Christmas Island red crab

The Christmas Island red crab, Gecarcoidea natalis, is a species of land crab that is endemic to Christmas Island and the Cocos (Keeling) Islands in the Indian Ocean. Although restricted to a relatively small area, it has been estimated that 43.7 million adult red crabs lived on Christmas Island alone, but the accidental introduction of the yellow crazy ant is believed to have killed about 10–15 million of these in recent years. Christmas Island red crabs eat mostly fallen leaves and flowers, but will occasionally eat other animals, including other red crabs if the opportunity arises.

The Crab Migration 
 At the beginning of the wet season (usually October / November), most adult red crabs suddenly begin a spectacular migration from the forest to the coast to breed. Breeding is usually synchronized island-wide. The rains provide moist overcast conditions for crabs to make their long and difficult journey to the sea.

During peak migration times, sections of roads where crabs cross in high numbers may be closed to vehicles for short periods of time. You can park your vehicle and carefully walk amongst the moving sea of crabs as they relentlessly make their way to and from the shore. The easiest places to watch the crab migration and the females spawn are at Drumsite, Flying Fish Cove, Ethel Beach and Greta Beach. Updates on public notice boards and local radio give crab movements during the migration.

Here is some pictures 

Here is a video of the migration