Coral reefs are massive underwater structures made from the hard limestone exoskeletons of thousands of tiny living organisms (coral polyps) produced one on top of another in warm, clear, shallow ocean waters. Living polyps extend upward and outward from the coral colony center and live on top of the old dead exoskeletons. Coral reef communities are crowded with other animals representing virtually every major animal phylum. Space is at a premium on reefs, corals, seaweeds (various forms of algae), sponges, or other organisms cover virtually every surface. Because both corals and algae require light to survive, access to light, like space, is also a resource subject to competition.

Fast-growing, branching corals can grow over slower-growing, encrusting, or massive corals and deny them light. In response, the slower-growing forms can extend stinging filaments from their digestive cavity and kill their competitor’s polyps. Undamaged polyps on the faster-growing, branching coral, however, may grow very long sweeper tentacles, containing powerful nematocysts (stingers) that kill polyps on the slower-growing form. The faster-growing form repairs the damage and continues to overgrow its competitor. In addition to sweeper tentacles and stinging filaments, corals have several other mechanisms available for attack or defense.

In general, slower-growing corals are more aggressive than fast-growing species. In cases where a competitor cannot be overcome, however, corals may survive by taking advantage of differences in local habitats. Massive corals are generally more shade tolerant and able to survive at greater depths. Therefore, on many reefs it is the fast-growing, branching corals that ultimately dominate at the upper, shallower portion of the reef, whereas more massive forms dominate in deeper areas.

Corals also must compete with other reef organisms, each with its own strategies for survival. Sponges, soft corals, and seaweeds (algae) can overgrow stony corals and smother them. Algae are competitively superior to corals in shallow water but less so at depth. Survival of coral in shallow water, therefore, may depend on grazing by plant-eating echinoderms (starfish and sea urchins) and fishes. In Jamaica, overfishing removed most of the plant-eating fish from coral reefs. Initially, algal growth was kept in check by grazing sea urchins, but in 1982, a pathogen reduced the population by 99 percent. Without grazers, the algae were able to completely overgrow the coral.

Competition may occur among other reef communities. Grazing by urchins and fishes is important in preventing seaweeds from overgrowing the reef. The dominant algae on a healthy reef are usually fast-growing filamentous forms or coralline algae, well protected by calcification (hardening) and the production of noxious chemicals. These algae are inferior competitors to larger, fleshier seaweeds, so grazing by urchins and fishes on the larger seaweeds allows these algae to persist. Grazing on plants is greatest in the shallow reef areas but decreases with depth, where lower temperatures and light reduce algal growth. The reef is, therefore, a mosaic of microhabitats with different levels of grazing and different algal communities.

An additional complexity arises from the activity of damselfish. Because they are territorial, many damselfish species exclude grazers and other species from certain areas of the reef. Algae grow rapidly in these territories, providing habitat for many small invertebrates but overgrowing the corals. Branching corals tend to dominate in damselfish territories because they are upright and faster growing than the more massive or encrusting forms.

Although less studied than on rocky shores, predation almost certainly has a significant influence on the community structure of coral reefs. Fish and other predators may preferentially prey on such competitors of corals as sponges and gorgonians, giving competitively inferior reef corals an advantage in securing space. Many species of fish, mollusks, and crustaceans also feed directly on coral polyps. Several surgeonfish and parrotfish may actually pass coral skeletons through their digestive tracts and add sediment to the reef. Both fish and invertebrate corallivores (coral-feeding organisms) seem to attack faster-growing, branching species preferentially, perhaps preventing slower-growing forms from being overgrown. Corallivores, however, rarely ever completely destroy a coral colony except in cases where tropical storms or humans have already done severe damage. The fact that almost all small invertebrates on reefs are so well hidden or highly camouflaged is another indicator of how prevalent predation is on reefs and its importance in determining reef structure.

珊瑚礁是由成千上万的微小生物（珊瑚虫）的坚硬石灰岩外骨骼构成，并在温暖，清澈，浅浅的海洋中彼此重叠，从而形成的巨大水下结构物。 活着的珊瑚虫从珊瑚群落的中心向上和向外伸展，并在老死的生物外骨骼上面生活。 珊瑚礁群落满是几乎代表所有主要动物类别的一些海洋动物。礁石，珊瑚，海藻（各种藻类），海绵或其他生物几乎覆盖了珊瑚的每个面，因此生活的空间都很重要。 因为珊瑚和藻类都需要光照才能生存，所以就像空间一样，这样的光线对于海洋生物而言也是一种竞争的资源。 生长迅速的珊瑚虫可以在生长缓慢，结壳或块状珊瑚上生长，并使它们不发光。但是，生长缓慢的珊瑚可以将其消化腔中产生刺痛的细丝向四处扩散并杀死其对手的息肉。 然而，生长速度较快的分枝珊瑚上，未损伤的息肉可能会长出非常长的触角，其中含有强大的刺激性线虫囊（刺激物），能够杀死生长缓慢的珊瑚虫。 这种快速增长的形式可以修复损害，并继续扩大超过其竞争对手。 除了清扫触角和刺痛的细丝之外，珊瑚还有其他几种可用于攻击或防御的机制。 一般来说，生长较慢的珊瑚比速生的珊瑚更具侵略性。 然而，在无法战胜竞争对手的情况下，珊瑚可以通过利用当地栖息地的差异而存活下来。块状珊瑚通常耐荫性更强，能够在更深的地方生存。 因此，在许多珊瑚礁上，生长速度最快的分支珊瑚最终在礁石的上部较浅的部分占优势，而在更深的地区，块状珊瑚礁占优势。 珊瑚也必须与其它的珊瑚礁生物竞争，而每个珊瑚礁都有自己的生存策略。 海绵，软珊瑚和海藻（藻类）可以覆盖多石珊瑚，并使它们窒息。海藻在浅水中比珊瑚更有优势，但在深度上不如珊瑚。 因此，在浅水中生存的珊瑚可能依靠吃植物性棘皮动物（海星和海胆）和鱼类为生。 在牙买加，由于过度捕捞，使得珊瑚礁的大部分藻食性鱼类中消失了。 最初，人们通过投放海胆来控制藻类生长，但在1982年，当地的一种病原体使水中的生物减少了99％。 没有了食草动物，藻类就能够完全长满珊瑚。 其他珊瑚礁群落也可能会发生竞争。 人们投放海胆和鱼类对于防止海藻生长过长是非常重要的。 生长在状态良好的珊瑚礁上的主要藻类通常是生长迅速的丝状藻类或珊瑚藻，它们受到钙化（硬化）和有害化学物质的良好保护。 这些藻类是较大的，肉质海藻的对手，因此，在较大的海藻上，投放海胆和海鱼，可以使这些藻类继续存活。在浅水区投放植物是最好的办法，因为随着深度的下降，在低温和光照下将减少藻类的生长。因此，珊瑚礁是一个有着不同程度的投放和不同藻类群落的小栖息地。 雀鲷的活动会出现更多复杂的情况。 由于是他们的领地，许多雀鲷物种排斥来自珊瑚礁某些区域的掠食者和其他物种。这些地区的藻类生长迅速，为许多小型无脊椎动物提供了栖息地，但却使珊瑚过度生长。 分支状珊瑚往往在雀鲷地区占主导地位，因为它们比那些较大或结实的形态更直，生长的更快。 虽然人们对岩石海岸的研究较少，但几乎可以肯定的是，捕食对珊瑚礁的群落结构会产生重大影响。鱼类和其他食肉动物可能先捕食如海绵和柳珊瑚这类珊瑚，这使得具有竞争力的礁珊瑚在保护空间方面有优势。许多鱼类，软体动物和甲壳类动物也直接以珊瑚虫为食。一些刺青鱼和鹦嘴鱼实际上可以通过它们的消化道通到珊瑚骨骼，并将沉积物添加到珊瑚礁中。鱼类和无脊椎动物(以珊瑚为食的生物) 似乎更倾向于攻击生长速度更快的分支物种，也许可以防止生长缓慢的物种过度生长。然而，珊瑚虫很少能完全摧毁珊瑚群落，除非热带风暴或人类已经造成了严重破坏的情况。事实上，珊瑚礁上几乎所有的小型无脊椎动物都隐藏得很好或者高度伪装，这是另一种迹象，表明在礁石上的捕食是多么普遍，以及确定珊瑚礁结构的重要性。