Spermatophores and Plug Substance of the Marine Shrimp Trachypenaeus similis (Crustacea: Decapoda: Penaeidae): Formation in the Male Reproductive Tract and Disposition in the Inseminated Female
Sperm are packaged into many small spermatophores of variable size in the median vas deferens (MVD) of the male. A substance is intermixed with sperm in the proximal coils of the MVD, separating groups of sperm that will be ejaculated as spermatophores. Most of the ejaculatory duct is occupied by a chamber filled with a transparent, viscous fluid termed the "plug substance." When males are artificially ejaculated, spermatophores are emitted, followed by plug substance that quickly solidifies. This latter material fills and stoppers a space on the female, the median pocket, which serves as an antechamber to the apertures of the internalized seminal receptacles, where sperm from ruptured spermatophores are stored. The slit-like openings to the receptacles are functionally divided into a posterior aperture, stoppered by plug substance after insemination, a closed mid-section, and an anterior exit for sperm release during spawning. Direct insemination by an everted male gonopore is considered more likely than transmission of spermatophores and plug substance via the male gonopod (petasma). In addition to its hypothesized roles during insemination and sperm release, the mass of plug substance (mating plug) may act as a paternity assurance device that prevents subsequent inseminations by other males.