Fine structure of the spermatozoon of Perinereis macropus (Claparède, 1870) (polychaeta, Nereidae)

•Sperm cell constituted of three parts: head, midpiece and flagellum.•Primitive type of spermatozoon with an acrosomal extension.•Presence of an acrosomal rod and a fibrillar crown in the acrosomal complex.•The short midpiece contains nine to eleven mitochondria.•Presence of lateral protuberances on the membrane of the flagellum. The mature spermatozoa of Perinereis macropus were investigated by transmission electron microscopy. The spermatozoon is composed with a large anterior part (head), a short middle piece and a long flagellum. The head contains a large acrosomal complex with a convex acrosomal vesicle, a subacrosomal space, a fibrillar crown and an acrosomal rod which penetrates into the nucleus invagination. The later is U shaped (in longitudinal section). The short middle piece contains about nine to eleven mitochondria and a centriole associated with the flagellum. This centriole, slightly eccentric to the sperm axis, anchors to the plasma membrane by nine satellite rays of the pericentriolar complex. The axoneme has a “9+2″ arrangement of microtubules. In cross section, the flagellar membrane extends in two lateral protuberances, aligned with the axis formed by the two central microtubules of the axoneme. The spermatozoon of P. macropus conforms to the primitive type with an acrosomal extension. Nevertheless, the acrosome complex ultrastructure shows noticeable modifications from the basic form. This finding agrees with the previously observed reproductive pattern (broadcast spawning – free-swimming larvae), and may be helpful to classify the sperm type of P. macropus.