Biology and histopathology of Thelohanellus nikolskii Achmerov, 1955 (Myxosporea, Myxozoa), a protozoan parasite of the common carp (Cyprinus carpio).

Thelohanellus nikolskii, a myxosporean originating from the Far East, arrived in Hungary through natural waters from neighbouring countries which it had originally reached because of the introduction of the Amur wild carp. T. nikolskii is a specific parasite of carp which produces large cysts on the fins of fry. The cysts usually appear on the fish at the beginning of July, and in August and September cysts containing the mature spores burst. The cysts are easily seen even by the naked eye and are surrounded by a thick capsule composed of connective tissue strengthened by cartilaginous elements. Due to the breaking down of the fins the parasite has pathological significance in fry.