Most of the reef colony is actually dead. Only the upper layer is covered by a thin changeable 'skin' of living coral. The tiny, transparent polyps of stony corals are the master builders of the sea, erecting their architectural masterpieces upon the remains of their predecessors. Just as the rainforest has distinct and myriad life forms in each layer of the canopy, so coral-dependent species are linked to the type of coral and its supporting cast of plants and animals. The secret to the reef's diversity of species lies in its multi-story architecture.
Reefs are like inner-city apartment blocks, where each one room flat is occupied to two organisms.
Resident reef fishes which forage during the day, share their living space with other, nocturnal species. While daytime fishes are feeding, their flat is occupied by a fish or other reef dweller, say an octopus, which is only active at night. During the night, the dayshift sleeps. This sharing of quarters allows a reef to shelter two separate populations of marine organisms. Coral reefs are tightly woven ecosystems, with incredibly complex interlinkages.