Species diversity of the genus Thelohanellus Kudo, 1933 (Myxozoa: Bivalvulida) parasitizing fishes in Indian subcontinent.

A synopsis of 52 nominal species of the genus Thelohanellus Kudo, 1933 reported to infect freshwater fishes in India is presented. These species infect various organs such as gills, fins, scales, skin, muscles, gall bladder, kidney etc. The diagnostic features of the genus include tear-shaped myxospores with smooth shell valves and single polar capsule. The members are histozoic in their nature of parasitism infecting fresh water fishes. The present manuscript is aimed to compile the list of reported species in India along with essential information on morphometrics of vegetative stage, host, organ preference and pathogenic effects if any. Maximum number of species are recorded from state of West Bengal (25) followed by 23 in Punjab (out of which 8 were already reported from other states of India), Andhra Pradesh (7), Orissa (3), Manipur (1) and Haryana (1). Majority of the species have been recorded to infect major and minor carps (43 species) and 9 species in cat fishes. Out of 43 species infecting Indian major carps, T. batae infected the epithelial lining of gill filament of Labeo bata and also the wall of the duodenum and pectoral fin of Wallago attu in Harike Wetland (Punjab). Organ distribution of the species belonging to the genus Thelohanellus indicated that the plasmodia of 27 species were located in the gills, 14 species in the fins, 7 species in the gall bladder, 4 species in muscles and 3 species in kidney, 2 each in scale and skin on the head region and 1 species each in brain, eye, spleen, liver and wall of the duodenum. Recent studies conducted by the authors and also by few earlier studies in West Bengal have documented many species of Thelohanellus to cause serious gill haemorrhagic disease and dermal lesions. Histological studies on the gills of cultured carps in Punjab infected with members of Thelohanellus have indicated that these parasites cause 15-20 % damage to respiratory surface leading to localized loss of function.