Peculiar features of the sperm of a calanoid copepod Arctodiaptomus salinus

The spermatophore and spermatozoon of the calanoid copepod A. salinus, were investigated by light and electron microscopies. Mature spermatozoa were found within the spermatophore in the seminal vesicle at the end of the deferent duct. The cells measured 2–3 μm in diameter and were almost spherical; the nucleus was mixed with the cytoplasm as no envelope was present to separate them. They lack flagellum and any acrosome-like structure. The dispersed chromatin forms small granules surrounding the mitochondria, characterised by a small number of highly modified cristae. Under the plasmatic membrane, a group of electron-dense lamina enclose the whole cytoplasm. In the final phase of spermiogenesis, the dismantling of the nuclear envelope, the formation of chromatin granules and the modification of mitochondria occur. This process takes place simultaneously with the synthesis of the components of the spermatophore by the epithelial cells of the deferent duct. All these characteristics of the sperm of A. salinus identify it as being a peculiar case among copepoda.