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Sperm ultra-structure of Odontosyllis ctenostoma (Polychaeta: Syllidae) with inferences on syllid phylogeny and reproductive biology
Luigi Musco, Adriana Giangrande, Miriam Gherardi, Elena Lepore, Maria Mercurio and Margherita Sciscioli

The analysis of the complex reproductive patterns of Syllidae may drastically change the taxonomic hierarchy of the family. To further contribute to the knowledge of Syllidae we have described the sperm ultra-structure and some steps of spermiogenesis of Odontosyllis ctenostoma Claparède, 1868, the first non interstitial eusylline investigated. The mature sperm has a bell-shaped acrosome and contains electron-dense granular material and thin filaments. The barrel-shaped nucleus bears two depressions: one anterior facing the acrosome and the other posterior partially containing the distal centriole and up to six mitochondria. Odontosyllis ctenostoma spermatozoa can be ascribed to the ect-aquasperm type typical of species practising external fertilisation. This morphology is not in complete accordance either with the particular brood protection reported for the species or the egg size. The sperm is similar to those of the Syllinae species thus far investigated, but the acrosome resembles that of the exogonine spermatid. Some authors consider Odontosyllis to be phylogenetically closer to Syllinae, though it shares epigamy with Exogoninae. Others have hypothesised that the exogonine sperm could have been derived from sylline sperm by simplification. In our hypothesis this could have happened through a gradual passage from an Odontosyllis-like eusylline ancestor.

Keywords: sperm morphology, ultrastructure, Polychaeta, Eusyllinae, reproduction, phylogeny
Contents of this volume Sci. Mar. 72(3) : 421-427 Back PDF
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