The morphology of the male reproductive system, spermatogenesis and the spermatozoon of Daphnia magna (Crustacea: Branchiopoda).

This study analyses the histological and cellular morphology of the testis and sperm development in the male Daphnia magna Straus 1820. Due to the rarity of males and predominately parthenogenetic lifecycle of Daphnia, there has been limited detailed information on males in contrast to the well-studied female. Using light and electron microscopy approaches, we describe the morphology of the testis during the progression from an immature to mature testis. The testis has an encasing muscular mesh sheath outside the basal lamina, beneath which is a thin somatic epithelial cell layer. Internal to the epithelium are the spermatogonial stem cells and subsequent syncytial clusters of the germ cells as they progress through spermatogenesis; spermatozoa occupy the entire testis in sexually mature D. magna. We describe the structure of developing and mature spermatozoa; mature spermatozoa are non-flagellated, ovoid in shape with plasmalemma filapodia and are encased in an extracellular capsule.