A new species of the genus Henneguya Thélohan, 1892 (Myxozoa: Myxobolidae) parasitizing the gill lamellae of the giant gourami Osphronemus goramy Lacepède (Anabantiformes: Osphronemidae) from the Mekong River Delta, Vietnam.

During a survey of myxosporean parasites infecting freshwater fishes in the Mekong River Delta, Vietnam, myxospores of the genus Henneguya were discovered in the gills of the giant gourami Osphronemus goramy Lacepède (Anabantiformes: Osphronemidae). White, spherical plasmodia were found within the gill lamellae of 10 out of 25 examined fish (40% prevalence) and measured 650-950 μm in diameter. Mature myxospores are elongate with two equal caudal appendages, and measure 68.5 ± 3.4 (64.8-79.4) μm in total length. The spore body measures 16.3 ± 0.9 (15.0-18.1) μm in length, 5.6 ± 0.3 (5.1-6.0) μm in width, and 4.8 ± 0.2 (4.5-5.0) μm in thickness. Each caudal appendage is 51.6 ± 3.3 (46.5-58.9) μm in length. The spore wall is smooth, and two equal ovoid nematocysts, each 6.1 ± 0.3 (5.6-6.6) μm long and 1.6 ± 0.2 (1.3-1.8) μm wide, are located anteriorly. Histological sections revealed no inflammatory response but showed local deformation of the gill lamellae at infection sites. Phylogenetic analysis of the SSU rDNA sequences placed this species in a distinct clade closely related to Myxobolus neurophilus, Myxobolus pharyngeus, and Henneguya voronini. Based on morphological and molecular evidence, the parasite is described herein as a new species, Henneguya luci n. sp.