The soldier crab Mictyris brevidactylus breeds in winter. Large females spawned over a few days in early winter after most had decalcified their vulvar opercula. Some females spawned in their first winter, less than 1 year after settlement. The crabs probably copulate during a few days before spawning while the vulvar opercula are decalcified. During the daytime low tide in the breeding season, large males emerged and fed on the sediment surface. Females, on the other hand, made a sand roof above themselves using sand pellets from which they had sorted out food particles, and they fed while hiding under it. After the males had ceased their surface activities, they entered the sand tunnel made by the female. The roof was connected with a hollow shaft, which had been made when the crab had ascended to the surface. Before the habitat was submerged, the shaft was plugged from within with sand to form an air chamber, which suggests that the male and female descend together in the air chamber and copulate there. It is possible that the crab uses the sand roof as a visible signal to attract mates.