Major Reef Types

Charles Darwin was the first to describe specific reef types formally. Later researchers have added some to his original three.

• Barrier reefs: Separated from the shore by a wide channel, barrier reefs develop along the edge of the continental shelf. The organisms found on the seaward side often differ markedly from those in shoreward waters.

• Fringing reefs: Barrier reefs that hug the shore are usually called fringing reefs. The channel is absent, and coral growth may be exposed at low tide. Many saltwater aquarium species are collected from this, the most common type of reef.

• Atolls: When fringing reefs encircle an island and the island subsequently subsides under the ocean, an atoll is formed. These ring-shaped reefs have a central lagoon area with species that thrive in bright shallow water.

• Patch reefs: Separate from the main reef structure, patch reefs develop for a variety of reasons. Any suitably stable spot may be colonized by corals, and a new patch reef begins. Research has shown that certain key "founder" species typically start building the patch reef, and the biotope then becomes more complex and the reef framework grows.

• Bank/barrier reefs: Along the Florida Keys, reefs run parallel to the shore as with barrier reefs, but the intervening channel is narrow and shallow compared to other barrier reefs. The term bank/barrier reef is sometimes used to describe this unique topography.


On a given reef, patterns of coral zonation can be recognized. Certain corals tend to be found in certain areas of the reef. Factors influencing zonation include depth, especially as it affects penetration of sunlight, and water movement, whether forceful or relatively calm. Other local conditions, such as exposure of the reef at low tide or the increases in salinity and temperature that occur in shallow lagoons, further demarcate zones of diversity. First walking along the shore, then wading, and finally swimming, let's explore each of the major zones.

Tide Pool

Tide pools form when a tide recedes and water remains in depressions waves have sculpted in the soft coral rock. Various species routinely find themselves trapped in these pools with no option but to wait for the return of the tide. Even though the tropical sun can heat the water, increase salinity by evaporation, and thus lower the oxygen content, tide pools may be remarkably diverse. As a rule, the larger the pool, the more diverse it is. Excavations along highway U.S. 1 in the Florida Keys have created some spectacular man-made tide pools.

Iron Shore

When no beach exists and waves break directly upon rocks, we have an iron shore. Even in this harsh environment, corals and other organisms thrive, clinging tenaciously to the substrate.


Moving further seaward, we encounter the lagoon, a shallow, relatively calm and sediment-lined area, where collectors take many aquarium animals. Corals adapted for growing on soft sediment or sand, Catalaphyllia and Fungia, for example, grow here, along with dozens of sponges, soft corals, anemones, crustaceans, and echinoderms. Because nutrients become concentrated in the lagoon, plankton flourishes, feeding millions of hungry mouths. Certain portions of the lagoon accumulate a thick blanket of mud. Turtle grass, Thalassia, takes root in the sediment and covers acres. The turtle grass beds provide yet another biotope, exploited by such familiar creatures as Condylactis anemones and the giant seahorse, Hippocampus erectus.

Mangrove Swamp

Sometimes the shore edge of a lagoon is fringed with mangrove trees, creating yet another biotope. The mangrove's prop roots extend into the ocean, sheltering vast numbers of tube worms, sponges, and other invertebrates. These areas also act as fish nurseries, with juveniles of many species found there. Like the lagoon, because of its accessibility from shore, the mangrove swamp provides many aquarium species.

Back Reef Slope

Paddling further out, we find that the bottom begins to slope upward, and the amount of coral growth increases noticeably. We have reached the back reef slope, where, protected from the fury of the open ocean by the reef itself, another rich community of fish and invertebrates appears. Growth-forms, both delicate and massive, appear among the corals.

Reef Flat

At the top of the back reef slope, the bottom levels out again, often only a few feet below the surface of the sea. We have reached the reef flat and could wade again, except our feet might be cut to ribbons by the thousands of corals. Calm shallow water under the blazing sun may allow a coral garden to develop, or, on occasion, this zone is strikingly empty, strewn with coral rubble from the reef crest. Coral gardens sometimes grow so close to the surface that they are left high and dry at low tide. Corals from such areas often have bright pigments that protect them from the sun's harsh rays.

Reef Crest

Corals found on the reef crest, where waves break across the reef's highest point, often grow as branching forms highly prized by some aquarists. Often brightly colored, reef crest corals thrive on high levels of sunlight and pounding currents.

Upper Fore Reef

Surmounting (somehow) the reef crest, we arrive in the clear water of the open ocean as the reef drops away below our feet. Up here, where the shallow water permits maximum light penetration, we find the greatest abundance and diversity of both invertebrates and fishes. Here we might see a shoal of Pseudanthias or Chromis numbering in the thousands. Stony and soft corals, sponges, gorgonians, and dozens of other encrusting species vie for space in the sun. On and among them, crustaceans, worms, echinoderms, and small colorful fish seek shelter and food. Tangs, angelfish, butterflyfish, and wrasses patrol back and forth, feeding on algae and invertebrates. Here and there, lionfish, groupers, and other predators may lurk. In this reef zone, most experienced saltwater aquarists find great inspiration.

Deep Fore Reef

Light becomes more and more dim as we (hopefully now wearing scuba gear) descend the face of the reef. With each fathom of descent, we find new species. In deep water, corals become thin and flat, spreading out to capture what little light is available. Some favored aquarium fish live here, including the flame angelfish (Centropyge loriculus) and blackcap basslet (Gramma melacara).

Captivated, we might be tempted to spend hours on the deep fore reef, but it is time to return to the surface. The dive boat waits above. Soon, at home, we can begin to recreate in an aquarium one of the reef's memorable sights.

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