Crabs are crustaceans which have their reduced abdomen entirely hidden under the thorax and are covered with a thick exoskeleton and they have a single pair of claws. You can find crabs in every ocean, by they prefer fresh water. About the size, you can find from tiny crabs to huge ones (Japanese spider crab). It is also very common to see crabs moving sideways due to the special articulation of their legs (although others also move forwards and backwards).