LAUSR

Abstract The ultrastructural characters of mature spermatozoon of the digenean Haplosplanchnus caudatus were described by means of transmission electron microscopy (TEM). Live parasites were collected from the digestive tract of flathead grey mullet Mugil cephalus (Teleostei: Mugilidae) caught off the Gulf of Gabes in La Chebba (Tunisia). The present study provides the first ultrastructural data on a digenean belonging to the superfamily Haplosplanchnoidea. The spermatozoon of H. caudatus is a filiform cell which exhibits: (i) two axonemes with the 9+‘1’pattern of trepaxonematan Platyhelminthes having about the same length, (ii) an external ornamentation of the plasma membrane associated with cortical microtubules and located in the anterior part of the sperm cell, (iii) spine-like bodies, (iv) two mitochondria containing large mitochondrial matrix granules, (v) a nucleus, and (vi) granules of glycogen irregularly distributed. Moreover, the spermatozoon of H. caudatus displays the Quilichini et al.’s cryptogonimid type of posterior extremity. The mitochondrial matrix granules are described for the first time in a digenean sperm cell.