In the mating system of Ilyoplax pusilla, wandering females respond to courting, burrow-holding males that wave their chelipeds and enter their burrows. They pair in plugged burrows where mating occurs. Female I. pusilla spawned within 3 days of pairing, after which the male left. The opercula of females became decalcified, before, or more often after, pairing. Decalcification normally follows responsiveness to male courtship, but it is not necessary to produce female receptive behaviour, and pairing with a male in his burrow is not essential to evoke decalcification. After spawning, the numbers of calcified females gradually increased, and most ovigerous females were recalcified before larval release. The percentage of decalcified, single, non-ovigerous females in the population peaked near the full and new moons. The peak percentage of decalcified, non-ovigerous, paired females occurred 2 days later than that of decalcified, single, non-ovigerous females. Decalcified females did not always enter male burrows for mating. Some females remained in their own burrows and laid eggs fertilized with stored sperm. Spawning with stored sperm may be an adaptation to avoid the risk of predation and it enables females, not evicted by neighbouring males, to reproduce.