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.

Oligochaetes living in the Mijares River close to the Sitjar reservoir, and polychaetes from the brackish and marine waters of a channel flowing into the Mediterranean Sea, both in the province of Castellón (Spain), were examined for the presence of actinosporea. An aurantiactinomyxon was isolated from 60 specimens of the oligochaete Branchiura sowerbyi collected from the river, but no actinospores were isolated from 160 polychaetes collected from the sea channel. The aurantiactinospores were detected by the cell-well plate method. The detected species are not identical with any of the aurantiactinomyxon forms hitherto described in the literature. This is the first report on the occurrence of an actinosporea in Spanish waters.

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