These differing zones result from interactions between many factors, including sea floor depth profile, water movement, light penetration, and other factors that affect hard coral species distribution and abundance.
There are numerous ways to classify coral reef zonation patterns, but perhaps the simplest and most practical scheme recognizes three primary, easily recognizable kinds of areas found in most coral reef ecosystems. These primary coral reef zones are called the (1) fore reef, (2) reef crest, and (3) back reef .
The three primary coral reef zones typically found in coral reef ecosystems. Modified from graphic provided by U.S. Geological Survey.
The comparative extent and development of each of the three primary coral reef zones often varies considerably at different geographic locations and even within different parts of single reef systems. In some places, one zone may abruptly transition to another, while in other places such transitions are more gradual. At some locations an entire zone may be poorly developed or entirely missing. .
Because each zone differs from the others in terms of numerous factors that have profound effects upon the distribution and abundance of hard corals and other types of marine life, each of these zones represents a different type of coral reef habitat for fishes, invertebrate animals, and plants. Even so, many species occupy more than a single zone, and highly mobile species such as reef fishes often move freely between zones.
Many reef scientists routinely further subdivide the three primary zones into a number of subzones. This is useful in certain ecological contexts, particularly in terms of describing the often uneven distribution or abundance patterns of corals and other types of marine life within a primary zone.
Finally, it should be understood that there is by no means universal agreement among reef scientists regarding the classification of coral reef zonation patterns or even the names given to different zones and subzones. This can often be a source of confusion and frustration for professional scientists as well as the general public.
In particular, it should be noted that scientists studying Indo-Pacific coral reefs often use a somewhat different classification system and terminology than do their colleagues in the greater Caribbean region. This is largely due to certain systematic differences in the structure of coral reef environments between the two regions.