How is a giant sperm ejaculator formed? Development of the Zenker organ after the last moult in Pseudocandona marchica (Crustacea, Ostracoda, Candonidae)

'Giant sperm', in terms of exceptionally long spermatozoa, occur in various taxa in the animal kingdom, particularly in arthropods. In freshwater ostracods (Cypridoidea) reproducing with giant sperm, males are equipped with a pair of specialized ejaculators, or Zenker organs, located in the distal part of vas deferens.We observed adult postmoult specimens of a cypridoidean ostracod, Pseudocandona marchica, by SEM, TEM and CLSM and elucidated the formation process of their Zenker organs. This investigation shows that most of the formation process of the Zenker organ takes places after the moult to adulthood. In P. marchica this process can be divided into five stages, referring to consecutive differentiation steps of epithelia, musculature, and chitinous supports. The evolutionary significance of the Zenker organ's morphology in freshwater ostracods is discussed in the light of its maturation process, as well as the relevance of Zenker organs without chitinous spines that were reported in previous studies.