The Britannica World Language Dictionary defines a reef as "a ridge of sand or rocks or especially of coral, at or near the surface of the water". The nautical definition of the phrase "at or near the surface of the water" is 6 fathoms (36 feet) or less at low tide. Obviously, reefs can be produced by natural processes like erosion and deposition of sediment, and also by the actions of organisms. We call the first type abiotic, the second type biotic. It is the biotic reefs that we are concerned with in this section.
Many types of organisms have produced reef-like structures in the long history of life in the sea. The most primitive organisms on Earth, the prokaryotic cyanobacteria, have been doing it for well over 3 billion years. The simplest eukaryotic organisms, the protists, have been important contributors to the building of reefs for nearly as long. Reef-building is indeed an ancient art form.
One important group of skeletal reef-builders with both sponge-like and coral-like characteristics arose during the Cambrian Period of Earth's history. Known as the Archeocyathids, their tenure on Earth was extremely short and they are no longer with us. The true sponges have always been important contributors to the construction of reefs, but during one period in Earth's history they were major players in the reef-building business. Corals make their appearance as major reef-builders during the Ordovician Period and although many forms of coral have come and gone, they are still the dominant constructors of reefs today. Finally, even unlikely creatures like clams and oysters have been important reef-builders in the past.
Some organisms are not so much reef-builders as reef-contributors. Various forms of red algae, bryozoans, and even crinoids and brachiopods have been important contributors who help "glue" together the framework structure of reefs.
The first life forms on Earth were simple bacteria. These single-celled prokaryotes comprise their own kingdom, initially known as the Kingdom Monera, but now recognized as the Kingdom Prokaryotae. The cyanobacteria are a major phylum within this kingdom. Consisting of photosynthetic blue-green bacteria (also known as blue-green algae), the cyanobacteria have made a tremendous impact on our planet. Because they photosynthesize, cyanobacteria were a major source of atmospheric oxygen during the Proterozoic Eon. Their abundance during this period of Earth's history is attested to by the presence of Proterozoic oil deposits. These ancient pools of oil are derived mostly from cyanobacteria!
Cyanobacteria were major reef-builders eons before the corals and sponges. The reef structures built by cyanobacteria are called stromatolites. They can still be found today in a few isolated locations in the tropics, usually in shallow waters of high salinity. But during the Archean and Proterozoic Eons, stromatolitic reefs were being constructed all over the world.
Stromatolitic reefs are not skeletal structures like modern-day coral reefs. They are more accurately defined as organosedimentary structures produced by the adherence of silt, sand, and clay to the sticky mucus secreted by the cyanobacteria. The mucus serves as protection against harmful ultraviolet radiation, but is also very efficient in trapping sediment. The cyanobacteria are also able to fix CaCO3 which helps to cement the components of the reef together. The stromatolite structure grows as the bacterial colony is covered by sediment which sticks to the mucilage on the surface. The cyanobacteria must now recolonize the surface of the sediment layer for photosynthesis to resume. The bacterial mat that forms on the surface begins to secrete mucus and the cycle starts all over again. In this way, huge stromatolitic reefs were eventually constructed in the ancient Precambrian seas of planet Earth.
The simplest and most primitive members of the Animal Kingdom are the sponges. These filter-feeding, benthic sessile organisms have no organs or true tissues. Comprising the Phylum Porifera, sponges are essentially masses of eukaryotic cells embedded in a gelatinous organic matrix. Their only hard parts are tiny, needle-like spicules composed of CaCO3, SiO2, or spongin, which give the sponge colony support and rigidity. Three of the main classes of sponges are defined by the composition of their skeletons: the Calcispongea possess a CaCO3 skeleton, the Demospongea have spicules composed of spongin (some demosponges have siliceous spicules supported by spongin), and the Hyalospongea have glass spicules composed of SiO2.
A fourth class of sponges employed all three materials when constructing their skeletons. Known as the sclerosponges, these important parazoans have siliceous spicules and spongin fibers supported by a basal layer of calcium carbonate. One order of sclerosponges in particular is noted for its massive reef-building projects during the early Paleozoic Era. Along with the colonial hard corals, the stromatoporoid sponges produced extensive reef systems in the shallow Silurian and Devonian seas of ancient Earth.
Approximately 70 genera of stromatoporoids are recognized in the fossil record. All are constructed of thin, horizontal layers (or laminae) supported by fibrous columns of calcium carbonate. This system of vertical pillars and branching, horizontal canals is a distinctive characteristic of the stromatoporoid sponges. The ancient surfaces of stromatoporoid reefs were sometimes smooth and unmarked, but more often bumpy and undulating. Those "bumps" on the surface of the reef sometimes bear another feature that is a hallmark of the stromatoporoid sponges. At the top of each bump is a star-shaped cluster of small, radiating canals known as astrorhizae.
During the Cambrian Period of Earth's history an entire phylum of reef-building organisms arose, flourished, and then disappeared from the fossil record. Known as the Archeocyathids, these simple "cup animals" possessed both sponge-like and coral-like characteristics. The calcareous skeleton of these invertebrates consists of a cone-shaped or in most cases double cone structure which is hollow and porous. These characteristics are commonly seen in the sponges. In the double cone species, the calcareous skeletons have radial walls or septa that connect the internal and external walls. These radial septa are a characteristic seen in the corals.
Archeocyathids were prolific reef-builders during their short tenure on Earth. They are found only in Cambrian limestones, usually in association with stromatolites and sponges. Archeocyathids were the first skeletal reef-builders on Earth. Their reef structures were modest in size compared to modern-day coral reefs, but some fossil Archeocyathid reefs have been traced in the rocks for hundreds of miles.
The reef-building corals are part of a large and varied phylum of organisms known as the Cnidarians. Originally called the Coelenterates, these invertebrates possess specialized stinging cells (nematocysts) which are unique to the phylum. The Cnidarians are dimorphic in that they display two body forms, the medusa and the polyp. Some groups occur only in the medusa form, others only in the polyp stage, while a few occur in both. The hydrozoans (hydroids, hydras, and the Portuguese Man-of-War) generally display a true dimorphism while the scyphozoans consist of jellyfish that are for the most part medusas throughout their lives. The cubozoans are also jellyfish but their square-shaped medusas and nektonic rather than planktonic lifestyles place them in a different class than the scyphozoans. Known as the "box jellyfish", this group of invertebrates includes some of the deadliest creatures on Earth. The Sea Wasp (Chironex fleckeri) is probably the most dangerous. The anthozoans, which include the anemones and the reef-building corals, live their entire lives as polyps, forsaking the medusa stage entirely.
The Cnidarian fossil record begins early with jellyfish impressions in ancient Precambrian sandstones and quartzites. Evidence of both jellyfish and sea pens can be found in Ediacaran fossil assemblages the world over. These trace fossils range in age from 545 to 670 million years old. The first stony corals appeared early in the Paleozoic Era, during the Ordovician Period. They quickly became the dominant reef-builders on Earth.
While the jellyfish and hydras lack hard parts, other Cnidarians are quite adept at constructing skeletons. Some of these are merely contributors to the reef structure, helping to bind or "glue" it together with their attached hard parts. Others are the actual builders of the reefs themselves.
As a group, the hydrozoans are insignificant reef-builders, but one family of these Cnidarians is an important contributor to the construction of reefs and has even had a stint at reef-building. The milleporids, which include modern-day fire coral, are colonial hydrozoans that produce a CaCO3 exoskeleton. They had their heyday as reef-builders during the Tertiary Period of Earth's history. Massive milleporid reefs occur in the limestone formations of that period. At no other time in Earth's history do we find such prolific reef-building by a hydrozoan.
Most anthozoans produce a hard exoskeleton, usually of CaCO3 but sometimes of SiO2, that attaches to the substrate and in time becomes part of the reef. At the very least, these creatures are minor contributors to the reef, but a few rank as the most important reef-builders in the history of life on Earth.
The sea pens are an ancient group of anthozoans whose fossil record extends into the Proterozoic Eon, some 670 million years ago. Known as the pennatulaceans, these filter-feeding invertebrates construct feather-like colonies composed of CaCO3. Because their hard calcium carbonate skeletons are attached to the substrate, the sea pens can be considered minor contributors to the framework structure of coral reefs.
The sea pens and the closely related sea fans and sea whips are classified as "soft corals". As such, they are only minor contributors to the reef. The sea fans and sea whips are sometimes called the "horny corals" but are better known as gorgonians. They are also referred to as the octocorals in allusion to the 8 tentacles associated with each polyp in the colony. (The hexacorals have only 6 tentacles per polyp.) Likewise, the black corals and wire corals are at best only minor contributors to the reef. Known as the antipatharians, these stick-like corals thrive in deep, dark waters which are unfavorable for the growth of reef-building corals.