Spermiogenesis in Helicometra fasciata is characterized by an unusual flagellar rotation. Indeed, a rotation of the axonemes, higher than 90°, occurs. Nevertheless, the general pattern is the same as in the other digenean trematodes examined. The zone of differentiation presents striated rootlets associated with two centrioles, and an intercentriolar body. The nucleus and mitochondrion migrate in the median cytoplasmic process. An asynchronic proximo-distal fusion takes place. The mature spermatozoon possesses five regions and two axonemes with the 9 + ‘ 1 ’ pattern, nucleus, mitochondrion, glycogen granules and two bundles of parallel cortical microtubules. Despite these features, there are some other peculiarities that distinguish the spermatozoon of H.fasciata. A centriolar derivative and a lateral expansion were observed in the anterior region. To discuss the phylogenetic relationships, comparison is made with Opecoeloides furcatus, the only other opecoeloid for which the reproduction characters were described.
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