Infection of Myxobolus turpisrotundus sp. n. in allogynogenetic gibel carp, Carassius auratus gibelio (Bloch), with revision of Myxobolus rotundus (s. l.) Nemeczek reported from C. auratus auratus (L.).

Infection of a Myxobolus species, previously identified as Myxobolus rotundus, was detected in 182 of 7892 (2.31%) allogynogenetic gibel carp, Carassius auratus gibelio, in a closed pond culture system in China. Morphological and molecular data showed that this myxosporean is a different species from M. rotundus parasitizing Abramis brama in Europe and is thus designated as a new species, Myxobolus turpisrotundus. M. rotundus (s.l.) ex C. auratusauratus is a synonym of M. turpisrotundus. Plasmodia of M. turpisrotundus develop in the subepidermal tissues of the body surface resulting in an unaesthetic appearance and causing severe economic losses. Prevalence of infection with the myxosporean plasmodia varied seasonally, increasing in winter and decreasing in spring. Prevalence was positively correlated to host size, but no host sex-specificity was found. No infection was observed in other fish species (grass carp, bighead carp and yellow catfish) reared in the same pond, suggesting that the parasite has a relatively strict host specificity. Plasmodia grew gradually as the parasite developed, and reached up to a maximum 5.6 mm in diameter. Plasmodia ruptured naturally to release the mature spores and host fish completely recovered with no mortality. Release of spores and regeneration of lesions were not correlated with water temperature. Histology showed that plasmodia developed sub-epidermally, and that the wall of the plasmodia was composed of a multiple complex structure, including layers of fibroblasts, a collagenous membrane, melanophores and a layer of cup-like cells of unknown derivation and function. The cup-like cells are in direct contact with pre-sporogonic stages located in the peripheral parts of the large plasmodia. No severe host inflammatory response was seen.