Coral Reefs: Ancient Cities of the Sea!

Amazing Fact: Coral reefs are over half a billion years old, making them the oldest complex ecosystems still thriving on Earth! These ancient cities of the sea existed before there was any life on land. Today, they support an astounding diversity of marine life, earning their reputation as the rainforests of the ocean.

The Living Reef

Coral reefs are spectacular to behold, lush gardens in the sea, supporting a staggering amount of marine life in a densely packed, thriving marine metropolis. In fact, coral reefs have the largest abundance and greatest diversity of life living together of any place on Earth, including the tropical rain forests. People often refer to coral reefs as “rainforests of the sea.”

A dive on a coral reef is a voyage to another world. The surrealistic landscape is shaded in blue and surrounded by life. The coral reef is a gathering place in the ocean. It is an oasis in a desert, a place which gives shelter and food in an ocean where these things are rare. In fact, the entire tropical ocean ecosystem depends on the reef for sustenance.

Nature’s Master Builders

The reef itself is a living, growing organism—colonies of tiny animals all working together to create the largest structures on Earth. A coral reef is a hard structure made by the combined efforts of thousands of coral polyps.

A coral polyp is a tiny animal, with a circle of tentacles around its mouth. They are sessile, benthic animals, meaning that they live on the bottom, fixed in one position. Unlike anemones, which are solitary animals, coral polyps grow in colonies. These colonies can become quite large—a brain coral colony can reach over 10 feet across and may contain many thousands of individual polyps.

A coral reef may stretch hundreds of miles across, but it is constructed by polyps only a quarter of an inch or less in size!

Building the Skeleton

The reef is built up on a skeleton that the coral polyps make of calcium carbonate (limestone). Ever so slowly, over hundreds or thousands of years, the coral polyps add limestone to their skeletons in layers, and grow outward and upward, expanding the reef.

The coral polyps extract calcium from the seawater and combine it with carbon, produced as a by-product of respiration, to produce the limestone. Hard corals, also called reef-building corals, produce reefs with their rigid, rock-hard skeletons.

Growth Rates: Staghorn corals can grow as fast as 6 inches a year, making them among the fastest growing corals in the world. Most other corals grow less than an inch per year, making them slow to recuperate from reef damage. Judging by the size of many slow-growing coral colonies, some of them are well over one thousand years old!

Types of Corals

Hard Corals (Reef Builders)

Hard corals produce rigid, boulder-like reefs. They include species like brain coral, star coral, and staghorn coral. These are the primary architects of the reef structure, slowly building massive underwater mountains of limestone over millennia.

Although living coral hides its skeleton under living tissue, a dead piece of coral reveals just the white skeleton underneath.

Close-up of soft coral polyps and spicules.

Soft Corals

Not all corals produce rigid, boulder-like reefs. There are many species of soft corals, which look like trees or bushes, flexing in the currents. Many a snorkeler or diver has mistaken these animal colonies for plants!

For a skeleton, most soft corals manufacture tiny needle-like splinters called spicules out of calcium carbonate. These spicules are embedded within the colony to give it strength, yet still allow the colony to flex and bend with the currents.

Types include:

Branching soft corals – Tree-like appearance
Gorgonians – Sea whips with bush-like appearance
Sea fans – Brilliantly colored fan-shaped corals

Reef Facts

500+ Million Years Old

Among Earth’s oldest ecosystems

25% of Marine Species

Supported by reefs covering <1% of ocean

1000+ Years Old

Some coral colonies are ancient

65°F Minimum

Temperature needed for reef growth

Coral Reef Gallery

Coral as seen from the surface in shallow water, Malaysia.

Close-up of interconnected polyps of Brain coral. The green zooxanthellae are quite obvious in this shot!

Close-up of star coral polyps

Soft coral on a wall, Philippines.

Close-up of soft coral polyps and spicules.

A colorful Indonesian reef.

Star coral spawning, Caribbean.

The Secret to Success: Zooxanthellae

In an area with this much diversity and life, it is easy to think that the tropical oceans are highly rich in nutrients. However, compared to the cold, murky waters of the temperate seas, coral reefs live in nearly sterile water. The waters of the Caribbean are clear, unclouded by plankton. In an ocean with very low food resources, all forms of life have to struggle for survival, and coral is no exception.

A Perfect Partnership

Because of the low plankton density, many corals simply cannot get enough nourishment by catching plankton with their nematocyte-laden tentacles. Many species of coral have solved this problem with a symbiotic relationship.

These corals, called hermatypic corals, have multitudes of tiny, single-celled plants (actually, a type of dinoflagellate) living in their tissues. The plants, called zooxanthellae (zoo-zan-THEL-eye), give many corals their greenish to brownish color.

How It Works:

Zooxanthellae use sunlight to produce sugars through photosynthesis
The coral shares in this production, getting energy from excess sugar
The coral may also absorb oxygen given off during photosynthesis
In return, the plants use carbon dioxide and waste products from the coral

It’s an ideal mutually symbiotic relationship, and it works well except for one limitation: the sun. Hermatypic coral must be exposed to sunlight for photosynthesis to occur. This limits the depth to which hermatypic corals can live to less than about 75 meters (250 feet).

Climate Change Threat: Rising ocean temperatures cause coral bleaching, where stressed corals expel their zooxanthellae, losing their color and primary energy source. Without their algal partners, corals can starve and die.

Where Reefs Grow

A few varieties of small corals live in cold waters, but reefs cannot grow there. Biologists believe that cold water inhibits the coral’s ability to form associations with zooxanthellae which facilitates calcification essential for building reefs.

Coral reefs are therefore only found in ocean regions where the water is always above approximately 65 degrees Fahrenheit, or roughly between the Tropic of Cancer to the north and the Tropic of Capricorn to the south.

Reef Ecosystems

An Ocean Oasis

The importance of the coral reef in the tropical ocean ecosystem cannot be stressed enough. Coral reefs provide nesting areas and hiding spots for fish and invertebrates, making the reef into a teeming metropolis of life. This, in turn, attracts larger predators to the area looking for food.

A coral reef may be packed with fish and other marine life, while only a hundred yards from the reef, there is hardly a thing to be found. The reef truly is an oasis in the desert of the tropical ocean.

Artificial Reefs

For a new coral reef to form, a coral larva has to settle down on an appropriate surface and start to grow. A solid foundation could be as simple as a small rock on the sand, or a barren piece of dead coral skeleton. Sometimes, however, the most unlikely objects can form the basis for reef growth.

Many man-made objects from oil rigs to shipwrecks start new reefs by providing a foothold. In many places all over the Caribbean, artificial reefs are created by the intentional sinking of old ships onto barren patches of underwater terrain.

Within just a few years, soft corals and sponges will grow on the wreck. A hundred years down the road, the coral will completely overgrow the ship, creating a beautiful and life-sustaining coral reef to endure for thousands of years, long after the ship itself has rusted away.

A Living Wonder

Coral reefs are among nature’s greatest spectacles. They form communities of startling complexity and help to create an entire ecosystem in oceans with low nutrient resources. Throughout their life stages, corals act as food for other animals, shelter for other animals and producers of the greatest examples of natural architecture in the world.

All this and more is done by tiny animals less than a centimeter across. Perhaps this is why the coral reef truly is a living wonder.

Reef Facts

500+ Million Years Old

Among Earth’s oldest ecosystems

25% of Marine Species

Supported by reefs covering <1% of ocean

1000+ Years Old

Some coral colonies are ancient

65°F Minimum

Temperature needed for reef growth

Coral Reef Gallery

Coral as seen from the surface in shallow water, Malaysia.

Close-up of interconnected polyps of Brain coral. The green zooxanthellae are quite obvious in this shot!

Close-up of star coral polyps

Soft coral on a wall, Philippines.

Close-up of soft coral polyps and spicules.

A colorful Indonesian reef.

Star coral spawning, Caribbean.

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