Abstract

The breeding migrations of the Christmas Island red crab were monitored at plateau and shoreline locations from November 1979 to May 1983. Up to three migration sequences, each resulting in egg release, occurred each year in a breeding season lasting up to 3 months. In the main sequence migration from the plateau to the shore began at the onset of the wet season and lasted 9-18 days. On arrival at the sea, crabs immersed in salt water in large numbers before retreating to the shore terrace rainforest, where males fought over possession of burrows. Crabs courted and copulated near or in the burrows; they copulated in a hard-shell condition with the male generally underneath. At the conclusion of mating activity, males again dipped in salt water before returning to the plateau. Females remained in burrows on the shore terrace for 12-13 days until eggs were nearly ripe; they then moved to shoreline rocks. They released their eggs at night on the turn of the high tide between the last quarter of the moon and the new moon, sometimes from cliff faces, then returned en masse to the plateau. Zoeae hatched immediately on contact with salt water, and baby crabs emerged at the first crab stage 27 days after egg release; successful emergence varied considerably between seasons. Baby crabs migrated inland, taking 9 days to reach the plateau. Most migration activity was diurnal, the main exceptions were egg release and dipping activities.