Morphological and genetic differences among actinosporean stages of fish-parasitic myxosporeans (Myxozoa): difficulties of species identification

The occurrence and morphology of actinosporean stages of myxosporeans were studied at a fish farm and in the River Tisza in Hungary. The 43 samples sequenced belonged to 10 'genotypes', from which six were determined as new actinosporean types. Based on DNA sequence analysis, the assumed actinosporean developmental stages of four myxozoan species were identified. The raabeia type collected from the worm Branchiura sowerbyi was identified as Myxobolus cultus Yokoyama, Ogawa & Wakabyashi, 1995. This was the first occurrence of this parasite in Hungary. Aurantiactinomyxon type 'A' was assigned as a developmental stage of Thelohanellus hovorkai Achmerov, 1964, triactinomyxon type 'D' was identified as Myxobolus sp. from the fins of roach Rutilus rutilus, while the DNA sequence of the guyenotia type actinosporean was 99.9-100% identical with Sphaerospora sp. from the kidney tubules of goldfish Carassius auratus auratus. Partial 18S rDNA sequences of the myxosporeans Thelohanellus hovorkai, T. nikolskii Achmerov, 1955 and Myxobolus sp. from Rutilus rutilus were new additions to GenBank. The DNA sequence analysis revealed that, in the case of actinosporeans, different 'morphotypes' can belong to the same 'genotype'. This study confirmed that actinospore classification based solely on traditional morphological features may lead to false conclusions, thus sequence analysis of the 18S rDNA and/or other genes is recommended in species and type descriptions.