The Opecoelidae sperm model and its contribution to phylogeny: Spermatozoon ultrastructural particularities of Allopodocotyle pedicellata (Plagioporinae, Digenea, Platyhelminthes)

The ultrastructure of the spermatozoon of Allopodocotyle pedicellata (Digenea, Opecoelidae), an intestinal parasite of the teleost Sparus aurata (gilt-head bream), is described by means of transmission electron microscopy (TEM). The mature spermatozoon possesses two axonemes of the 9+“1” trepaxonematan pattern, an anterior electron-dense material, two mitochondria, a nucleus, external ornamentation of the plasma membrane, spine-like bodies and parallel cortical microtubules distributed into two bundles. The anterior electron-dense material and the morphology of the anterior mitochondrion are the noteworthy characters that distinguish the spermatozoon of A. pedicellata from those of most opecoelids. However, a comparative study with the remaining opecoelids described so far, reveals similarities in their ultrastructural organization. Thus, the sperm model is established for the Opecoelidae and it is compared to mature spermatozoa of other digeneans, particularly those belonging to the family Opistholebetidae and the superfamilies Allocreadioidea and Brachycladioidea or their related sister groups. The morphology of the posterior spermatozoon extremity, the disposition of cortical microtubules and the association of external ornamentation with cortical microtubules and spine-like bodies appear as interesting elements with phylogenetic significance for the Opecoelidae.