These crustaceans have been misnamed for two reasons: First, they’re not true crabs, like blue crabs, in that they don’t have a uniformly hard exoskeleton and can’t grow their own shells.

The blue crab species Discoplax celeste is only found on Christmas Island. It is one of more than 20 species of land crab on the island. With more than 150 million red crabs on Christmas Island ...