Since penaeids usually moult at night, mating in most closed-thelycum species must also be nocturnal. This has been confirmed for P. merguiensis, P. japonicus, and P. monodon. In P. vannamei- an openthelycum species - mating takes place around sunset (Dall et al., 1990).

Closed-thelycum species are mating between intermoult males and recently moulted females. In P. vannamei, which has an open thelycum, both sexes were in the intermoult stage, the females in late intermoult. As the time period when females of closed-thelycum species can mate is very short, it would be advantageous for males or females to be attracted to one another around the time the female moults (Dall et al., 1990).

In the first phase of mating and copulation, female P. japonicus and P. monodon move around after moulting, occasionally swimming up 20-40 cm and then resting on the bottom. During this period one or more males follow the female. In all three species, the male then moves to below the female which grasps his carapace with her pereopods, while continuing to swim. In P. paulensis, the female stays on the bottom and does not swim around before or during mating. In P. monodon the position is maintained for between 20 and 120 min. If the male is dislodged it is replaced by another. In the second phase, the male turns upside down below the female and they grip each other with their pereopods. It is difficult for another male to dislodge the successful male from this position but when this does happen, the dislodged male reverts to the first phase and follows the female. In P. monodon and P. paulensis, the third phase is marked by the male continuing to hold onto the female but rapidly turning perpendicular to the female's body. The male arches his body around the female and then appears to squeeze the female and simultaneously flick is head and telson; spermatophore transfer probably takes place at this point. The male then separates from the female and swims away. In P. japonicus, the male does not rotate but remains aligned with the female. Courtship and mating in P. monodon takes 30 min to 3 h, far longer than in P. japonicus, in which it takes approximately 10 min (Dall et al., 1990).

Mating in P. vannamei, an open-thelycum species, is similar to that of closed-thelycum species. Males follow a female on the bottom and attempt to get underneath her from behind. The female then swims off the bottom followed by one or more males. The chasing male turns upside down and grasps the female from underneath by means of its pereopods. This position is maintained for 1 or 2 s during which spermatophore transfer occurs. Male P. vannamei do not rotate their bodies but, like P. japonicus, remain parallel to the female (Dall et al., 1990).