Reefs may be classified in terms of their origin, geographical location, depth, and
topography. For example a tropical coral fringing reef, or a temperate rocky intertidal reef.
Biotic A variety of biotic reef types exists, including
oyster reefs and
sponge reefs, but the most massive and widely distributed are tropical
coral reefs. Cyanobacteria do not have skeletons, and individual organisms are microscopic. However, they can encourage the precipitation or accumulation of calcium carbonate to produce distinct sediment bodies in composition that have relief on the seafloor. Cyanobacterial mounds were most abundant before the evolution of shelly macroscopic organisms, but they still exist today.
Stromatolites, for instance, are microbial mounds with a laminated internal structure. Whereas,
bryozoans and
crinoids, common contributors to marine sediments during the
Mississippian period, produce a different kind of mound. Although bryozoans are small and crinoid skeletons disintegrate, bryozoan and crinoid meadows can persist over time and produce compositionally distinct bodies of sediment with depositional relief. The
Proterozoic Belt Supergroup contains evidence of possible
microbial mat and dome structures similar to stromatolite reef complexes.
Geologic Rocky reefs are underwater outcrops of rock projecting above the adjacent unconsolidated surface with varying relief. They can be found in depth ranges from
intertidal to deep water and provide a substrate for a large range of sessile benthic organisms, and shelter for a large range of mobile organisms. Corals, including some major extinct groups
Rugosa and
Tabulata, have been important reef builders through much of the
Phanerozoic since the
Ordovician Period. However, other organism groups, such as calcifying algae, especially members of the red algae (
Rhodophyta), and molluscs (especially the
rudist bivalves during the
Cretaceous Period) have created massive structures at various times. During the
Cambrian Period, the conical or tubular skeletons of
Archaeocyatha, an extinct group of uncertain affinities (possibly sponges), built reefs. Other groups, such as the Bryozoa, have been important interstitial organisms, living between the framework builders. The corals which build reefs today, the
Scleractinia, arose after the
Permian–Triassic extinction event that wiped out the earlier rugose corals (as well as many other groups). They became increasingly important reef builders throughout the
Mesozoic Era. They may have arisen from a rugose coral ancestor. Rugose corals built their skeletons of
calcite and have a different symmetry from that of the scleractinian corals, whose skeletons are
aragonite. However, there are some unusual examples of well-preserved aragonitic rugose corals in the
Late Permian. In addition, calcite has been reported in the initial post-larval calcification in a few scleractinian corals. Nevertheless, scleractinian corals (which arose in the middle Triassic) may have arisen from a non-calcifying ancestor independent of the rugosan corals (which disappeared in the late Permian).
Artificial An artificial reef is a human-created underwater structure, typically built to promote
marine life in areas with a generally featureless bottom, to control erosion, block ship passage, block the use of
trawling nets, or improve
surfing. Many reefs are built using objects that were built for other purposes, for example by sinking oil rigs (through the
Rigs-to-Reefs program),
scuttling ships, or by deploying
rubble or
construction debris. Other artificial reefs are purpose built (e.g. the
reef balls and
Electrified Reefs). Shipwrecks become artificial reefs on the seafloor. Regardless of construction method, artificial reefs generally provide stable hard surfaces where
algae and invertebrates such as
barnacles, corals, and
oysters attach; the accumulation of attached marine life in turn provides intricate structure and food for
assemblages of fish. == See also ==