Coral reefs are among the most diverse communities on Earth. This diversity occurs at low (species) and high (phylum) taxonomic levels and includes cryptofauna living within the substrate (for example, boring sponges, worms, and bivalves; sessile encrusting bryozoans, sponges, tunicates, and worms; motile worms, mollusks, echinoderms, and crustaceans), sessile epifauna on the substrate surface (for example, scleractinian corals, sponges, and coralline and fleshy algae), and suprabenthic fishes in the overlying water column. Approximately 93,000 species have been described globally on modern reefs, although 500,000 to 1 million species are estimated to inhabit reefs (in comparison with 20 million species in tropical rainforests). In addition to the tropical location of coral reefs, a number of other factors appear to be responsible for this high biodiversity, including elaborate biologically generated physical heterogeneity, sophisticated specializations, common sibling species (morphologically similar), and coevolved associations among species.