Sperm structure of Rhopalura littoralis (Orthonectida)

The fine structure of the spermatozoon in the orthonectid Rhopalura littoralis is described. This is the first fine structural description of an orthonectid sperm. The spermatozoon contains a slightly elongated nucleus and two centrioles orientated along the longitudinal axis of the sperm. The proximal centriole bears one rootlet. A single mitochondrion is present in the mid-piece region. An acrosome is absent. The sperm tail is a simple flagellum with 9 +2 structure. We consider the orthonectan spermatozoon to be closer in structure to of those of Porifera, Cnidaria, and Annelida, than to Aschelminthes and Platyhelminthes, to which they have previously been allied.