The resurrection of Myxobolus toyamai with a validation of a stunted polar capsule based on morphological evidence

• The presence of the small polar capsule of Myxobolus toyamai was investigated.
• Potassium hydroxide and drying resulted in the extrusion of the two polar filaments.
• Transmission electron microscopy revealed the filament coiled in the small capsule.
• The 18S rDNA of the present isolate was almost same as M. toyamai and Thelohanellus toyamai.
• M. toyamai is a valid scientific name and T. toyamai is a synonym of M. toyamai.

Myxobolus toyamai has long been known as a gill parasite of the common carp Cyprinus carpio in Europe, Asia and North America. Recently, a controversy was raised regarding whether this organism was synonymous with Thelohanellus toyamai because of ambiguity regarding the validity of the second stunted polar capsule. In the present study, detailed morphological examinations were conducted to reconfirm the presence of the small polar capsule of M. toyamai. The addition of potassium hydroxide or the application of drying treatment to fresh spores resulted in the extrusion of long and short polar filament from the spores. Transmission electron microscopy of M. toyamai plasmodia revealed that the polar filament coiled inside the small capsule. Molecular analysis of the 18S rDNA identified that the present myxosporean isolate was the same organism as the American and Japanese isolates that have previously been registered as T. toyamai and M. toyamai, respectively (99.5–99.8% similarities). These results demonstrate that the second small polar capsule was indeed present in the myxosporean isolate, which indicates that M. toyamai is a valid scientific name.

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