There is uncertainty regarding the validity of Myxobolus heterosporus Baker, 1963. The present study revises the taxonomy, using specimens isolated from plasmodia situated in the infected cornea of Oreochromis aureus, O. niloticus or Tilapia zillii inhabiting the River Nile, Egypt. In addition, histological effects of the parasite on the infected tissue were examined. The spores of M. heterosporus had a variety of shapes expressing remarkable heteromorphism. Five main Myxobolus-like spore types and tailed-spores were found. All forms were photographed, measured, sketched and described. Light and electron microscopy supported that spores of a Myxobolus-like morphology co-existed with so-called tailed-spores in one plasmodium. Some transitional stages from Myxobolus-like spore types to tailed-spores were observed. Therefore, some tailed-spores may be simply heteromorphs of Myxobolus.
KEY WORDS: Myxosporea · Revision · Myxobolus heterosporus · Heteromorphism · Plasmodium · Cornea · Oreochromis aureus · O. niloticus · Tilapia zillii · River NileResale or republication not permitted without written consent of the publisher Dis Aquat Org 63: 205-214, 2005 (2) Baker described Myxobolus heterosporus from what he called trophozoites and spores adjacent to blood capillaries and sinusoids of liver, spleen and from the kidney. In addition, Baker (1963) and Landsberg (1985) reported an absence of cysts in spleen and kidney where melano-macrophage centers were accumulation sites for spores. The variation between the 3 spore types of M. heterosporus and the inadequate description of the trophozoites led Landsberg (1985) to assume that Baker probably confused melano-macrophage centers and trophozoites. In addition, Landsberg (1985) reported that the spore formation occurs elsewhere and that spleen and kidney might act as concentrating areas for scattered spores, thus possibly providing useful knowledge of myxosporean infections. Dyková (1982) suggested that melano-macrophages are able to transport mature spores to the melano-macrophage centers where they are destroyed. Therefore, the myxosporean parasite should be described from its original plasmodium (Lom & Arthur 1989, Molnár 1994. This was discussed by Landsberg (1985), Hegazy (1999) and El-Mansy & Abdel-Ghaffar (2003), who reported that various spores may come from their original plasmodia to the kidney via blood. In this case, the kidney infection may be used as diagnostic evidence for the presence of myxosporeans but could not be used as taxonomic evidence, and further studies to detect their exact original sites is necessary.(3) Unclear drawings and the lack of some important characters such as the number of polar filament coils led to an incomplete description. Therefore, the species described by Baker should be considered as of uncertain taxonomic status. Lom (1969) considered the number of polar filament coils an important taxonomic character.(4) In addition, all descriptions and dimensions of spores were based on fixed material which led to atypical for...