Concepts

REEFS

	A reef is a structure built by organisms that rises above the surrounding seafloor.
Reefs throughout geologic history share a common set of environmental features. Reefs are built in warm shallow seawater in the tropics and subtropics. Reefs occur only in waters that are relatively free of suspended, land-derived sediment, which allows sunlight to penetrate to the reef surface, permitting photosynthetic organisms to live. Reefs are characterized by high biodiversity and include five major types of organisms.
	Reef constructors help to build the reef by forming a framework of hard skeletons.
	Reef bafflers have upright fronds or stick-like growth forms that interfere with currents and trap sediment on the reef surface.
	Reef binders, represented here by white-colored algae, grow over and around loose sediment and skeletons of reef organisms and literally bind them together.
	Reef dwellers consist of a variety of species that live in and among the constructors and binders, but they do not directly build the reef framework.
	Reef destroyers bore into or scrape away parts of the reef surface, converting hard reef framework into loose particles of sediment. These photos show holes in a coral produced by the bivalve at the far left.
The reef community is characterized by complex interactions among these types of organisms. Corals, which are common reef constructors, form a rigid framework that offers habitats for reef-dwelling bivalves, which may cement to coral heads or nestle in cavities between them. Certain sponges, worms and other bivalves act as reef-destroyers by boring into the coral framework and producing loose particles of broken coral. Baffling organisms, such as sea-fans, may concentrate these sediment particles on particular parts of the reef, where organisms such as calcareous algae can bind them to create a new type of rigid surface. Silurian reefs of 425 million years ago share many features in common with those of today, including tropical to subtropical distribution, growth in shallow water, and relatively high biodiversity. Silurian and modern reefs also show important differences, however. Binders appear to have been more important in Silurian reefs, and destroyers less important, than in reefs of today. The actual groups of organisms that occupy various functional roles in the reef community are also different. For example, stromatoporoids, an extinct type of sponge, are important constructors in Silurian reefs but are unknown in those of today.