Sperm ultrastructure of Prodistomum polonii (Digenea, Lepocreadioidea), an intestinal parasite of the Atlantic horse mackerel, Trachurus trachurus (Teleostei, Carangidae), from the Gulf of Gabes, Mediterranean Sea

Abstract

The present study describes the ultrastructural organization of the mature spermatozoon of the lepocreadioid digenean Prodistomum polonii (Lepocreadiidae) by means of transmission electron microscopy (TEM). Live digeneans were collected from the digestive tract of the Atlantic horse mackerel Trachurus trachurus (Teleostei, Carangidae) captured in the coastal zone of the Mediterranean Sea, off La Chebba (Tunisia). The study reveals that the mature sperm cell of P. polonii is a filiform cell, which is tapered at both extremities. It exhibits the Bakhoum et al.’s type III of the digeneans spermatozoon characterized by the presence of: two 9+‘1’ axonemes, external ornamentation of the plasma membrane associated with cortical microtubules and located in the posterior part of the anterior region, two bundles of parallel cortical microtubules with their maximum number (15 microtubules) located in the middle part, and the presence of two mitochondria (one of them of moniliform type). According to the location of the external ornamentation, the male gamete of P. polonii presents a Quilichini et al.’s type 2 spermatozoon. Moreover, in the anterior spermatozoon extremity, there is a discontinuous and submembranous layer of electron-dense material. Whereas, the posterior spermatozoon extremity belongs to the Quilichini et al.’s cryptogonimid type. Our results are compared with the available data on digenean spermatology, in particular with species belonging to the superfamily Lepocreadioidea to highlight the potential criteria useful for phylogeny.