Crabs inhabit all the seas and oceans of the planet, but they reach the greatest variety in the tropics. The habitat of these crustaceans has a very wide range: crabs can be found in the shallows of the seas and oceans, among the thickets of corals on reefs, at depths of up to 5000 m, in caves, in the tidal zone, mangrove thickets and even in the depths of islands away from the shore. The vast majority of them live in salt water, about 850 species live in fresh water.
Crabs, which spend a long time on land, store water under the armor or they develop organs like lungs. Underdeveloped gills in them almost do not work and with constant immersion in water such individuals die. Species that live on the bottom are often active in the dark, land crabs are most active during the day.
When moving these crustaceans are never put on the ground at the same time, both legs of one pair, which gives their gait stability, but the small length of the body and a large number of legs make it uncomfortable to move forward, so crabs prefer to walk sideways. At the same time, this does not in the least prevent them from developing a decent speed, for example, a grass crab for 1 second crosses 1 m! But these animals swim badly and reluctantly.