Coral reefs form in clear warm tropical seas and are particularly well developed in the South Pacific. They are formed by accumulation in calcarious skeletons of tiny colonial animals called corals over generations. Coral reefs usually form along the shallow submerged shelves and they are limited to a depth up to which sunlight can diffuse. Coral reef communities in terms of species diversity, number of organisms, brilliance of colours and interesting life forms are comparable with tropical forest communities.