A reef is defined as a carbonate buildup mainly constructed by organisms and organic actions and stunding above the sea (or lake) floor. Organic actions contributing to reef include:(1) forming reef framework by erect

inarticulate and calcified organisms

(2) baffling sediments by erect

noncalcified or calcified organisms

(3) supplying sediments to reefs by calcified organisms except frame-formers

(4) adhering sediments by organisms such as algae which can secrete mucus

(5) strengthening reef construction through encrustment of sheet-like or filamentous organisms on frame-formers or bafflers

(6) binding non-consolidated sediments by sheet-like or filamentous organisms

(7) covering sediments by creeper organisms. On the contrary

organisms can destroy reefs by boring or eating the soft tissue or skeletons of the reef-forming organisms. According to their composition and structure

reefs are classified into:(1) frame-reefs

(2) baffle-reefs

(3) mounds (including limemud mounds).