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.

Myxobolus pseudodispar (Gorbunova) is a common parasite of the muscle of roach, Rutilus rutilus L., whereas its actinosporean development occurs in two oligochaete alternate hosts. This paper reports the complete developmental cycle of this parasite in the oligochaete alternate host Tubifex tubifex and the roach. In laboratory experiments, parasite‐free T. tubifex specimens were infected by myxospores of M. pseudodispar collected from roach in Lake Balaton. Parasite‐free roach fingerlings were infected with floating triactinospores (TAMs) released from oligochaetes on day 69 after challenge. Young plasmodia and spores in roach were first recorded on day 80 post‐exposure (p.e.). Myxospores collected from experimentally infected roach initiated a new development in T. tubifex and the resulting TAMs infected roach. No infection of roach resulted from feeding oligochaetes containing mature triactinospores.

The development of Myxobolus macrocapsularis Reuss, 1906, a myxosporean parasite of the gills of common bream Abramis brama L., was studied in experimentally infected oligochaetes. In 3 experiments uninfected Tubifex tubifex Müller and Limnodrilus hoffmeisteri (Claparéde) were exposed to mature myxospores of M. macrocapsularis. In all experiments, typical triactinospores developed in T. tubifex specimens but no infection was found in L. hoffmeisteri. Triactinospores were released from oligochaetes 66 to 99 d after initial exposure. At that time pansporocysts containing 8 triactinospores were located in the gut epithelium of experimental oligochaetes, but free actinosporean stages were also found in the gut lumen of the oligochaetes. Each triactinospore had 3 pyriform polar capsules and a barrel-shaped sporoplasm with 32 secondary cells. The spore body joined the 3 caudal projections with a stout style.