Ultrastructure of spermiogenesis and the spermatozoon of Opecoeloides furcatus (Trematoda, Digenea, Opecoelidae), a parasite of Mullus barbatus (Pisces, Teleostei).

Spermiogenesis and mature spermatozoa of Opecoeloides furcatus (Digenea, Opecoelidae) are described by means of transmission electron microscopy. Spermiogenesis in this fluke matches the general pattern of digenetic trematodes. Striated rootlets associated with the two centrioles and an intercentriolar body are present in the differentiation zone. Flagellar rotation of two flagella and their proximodistal fusion with a median cytoplasmic process also characterize the spermiogenesis of O. furcatus. Nevertheless, asynchronicity is reported for the proximodistal fusion of the two flagella. Mature spermatozoa of O. furcatus are filiform, tapering at both ends and they present all the characteristic features found in the Digenea gamete: two flagella, mitochondrion, nucleus and two bundles of parallel cortical microtubules. Nevertheless several peculiarities distinguish the mature spermatozoon of O. furcatus from the gamete of other digenetic trematodes.