The petasma is formed by the endopodites of the first pair of pleopods which are modified as interlocking structures for spermatophore transfer. The appendix masculina are on the endopodites of the second pair of pleopods and serve to separate the petasma into two component halves. The thelycum may be "open" or "closed", depending on the species. "Closed" thelyca are those where the spermatophore is placed by a male in the groove below the plates while the female is in the soft exoskeleton stage following molting. The spermatophore is stored for some time before spawning. "Open" thelyca are not enclosed by plates, and the spermatophore must be placed on it by a male when the female's exoskeleton is hard; usually within hours of spawning. The presence of a spermatophore on the female is evidence that she has successfully mated. Open thelyca are found in some shrimp species endemic to the Western Hemisphere, such as P. stylirostris and P. vannamei; while closed thelyca are characteristic of most Asian species, such as P. monodon, P. chinensis, P. indicus and P. merguiensis (Bailey-Brock & Moss, 1992).

Internal organs of the male reproductive system include a paired testes, vas deferens and terminal ampoules for spermatophore storage. The female reproductive system includes paired (but partially fused) ovaries that extend from the mid-thorax to the posterior end of the abdomen, and oviducts terminating adjacent to a single thelycum. (Bailey-Brock & Moss, 1992).