Twenty terrestrial and intertidal species of crab have been described here, of which thirteen are regarded as true land crabs, being dependent on the ocean only for larval development. Robber crabs, known elsewhere as coconut crabs, also exist in large numbers on the island. The annual red crab mass migration (around 100 million animals) to the sea to spawn has been called one of the wonders of the natural world. This takes place each year around November – after the start of the wet season and in synchronisation with the cycle of the moon. Once at the ocean, the mothers release the embryos where they can survive and grow until they are able to live on land.