大象传媒

The arthropods have hard, jointed skeletons. Armour gave the arthropods the staying power they needed to make it on land. Some crabs have now evolved to live far from the sea. Blue land crabs in Florida, for example, are found up to three miles from the beach. Crabs are actually well adapted to life on land. Their jointed, outer skeletons may have evolved in the sea but it also supports their weight on land making it easy for them to get around. It also stops them from drying out. In the Indian Ocean there is a land where the crabs have really made themselves at home. Christmas Island is completely overrun with red land crabs. Around 120 million live on this small island, outnumbering the human residents by 300,000 to one. Once a year vast numbers head for the beach, unable to shake off their ties to the sea. As they make their journey to the water the females clasp thousands of soft eggs underneath them. Like all land crabs they have to return to the water to release their eggs. Their young must grow up in the sea.