Sperm ultrastructure of Helicometra epinepheli (Platyhelminthes, Digenea, Opecoelidae), parasite of Epinephelus fasciatus (Pisces, Teleostei).

This paper describes the ultrastructure of the mature spermatozoon of the digenean Helicometra epinepheli. The ultrastructural elements observed are: two axonemes with a 9+"1" pattern of Trepaxonemata, four attachment zones, a nucleus, two mitochondria, external ornamentation of the plasma membrane, spine-like bodies and cortical microtubules. A particularity of this spermatozoon is the presence of an apical cone. However, the spermatozoon presents the general pattern of the Opecoelidae and thus differs from Helicometra fasciata (which appears as an exception in this family) by several ultrastructural features: the presence of external ornamentation and spine-like bodies and the arrangement of cortical microtubules around the first mitochondrion in the region of the ornamentation. Each ultrastructural feature is discussed and compared with the literature to highlight characters which could be useful for phylogeny.