The Elusive Acrosome of Chiton Sperm

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Abstract
In our study of spermiogenesis in the lined chiton Tonicella lineata, we traced the formation and migration of small Golgi vesicles to the apex of the sperm, where they fused to form an apical granule. This apical granule and other Golgi secretions tested positively for acid phosphatase. In preliminary experiments on fertilization, sperm swam inside open hull (chorion) cupules down to the surface of the egg and penetrated it. No micropyle was observed. Serial 1μm sections of eggs fixed during fertilization demonstrated that the sperm nucleus had penetrated not only the hull but also the vitelline and oocyte membranes. Serial thin sections showed that the tip of the anterior filament of the sperm had fused with a single microvillus of the oocyte membrane, creating a membranous tube through which the nucleus had entered the egg cortex. We suggest that the apical granule of chiton sperm is an acrosome that enables the nucleus to penetrate the egg membranes.