Biology: Red Crab Migration.

On Christmas Island in the Indian Ocean a sea of red migrates from the forested plateau to the shore and back every year. This is the annual migration of the red crab.

The red crabs go to the shore to breed and to replace water and body salts lost in the trip to the shore. The total migration, to the shore and back, takes three lunar cycles or three months starting in early November. Along the way they face with dehydration, roads and railway tracks.
Though things like road closures and underpasses have been made to help the crabs along the way.

When they arrive at the shore the crabs dip into the water to rehydrate themselves, and then they mate. After that the males dip into the water a second time and either head back to the forest or stay on the beach and feed.

The females incubate their up to 100,000 eggs for 12 -13 days and then release them into the water. The young then mature in the water for 30 days and until the start there life on land.

Picture: Wikipedia
Sources: (Book) Ocean by The American Museum of Natural History.