Abstract:

The ultrastructural characteristics of spermatogenesis and spermatozoon of the barnacle Megabalanus volcano were observed by scanning and transmission electron microscopy. The results showed that its spermatogenesis included five developmental stages, i.e., spermatogonium, primary spermatocyte, secondary spermatocyte, spermatid, and mature spermatozoon. During spermatogenesis, a circular or oval nucleus extended along with an axial filament and bent in cytoplasm under the action of microtubules. The nucleus further elongated with longitudinal extension of a spermatid, and finally became filiform. Massive chromatin of nucleus underwent disintegration, fine granules, coarse granules, and eventually condensed into a high electron density homogenizer. The vesicles secreted by the Golgi complex coalesced to different sizes of precursors of accessory droplet, and finally fused into an accessory droplet of high electron density. Many small mitochondria fused to a slender one that was eventually positioned behind the nucleus. The spermatozoon (about 58.39 μm in length), which was divided into acrosome, neck, middle piece, and tail, was long filiform with an accessory droplet. The conical acrosome was located at the spermatozoon anterior, followed by a basal body. The neck was composed of a "9+2" type axial filament, and the filiform nucleus was parallel to the axial filament. The length of acrosome and neck was about 6.65 μm. The middle piece was about 42.95 μm in length. The spindle-shaped accessory droplet (about 9.33 μm long and 2.13 μm wide) was adjacent to the nucleus and axial filament, followed by a mass of glycogenosomes and a slender mitochondrion. The tail (about 8.79 μm in length) only had a typical "9+2" axial filament.

Key words: Megabalanus volcano, spermatogenesis, spermatozoon, ultrastructure, accessory droplet