Ultrastructure of the spermatozoon of the digenean Cricocephalus albus (Kuhl & van Hasselt, 1822) Looss, 1899 (Platyhelminthes, Pronocephaloidea, Pronocephalidae), parasite of “the hawksbill sea turtle” Eretmochelys imbricata (Linnaeus, 1766) in Senegal

The mature spermatozoon of Cricocephalus albus is filiform, tapered at both ends and shows the following features: 2 axonemes of different lengths presenting the 9 + “1” trepaxonematan pattern, 2 bundles of parallel cortical microtubules, a mitochondrion and a nucleus. Nevertheless, the particularity of the spermatozoon of C. albus is its anterior extremity with an apical electron-dense material associated with extramembranar ornamentation, a cytoplasmic dorsolateral expansion and spine-like bodies. To our knowledge, such an anterior extremity of the spermatozoon has not previously been described from a species of the superfamily Pronocephaloidea. Our study provides new data on the mature gamete of C. albus in order to improve our understanding of the pronocephaloidean phylogenetic relationships.