Involvement of aurantiactinomyxon in the life cycle of Thelohanellus testudineus (Cnidaria: Myxosporea) from allogynogenetic gibel carp Carassius auratus gibelio, with morphological, ultrastructural, and molecular analysis.

During the investigation of actinosporean fauna diversity from commercial fish ponds in Hubei Province, China, a novel aurantiactinomyxon type was found from Branchiura sowerbyi. Spore body of the aurantiactinomyxon was ellipsoidal in side view and triangular in apical view, 15.5 ± 0.5 (14.5-16.4) μm in diameter; three leaf-like caudal processes were approximately equal, measuring 13.2 ± 0.9 (11.5-16.2) μm long and 7.4 ± 0.4 (6.7-8.0) μm wide at the base; three polar capsules were located at the apex of spore body, globular in apical view, 2.2 ± 0.1 (2.0-2.3) μm in diameter, and pyriform in side view, 2.5 ± 0.2 (2.3-2.9) μm in length and 2.0 ± 0.2 (1.8-2.4) μm in width; a total of 32 germ cells were observed within the sporoplasm. Ultrastructural analysis showed that the development was asynchronous between pansporocysts but synchronous within a pansporocyst. The formation of sporoblast and the development of sporogonic stage were also described and discussed. The 18S ribosomal DNA sequences of the current aurantiactinomyxon type corresponded to that of a previously reported Thelohanellus testudineus, suggesting that the newly identified aurantiactinomyxon type is the actinosporean stage in the life cycle of T. testudineus.