The present study provides the first ultrastructural data on the digenean Gonocerca phycidis belonging to the family Gonocercidae. Live adults digenean were collected from the stomach of the marine fish Gephyroberyx darwinii (Teleostei: Trachichthyidae) caught in the Atlantic Ocean, off Dakar (Senegal). This is also the first report of G. phycidis from Senegal and from this fish. The mature spermatozoon exhibits (i) two axonemes with the 9 + ‘1’ pattern of the Trepaxonemata, (ii) a lateral expansion associated with external ornamentations and without cortical microtubules, (iii) two types of external ornamentation of the plasma membrane located in the anterior region of the spermatozoon, (iv) two bundles of cortical microtubules in the mitochondrial and nuclear areas of the spermatozoon, (v) one mitochondrion and (vi) a nucleus. The maximum number of cortical microtubules is located in the median part of the spermatozoon. Moreover, the mature spermatozoon of G. phycidis presents some peculiarities. The presence of a lateral expansion without cortical microtubules associated with external ornamentations is described here, for the first time, in digenean spermatozoa. The presence of two elongated bundles of cortical microtubules is also reported for the first time in the superfamily of Hemiuroidea. However, the ultrastructure of the posterior spermatozoon extremity confirms that already described in other hemiuroidean species.

The study of sperm characteristics has proven useful for elucidating interrelationships in several groups of Platyhelminthes, such as digeneans. Thus, in the present work, the ultrastructural organization of the mature spermatozoon of the digenean Artyfechinostomum malayanum (Echinostomatidae), a parasite of Rattus norvegicus (Rodentia: Muridae) from Dong Thap Province, Vietnam, was investigated for the first time using transmission electron microscopy. The male gamete of A. malayanum exhibits two axonemes of different lengths, showing the 9 + ‘1’ pattern of the Trepaxonemata, a nucleus, two mitochondria, two lateral expansions, two bundles of parallel cortical microtubules, external ornamentation, spine-like bodies, and granules of glycogen. Thus, the mature spermatozoon follows a Type V sperm model proposed for digeneans. We also highlight some noteworthy characteristics in Echinostomatidae with possible phylogenetic implications, such as two lateral expansions in the anterior region of the spermatozoon and two mitochondria.