilian Amazon. Twenty-one S. insignis were caught live from the Tapajós river, Pará State, Brazil, then examined for myxozoan infections. Cysts of a novel Henneguya species were observed in the connective tissue of the fins. Myxospores measured 48 ± 4.9 (39.5–60.8) µm total length, of which caudal appendages were 33 ± 4.5 (26.4–45.2) µm and spore body was 15 ± 1.6 (12.4–20.5) µm. The spore body was 4.0 ± 0.6 (2.7–5.3) µm wide × 3.2 ± 0.4 (2.7–3.6) µm thick, with two unequal polar capsules (nematocysts) 7.2 ±0.8 (5.2–8.3) × 1.5 ± 0.3 (1.0–2.2) µm for the larger capsule and 5 ± 0.7 (4.0–6.3) × 1.4 ± 0.2 (1.0–1.8) µm for the smaller capsule. Polar tubules had 8–13 turns. Generative cells, immature and mature myxospores were observed within plasmodia. Ultrastructure showed plasmodia surrounded by collagen fibers, with the plasmodial membrane having pinocytotic channels. Phylogenetic analysis of small subunit ribosomal DNA sequences showed that the new Henneguya species clustered as a sister taxon to Henneguya tietensis, a parasite of the gills of the prochilodontid fish Prochilodus lineatus, from the geographically distant Paraná–Paraguai River basin Ver menos