Red Beach - The March of the Red Crabs
What you see in the five first photos are baby Christmas Island Red Crabs, Gecarcoidea natalis (Decapoda - Gecarcinidae), emerging from the ocean, and making their way to the forest, as seen in January of this 2014.
These crabs are endemic to the Christmas Island. For most of the year the red crabs are found within forest, but each year these crabs must migrate to the coast to breed (last photo). The estimated population of adult red crabs on Christmas Island was 43.7 million in 2001, so the migration is really impressive.
The arrival of the monsoonal rains allows increased activity of red crabs and stimulates the annual migration. During this breeding migration red crabs, like other terrestrial gecarcinids, must abandon their home ranges and travel down to the coast to mate and spawn. The downward migration normally requires at least a week, and the crabs migrate mainly during the first few hours of the morning and in the late afternoon.
The males excavate burrows, which they must defend from other males, on the lowest shore terraces; mating occurs in or near the burrows. Soon after mating the males start the journey back inland to the forest, while the females lay their eggs and remain in the burrows for 2 weeks. At the end of the incubation period the females vacate their burrows and make their way to the coastal cliffs, which almost completely surround the island, to cast their eggs into the ocean. The females usually release their eggs into the sea toward dawn, around the turn of the high tide, and then they return to the forest.
Eggs hatch immediately in the sea and the larvae (now called megalope) live in the sea for about a month before returning to land as juvenile crabs. These juvenile crabs start a first migration to the forest as seen in the first photos.
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Photo credit: ©Kirsty Faulkner | Locality: Christmas Island (2014)