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What is a Coral Reef?

Coral reefs are among the most diverse communities on this planet, often described as “rainforests of the sea”. Reefs occur in clear, shallow waters throughout tropical regions across the globe. Formed by the calcium carbonate skeletons, the backbone of the reef is built by tiny coral animals that make up large coral colonies.

Coralline algae produce calcium carbonate, which cements the coral skeletons together, forming the continuous reef structure. The skeletons of tube worms, mollusks, and other organisms also become incorporated into the reef.

Pillar coral. Photo courtesy Florida Keys National Marine Sanctuary
Pillar coral. Photo courtesy Florida Keys National Marine Sanctuary
Tube coral. Photo courtesy South Florida Water Management District
Tube coral. Photo courtesy South Florida Water Management District


Importance

Reefs provide habitat and nursery areas for many organisms.

Florida coral reef biodiversity. photo courtesy NOAA
Florida coral reef biodiversity. photo courtesy NOAA

Coral reefs are important habitat and nursery grounds for fishes and invertebrates, including those of commercial and recreational value. They are closely associated with mangrove and seagrass communities, providing protection from wave and storm damage. Disturbances to reefs may result in upsetting the ecological balance of the reef as well as having indirect impacts on other nearby habitats.

Scientific Classification

The classification Cnidarian includes:

Zooxanthellae. Photo courtesy NOAA
Zooxanthellae. Photo courtesy NOAA

The reef-building corals can be identified by their stony skeletons made of calcium carbonate. A coral colony consists of thousands of individual coral animals, each similar in appearance to a small sea anemone with its base attached to a calcareous cup. Corals are armed with a ring of tentacles used to capture zooplankton from the surrounding water. Reef-building corals also contain symbiotic algae, called zooxanthellae, within their tissues. These single-cell algae have a mutualistic association with the coral hosts, a relationship that benefits both partners. The algae utilize carbon dioxide and nitrogen-based waste products released from the coral. In return, the algae perform photosynthesis, producing sugars and amino acids. These products are transported to the coral in support of its nutritional needs.

Hydrozoa

Fire coral (Hydrozoan). Photo courtesy NOAA
Fire coral (Hydrozoan). Photo courtesy NOAA
Coral polyps. Photo courtesy Florida Keys National Marine Sanctuary
Coral polyps. Photo courtesy Florida Keys National Marine Sanctuary


Milleporina, commonly referred to as fire corals, are hydrozoans. They are common throughout the Caribbean and Atlantic reefs with two species found in Florida waters: Milliporina alcicornis, a branching form, and Milliporina complanta, a bladed form. Fire coral is named for the nematocyst-containing defensive and food capturing polyps. Nematocysts contain a coiled barb, trigger, and neurotoxin. Upon stimulation, the trigger shoots the barb, which releases the neurotoxin upon entering the prey or predator.

Anthozoa

Anthozoans consist of several subclasses including:

Sea rod. Photo courtesy NOAA
Sea rod. Photo courtesy NOAA
Sea fan. Photo courtesy NOAA
Sea fan. Photo courtesy NOAA


Anthozoans include soft corals, gorgonians, stony corals, and anemones. The subclass Octocorallia consists of the soft corals including gorgonians. On Florida’s reefs, soft corals are common in various forms ranging from encrusting mats to large sea fans. Gorgonians, including sea fans, sea plumes, and sea rods, are well-adapted to wave action with flexible skeletal materials and strong holdfasts that attach to the bottom substrate. Zoantharia includes the order Scleractinia, the stony corals. These corals are distinguished by a stony, calcareous skeleton. Each coral polyp is enclosed in a calcium carbonate cup-like structure. These cups are cemented together, forming a coral colony of thousands or millions of polyps.

Stony Coral skeletons. Photo © Jan Bester
Stony Coral skeletons. Photo © Jan Bester

These polyps are withdrawn during the day, opening up at night to feed on zooplankton. During the day, the symbiotic algae located in the coral tissues, carry out photosynthesis. The algae release some of the compounds produced via photosynthesis to the host, while receiving nutrients from the coral in the form of waste products including ammonia and phosphates. Both symbiont and host benefit in this mutualistic association.


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