Ecology and behavior of Gecarcoidea natalis, the Christmas Island red crab, during the annual breeding migration

The terrestrial crab Gecarcoidea natalis is endemic to the forests of Chris tmas Island but must migrate each year to the coast to breed. During 1993 a nd 1995, radio-tracking, mark and recapture, and counting methods were used to establish the routes, walking speeds, direction of travel, and destinat ions of migrating crabs, as well as crab numbers and distribution. The dens ity of crabs ranged from 0.09 to 0.57 crabs per square meter, which gave a population estimate of 43.7 million adult crabs on the island. During the d ry season the crabs were relatively inactive but on arrival of the wet seas on immediately began their migration. The crabs generally walked in straigh t lines, and most crabs from around the Island traveled toward the northwes t shore instead of simply walking toward the nearest shore. The maximum rec orded distance walked by a red crab in one day was 1460 m, but the mean was 680 m per day in 1993 and 330 m in 1995. Comparing the 1993 and 1995 study seasons, there was a 3-week difference in the timing of the start of the m igration, but the spawning date was fixed by the lunar phase and took place 17 to 18 days after mating. In 1993, late rain prompted a "rushed" migrati on and crabs walked directly to their shore destinations; in contrast, in 1 995 most crabs made stops of 1 to 7 days during the downward migration. By giving the crabs a chance to feed along the way and minimizing the time tha t the population was concentrated near the shore, these stops may be import ant in ensuring that the animals have enough food after the long dry season . Furthermore, this behavior implies that the crabs are able to judge how f ar away they are from the shore during the migration.