Santa isn’t the only long-distance traveler in a red suit—Gecarcoidea natalis, the Christmas Island red crab, makes an impressive annual migration across Christmas Island, an island in the Indian Ocean named for the day it was discovered in 1643.
Gecarcoidea natalis is endemic to the island, and numbers around 44 million [1]. These solitary crabs are spread out over the island, and inhabit the rainforest, where there is high humidity. Each crab lives in its own burrow, and must avoid being under direct sunlight for extended periods of time. Such exposure can kill a crab because they need to maintain their body moisture above certain levels. So when it’s the dry season on the island, around the middle of the year, they remain in their burrows for up to a few months, sealing themselves and the humidity in with leaves at the entrance. They are diurnal (active during the daytime) and not picky-eaters at all, feeding on dried leaves, fruit, and even other crabs that have died [2].
As the yearly monsoons approach (beginning around November), the air gains moisture, allowing the crabs to travel farther without worrying about losing their own body moisture. Any and all sexually mature crabs (around four years of age and up) will at this point partake in a migration to the coast to spawn. The sheer number of crabs crossing the island has made this one of the most spectacular and famous migrations of animals. Male crabs leave first, traveling over 4 kilometers (2.5 miles), depending on where their burrows are on the island, in 5 to 7 days [1]. Upon arrival, they will go to the sea to gain back the moisture they lost during the long journey, and then proceed to dig densely packed burrows. A few days later, adult females arrive and mate with males in their burrows, after which the males return inland and the females stay a couple of weeks to lay their eggs and hold them in their abdominal brood pouch as they develop.
Spawning is highly correlated to the lunar cycle. Female crabs wait until the last quarter of the moon, when there is little difference between low and high tide, to line up at the edge of the water and release their eggs into the ocean. The crab eggs hatch in the water, and given their r-selected lifestyle (that is, where offspring are numerous and development is fast), they are likely to suffer high juvenile mortality from fish predators in the sea. But after a month, those that do survive develop into small crabs, which then make the journey back into the forest. Spawning for 2014 is predicted to happen right about now, but that is only a prediction, because it can happen anywhere between September and January [2].
However, there is a Grinch to this story—Anoplolepis gracilipes, the Yellow crazy ant, named for its color and “crazy” motion when provoked.
This invasive species of ant builds supercolonies, with crazy-high densities of 2,000 ants per square meter that displace the Christmas Island red crabs [3]. Not only do these ants take over land previously used by the crabs, but also eat the crabs themselves. The ants are generalists, eating a variety of things from honeydew to invertebrates and small vertebrates. Their weapon? Formic acid. The ants spray other organisms with it, blinding them and then killing them within a few hours [3][4]. As a result, crab numbers are declining, which is a problem for the forest. In particular, the forest depends on the crabs for several “presents”: the turnover of soil while building burrows, the dispersal of seeds from their diet, and the fertilization of the soil from the crab’s droppings.
By: Maria Robles Gonzalez; ed. by Liz Boatman
Learn More:
[1] Agnieszka, M. A., and S. Morris. “Ecology and Behavior of Gecarcoidea natalis, the Christmas Island Red Crab, During the Annual Breeding Migration.” Biological Bulletin 200.3 (2001): 305-320.
[2] “Red Crab Migration.” Christmas Island: A Natural Wonder. Christmas Island Tourism Association.
[3] “Pest animal risk assessment: Yellow crazy ant Anoplolepis gracilipes.” Department of Employment, Economic Development and innovation, Biosecurity Queensland. 2012.
[4] Squires, N. “‘Crazy ants’ threaten Christmas Island crabs.” The Telegraph. 2007.
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