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Coral reefs
Surprising facts about the rainforests of the ocean

Coral reefs are the rainforests of the ocean – bright spots of food and shelter in the vast, cold waters of the open sea. And like the rainforests, reefs have a profound impact on the overall health and well being of the planet and face serious threats to their existence.

As snorkelers and divers know, coral reefs teem with life. Colorful fish, turtles, invertebrates and other animals rely on reefs for food and shelter. They are the most biologically diverse ecosystems on earth and are home to nearly one quarter of the world’s marine species.

Not only do reefs serve as a home for marine life, they also are critically important to humans. The majority of the total world population lives in coastal areas and those people depend on reefs for food, income and protection. Coral reefs provide an income source through fishing and tourism. In addition, scientists have discovered that coral communities may contain compounds that can be manufactured into medicines to treat diseases like cancer and AIDS. Humans also depend on reefs for protection from storms and tidal surges.

Almost a thousand coral species currently exist and take fantastic shapes that resemble mushrooms, moose antlers, cabbages, table tops, wire strands, fluted pillars and wrinkled brains.

But what are they?

Corals: animals, vegetables or minerals?
Despite the rock-like or plant-like appearance of corals, they are animals – tiny creatures that live together in large colonies.

Coral are divided into two basic types: hard corals and soft corals. Hard corals have cup-like skeletons that form the foundations of coral reefs. Soft corals have bits of bone-like calcium carbonate scattered throughout their bodies. Coral reefs are formed by colonies of individual coral animals called polyps that live together in limestone skeletons. Each polyp is a soft-bodied invertebrate that resembles an upside-down jellyfish.

Although corals are animals, they have a symbiotic (mutually beneficial) relationship with algae that lives inside their tissues. These algae are known as zooxanthellae. The coral provides a protected environment for the algae to live in. In return, the algae produce sugars and other compounds that help nourish the coral. Coral also gets its color from the zooxanthellae living deep within the tissues of each polyp. Coral polyps receive about 90 percent of their food from the zooxanthellae. The other 10 percent of their food come from capturing plankton.

Coral polyps hunt by extending their tentacles and capturing plankton, which is microscopic plants and animals that drift on the ocean’s currents. Corals feed at night because they are less likely to become victims of predators. During the day, coral polyps pull in their tentacles and hide deep within their limestone skeletons.

Corals come in a wide variety of shapes including branching, elkhorn, mushroom and digitate (finger-like). Different types of coral have different growth patters, but several factors can affect the shape a colony takes: where the coral lives, wave action, light, temperature, the corals’ proximity to each other.

Corals can reproduce either sexually or asexually. During asexual reproduction, a coral polyp clones itself by forming a new “bud” next to the parent polyp. The newly formed polyp is an exact copy of the parent and will remain forever attached to the parent. Sexual reproduction, also called spawning, takes places when corals release eggs and sperm into the water to be fertilized. Fertilized eggs result in coral larvae. Once a larval coral is formed, it floats on the water’s surface for several days, sometimes weeks, until it finds a suitable hard surface on which to attach and start growing.

In the Flower Garden Banks National Marine Sanctuary, off the coast of Texas, mass coral spawning is as predictable as the phases of the moon. Each summer, eight to 10 nights after the last full moon of August, between the hours of 9 p.m. and midnight, the corals in the Flower Garden Banks begin spawning. Each coral species spawns on a different night to prevent hybridization. This mass spawning event has been compared to “an upside-down snowstorm.”

Unfortunately, human activities, including those associated with global warming, are threatening these rainforests of the ocean and the future of coral reefs is uncertain. Current estimates note that 10 percent of all coral reefs have been degraded beyond any hope of recovery. Around 30 percent of coral reefs are in critical condition and could die within 20 to 30 years. Experts predict that if current pressures are allowed to continue, 60 percent of the world’s coral reefs may be completely dead by 2050.

There are a number of ways for people to personally help protect coral reefs:

  • Do not buy saltwater tropical fish or other live reef animals, including coral, for a home aquarium unless you can be sure they were collected in a sustainable manner.
  • Do not buy dead or dried marine souvenirs such as coral, pufferfish or seahorses.
  • Do not feed or touch marine animals when swimming, snorkeling or diving. It disrupts normal feeding and mating behavior patterns and may introduce disease or aggression.
  • Participate in coastal or river clean-up projects such as the Tennessee River Rescue. Make sure trash always goes in the proper container.
  • Be responsible at home by making sure cars do not leak oil and use caution when applying pesticides or other chemicals to lawns. Water that comes into contact with surfaces contaminated by oil or pesticides will introduce those contaminates to river and ocean environments.
  • Try to conserve energy whenever possible. Take public transportation, walk or ride a bike whenever possible. This helps reduce global climate change. Coral reefs are sensitive to even slight increases in water temperature.
  • Support marine sanctuaries.

Because many of the world’s coral reef communities are in danger, removal of natural coral from the ocean should be avoided. The coral formations in the Aquarium’s Secret Reef exhibit are not live coral, but a reproduction created by pressing molds into wet concrete. The concrete was then painted to replicate coral.

The Tennessee Aquarium inspires wonder and appreciation for the natural world. Admission is $17.95 per adult and $9.50 per child, ages 3-12. Each ticket purchased helps support Aquarium conservation programs. The IMAX® 3D Theater is next door to the Aquarium. Ticket prices are $7.95 per adult and $5.50 per child. Aquarium/IMAX combo tickets are $21.95 for adults and $12.50 for children. Advance tickets may be purchased online at or by phone at 1-800-262-0695. The Aquarium, located on the banks of the Tennessee River in Chattanooga, is a non-profit organization. Open every day except Thanksgiving and Christmas, the Aquarium and IMAX are accessible to people with disabilities. Members enjoy unlimited visits and other benefits. Call 267-FISH to join.

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