From sperm plug formation to ovulation: Morphological and ultrastructural modifications in the seminal receptacle of the blue crab Callinectes danae

Abstract

In species of Callinectes a sperm plug is formed in the seminal receptacle (SR) during the sperm transfer, which hinders the insemination by other males. In this study, we evaluated the morphology, histochemistry, and ultrastructure of the seminal receptacle of juvenile and adult females of Callinectes danae along the different stages of ovarian development. We described the histochemical changes that take place inside the SR, from the sperm plug formation until fertilization. We also quantified the amount of seminal fluid received by the females during copulation. C. danae has a dorsal-type SR, which is common in Portunidae. The sperm plug is formed by the orderly deposition of seminal fluid of the anterior and median vas deferens, which in turn influences the secretion of the posterior region and produces chemically distinct strata. The dissolution of the sperm plug inside the SR occurs in synchrony with the spermatophore dehiscence and ovarian development. A holocrine secretion from dense layer cells dissolves the sperm plug. These cells have electron-dense and electron-lucid vesicles. The plug is completely dissolved when the females are at the intermediary ovarian development. At this stage, the SR dense layer folds and forms the lateral chamber. The dorsal oviduct, continuous to the dense layer, moves to a ventral-lateral position opposed to the vagina. During ovulation, the oocytes of mature females pass through the oviduct and the fertilization occurs almost above the vagina. The male ejaculates all its seminal fluid in a single copulation, leading to a total sperm depletion. Therefore, the whole sperm plug is produced by the males, through the combined secretions from three vas deferens regions. We show, for the first time, that the secretions from the anterior vas deferens also participate in the sperm plug formation. The female dissolves the plug, which is a crucial morphological change of the SR during the processes of fertilization and egg release. In species of Callinectes there was a clear co-evolution between the male and female reproductive systems: the male invests all its reproductive potential to build the sperm plug, and the female, to dissolve it.