RMA Khalifa, HA Hassan and HS Mohamadain
Three different common species of Rabbitfishes in the Red Sea region were found to be naturally infected by Hexangium sigani Goto and Ozaki, 1929. The encountered parasites were described morphologically and morphometrically by means of light and scanning electron microscopy. Present specimens presented and exhibited a wide range of variability inside the same host and at same locality and accordingly all previous synonyms of Hexangium sigani were presented, shown here and discussed with the previously described forms. These variations included testes position relative to each other and relative to ovary, body spination and uterus extension but these differences were considered to be of minor importance. The SEM disclosed well differentiated three forms of sensory papillae; oral papillae, genital papillae and body papillae which may reflect a variation in the functions they performed. Furthermore, the true nature of male genital system in all Hexangium spp. were reviewed and elucidated the absence of cirrus sac in all known species and probably some fibrous tissues may be around the seminal vesicle. Also, Key to species of Hexangium Goto and Ozaki, 1929 was added. Molecular data characterized Hexangium sigani within Microscaphidiidae and referred to an interrelationship between Microscaphidiidae & Mesometridae which in need to more future analyses to give a deeper understanding. It is worth mentioning that SEM study of this parasite was done for the first time from Egypt with an addition of many ultrastructural details; most of which are of taxonomical importance. For the first time, Siganus luridus represented a new host record of H. sigani.