They look a lot like crabs and many have the word ` crab ' in their name , but are not true crabs . Crabs are found in all of the world 's oceans , while many crabs live in fresh water and on land , particularly in tropical regions . Crabs vary in size from the pea crab , a few millimetres wide , to the Japanese spider crab , with a leg span of up to 4 m.