Coral reefs normally live in tropical regions.  Some reefs live in places that are cool, but never cold areas.  One very popular reef is the Great Barrier Reef.  It is off the northern coast of Australia.  This reef grows in waters ranging from about 60-80 degrees Fahrenheit.  There are also coral reefs in Mexico, Cuba, Florida, California, Hawaii, and Japan.  A reef cannot withstand temperatures below 40 degrees Fahrenheit because the fish cannot live there.  It's like a food chain:  no reef means no shelter, no shelter means less fish, which means eels will leave.  No eels and no sharks, and if it continues  there will be nothing left living there.  That is why reefs are important.