The most important criterion for the living of sea urchins is the salinity of water, they are not found in the salty seas (Black Sea, Caspian and part of the Baltic Sea). A large number of sea urchins live on coral reefs. The main part of the coral reefs is located in the Indian and Pacific oceans. Also hedgehogs are often found in coastal waters.
Very poisonous species of sea urchins live in the waters of the Atlantic, Pacific and Indian oceans. A person who has been pricked by a sea urchin gets a lot of poison, because of which paralysis may occur, and eventually death.
Most sea urchins prefer to settle on rocky surfaces. Easily climbing with the help of their needles and ambulacral legs on any rocks, hedgehogs seek refuge in them in the form of depressions. If there is no suitable shelter, then hedgehogs drill it themselves.