Just what is a coral reef? The answer to that question depends somewhat on one’s perspective.
In one sense, the term “coral reef” is used to simply refer to the actual rocky reef structures composed of compacted or deposited remains of countless tiny coral animals and algal material – the structures long dreaded by mariners.
This definition of “what is a coral reef?” may be thought of as the “microscale” view of a coral reef.
At the broader “macroscale” level, ecologists view coral reefs as the biologically richest and most diverse of all marine biomes on our planet.
At this larger scale, coral reefs are defined from an ecosystem perspective – mosaics of different yet ecologically interconnected habitat types.
This larger view of coral reefs is discussed in some detail in our section on the coral reef biome. Here, we focus upon the foundation of these larger ecosystems – the reefs themselves.
Although a wide variety of marine creatures ultimately contribute to the complexities of coral reef structure, most of the reef’s underlying solid framework is constructed by a combination of just a few highly specialized kinds of animals and plants.
The vast bulk of the reef base is formed by just one of the two types of reef-building (hermatypic) corals called the scleractinian (“hard”) corals.
In these animals, the body of each individual (called a polyp) is encased in a hard external skeleton composed of calcium carbonate formed by the animal itself from substances extracted from seawater.
As adults, hard coral animals exist as parts of large cohesive assemblages called colonies.
Each coral colony is composed of many thousands of genetically identical individual polyps.
Only the outermost layer of the colony is composed of living polyps. Succeeding generations are built upon the skeletal remains of their forbearers.
Occasionally, isolated colonies of hard coral may be found, but far more typically they grow in larger assemblages that we call coral reefs, composed of many colonies of the same or different species.
Hard coral colonies generally exhibit one of three basic growth forms, called massive, branching, or plate-like.
Massive (left), branching (center) and plate-like (right) are the most common growth forms of hard coral colonies.
The shape, size, and color of these colonies are subject to modification by local environmental conditions such as wave action, currents, prevailing winds, etc., often leading to substantial variability in the appearance of colonies of the same species.
A smaller contribution is made by a second group of hermatypic corals called the Milleporina (“fire corals”. etc.) that grow atop the underlying reef base.
Growing upon and amidst the coral colonies are an unusual group of plants, called coralline or calcareous algae. As with the reef-building corals, these plants also incorporate calcium carbonate into their bodies.
Coralline algae are major contributors to the underlying reef structure; in some cases these plants can comprise almost half of the biomass of the reef framework.
Coralline algae also play a pivotal role in helping to build and cement the reef into a strong cohesive structure.
Ultimately, a great variety of organisms add to the building and biodiversity of coral reefs. Carbonate particles contributed by marine organisms other than coral and coralline algae contribute to the reef structure, albeit to a far lesser degree than the main reef-building corals and algae.
On many shallow reefs of the Caribbean region, a superficial layer of sponges, octocorals, and other invertebrate animals and macroalgae grows from the underlying structural foundation of the reef.
Where such assemblages occur in abundance, they provide addeitional structural and biological complexity and diversity to coral reefs
In summary then, it might be simply said that coral reefs are uniquely beautiful and complex types of shallow ocean environments filled with a diversity of colorful fishes, corals, and all manner of other fascinating life forms. By far, they are the best places for the average marine life enthusiast to explore underwater.
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