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.
Remove, or disturb one key component of the reef's fabric, and it may trigger a nasty chain reaction of events. Parrot fish and urchins are the sanitary engineers of the reefs, cleaning the coral of excess algae. Sponges are also keystone species. They act as the lungs of the reef, cleaning and recycling the water. Remove them and a crucial ecosystem function is lost, with consequences for the entire community.