Supplemental description of Thelohanellus wuhanensis Xiao & Chen, 1993 (Myxozoa: Myxosporea) infecting the skin of Carassius auratus gibelio (Bloch): ultrastructural and histological data.

Thelohanellus wuhanensis Xiao & Chen, 1993 infects allogynogenetic gibel carp Carassius auratus gibelio (Bloch) forming numerous plasmodia in the skin, resulting in high mortality of juvenile host fish. Within this paper we supplement the identification of this pathogenic species with information on its ultrastructural and histological characteristics. Measurements of the spores corresponded to the original description, but asymmetry in the spore shape was recorded. Scanning electron microscopy revealed that the spore surface was smooth and the sutural ridge of the anterior end was asymmetric as one side was thinner. In addition, the sutural line was either straight or "S" like. Transmission electron microscopy showed that most spores were surrounded by a membranous sheath, while some sheaths were difficult to observe under light microscopy as the closeness of the sheath to the valves. The small electron-dense granules in the sporoplasm ranged from 0.66 to 1.14 μm in diameter, but their function was unknown. Histology showed that the plasmodia of T. wuhanensis developed in the stratum spongiosum of skin dermis. The presence of melanocytes around the plasmodia was responsible for the black spots on T. wuhanensis cyst. The plasmodia were curved, perhaps to expand the surface area in contact with adjacent host tissue, facilitating the attainment of nutrients from the host.