Ultrastructure of the spermatozoon of Parahemiurus merus (Linton, 1910) (Digenea: Hemiuroidea: Hemiuridae), a parasite of Sardinella aurita Valenciennes, 1847 and S. maderensis (Lowe, 1838) (Teleostei: Clupeidae) in the Senegalese coast

The mature spermatozoon of Parahemiurus merus (Digenea: Hemiuroidea: Hemiuridae) is filiform, tapered in both ends and exhibits the general pattern described in most digeneans so far: two axonemes of the 9+'1' pattern of trepaxonematan Platyhelminthes, nucleus, mitochondrion, external ornamentation of the plasma membrane and parallel cortical microtubules. However, the spermatozoon of P. merus presents some peculiarities. The presence of an external ornamentation of the plasma membrane not associated with cortical microtubules in the anterior part of the spermatozoon is described for a second genus in the family Hemiuridae. Cortical microtubules are disposed in only one side of the spermatozoon as described in other species of Hemiuroidea but, in P. merus, the number of cortical microtubules is much lower. The results obtained are compared with the available data on Hemiuroidea in order to improve the understanding of phylogenetic relationships of hemiuroids. Similarities found between spermatozoa of hemiurids and lecithasterids are in accordance with molecular evidence that group these two families in a same clade.