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Trichodinids are one of the main groups of fish parasites. These ciliates are pathogenic, causing damage to skin and gills, often leading to the death of the infested fish. Trichodinid ciliophorans are still a poorly studied group in India. A detailed account is given about the infestation and identification of trichodinids from the skin and gills of two economically important fishes in India, Rastrelliger. kanagurta Cuvier, 1816 and Oreochromis mossambicus Peters, 1852. Dried slides were prepared from the smears and impregnated with silver nitrate (2%). Morphometric characteristics were made, and schematic drawings of the denticles were prepared using photomicrographs produced from the slides. O. mossambicus was found positive for two species, viz., Trichodina magna Van As and Basson, 1989 and Paratrichodina africana Kazubski and El-Tantawy, 1986. R. kanagurta was found to be infested with Paratrichodina sp. Lom, 1963. Full descriptions of collected species are given. Seasonal variations in the rate of parasite infestation were observed in T. magna and P. africana from O. mossambicus. These parasites showed a significant increase during post-monsoon while a decrease during monsoon. No seasonal variation was observed in Paratrichodina infestation in R. kanagurta. The infection induced excessive mucus secretion, paleness in gills, and multifocal whitish areas and lesions that probably permitted entry of opportunistic bacteria, which eventually caused ulcers and death. The present study reports the first record of a trichodinid sp. on R. kanagurta and T. magna on O. mossambicus.
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