Ultrastructure of the spermatozoon of Helicometroides atlanticus (Digenea, Monorchiidae), an intestinal parasite of Parapristipoma octolineatum (Pisces, Teleostei) in Senegal

• Ultrastructure reconstruction of the spermatozoon of Helicometroides atlanticus.
• Lacks of spine-like body and cytoplasmic expansion.
• Presence of an electron-dense material in the anterior extremity.
• Presence of two mitochondria.
• Maximum number of cortical microtubules (35) in the middle part of the spermatozoon.

The mature Helicometroides atlanticus spermatozoon exhibits an anterior electron-dense material, two axonemes of the 9 + “1” pattern, a nucleus, two mitochondria, extramembranous ornamentations located at the level of the first mitochondrion and cortical microtubules arranged into two fields: a ventral field corresponding to the mitochondrial side and a dorsal field corresponding to the nuclear side. It lacks of spine-like body and a cytoplasmic expansion as in Monorchis parvus the only species of Monorchioidea which spermatozoon has been described until now. Nevertheless, it is distinguished from M. parvus by the presence of an anterior electron-dense material and two axonemes which appear one after the other.

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