In the present study, the morphology and morphometric characterization of Heterobothrium lineatus, a monogenean gill parasite infecting the gills and wall of the bronchial cavity of the tiger pufferfish Tetraodon lineatus, were described by means of light and scanning electron microscopy for the first time from the River Nile at Qena Governorate, South Valley, Egypt. In wet mount preparation, the adult worms exhibited an elongated body with anterior pointed and posterior broad ends. The adult worm measured 1.15-1.76 mm (1.53 ± 0.2) in length and 0.30-0.42 mm (0.35 ± 0.02) in width. Light and scanning electron microscopic observations showed the presence of two buccal organs situated anteriorly around the mouth opening. The opisthohaptor was subdivided into four pairs of clamps but had no isthmus separating it from the body proper. The present Heterobothrium species differs from all other described species in the genus, by its lower dimensions of the worm measurements and the presence of a copulatory organ armed with 12-15 genital hooks. Furthermore, it is easily distinguished from Heterobothrium tetrodonis and Heterobothrium okamotoi by the absence of a distinct isthmus, and resembles Heterobothrium lamothei described from the gills of Sphoeroides testodineus in Mexico in its general appearance and the presence of rectangular haptor with the fourth pair of clamps smaller than the previous ones.

Emperor fishes, Lethrinus nebulosus are commercially important fishes in Saudi market. Despite of importance of these fishes, there is not a comprehensive report about parasitic fauna of these fishes. The aim of this research article is to determine the parasitic helminthes infecting these fishes in Saudi Arabia. For this purpose, 28 emperor fishes have been bought and examined by routine parasitological methods to find that 28.6% emperor fishes revealed intestinal trematode infections. Two trematodes were recognized in the fishes, Neolepidapedon (Neolepidapedon) macrum Overstreet, 1969 was found in 7 fishes (25%) and Opisthomonorcheides decapteri Parukhin, 1966 was found infecting one fish only. Scanning electron microscope had shown absence of spines and revealed scale-like structure. Saudi Arabia is considered a new locality of those two trematodes and Emperor fishes as new hosts.