Although coral reefs cover less than 1% of Earth’s surface, they are among the most diverse and influential ecosystems on the planet. Reefs provide resources for many species, including significant benefits for humans.

Additional Information: An ecosystem is a community of organisms interacting with the environment in a particular area. An ecosystem includes both living components, such as plants and animals, and nonliving components, such as water or rocks.

Coral reefs are known for their high biodiversity, meaning that they contain a large number of different species. An estimated 25% of all the species in the ocean depend on coral reefs for food, shelter, or other resources.

Additional Information: Biodiversity is the variety of species in an ecosystem. Ecosystems with higher biodiversity provide habitats for a variety of organisms and can produce more types of food and other resources than ecosystems with lower biodiversity can. They may also be better at handling environmental stress.

Below are just some of the many types of animals that use coral reefs. Learn how these animals depend on the reef ecosystem by hovering over the images below.

Crustaceans

Many crustaceans, such as crabs and lobsters, depend on coral reefs for food and shelter. Example: During the day, dwarf reef lobsters hide in crevices in the corals. At night, the lobsters emerge to scavenge for food around the reef."

Fish

Coral reefs support over 4,000 fish species by providing food and shelter from predators. Fish may also use reefs as spawning grounds for breeding and as nurseries for their offspring. Example: Parrotfish eat seaweed that grow on the surface of corals. These fish are named for their beaklike mouths, which they use to scrape seaweed off the reefs. This helps keep the growth of seaweed in check.

Sea Anemones

Sea anemones often attach themselves to coral reefs in order to catch small passing fish. Example: The giant Caribbean sea anemone lives on shallow coral reefs. It captures its prey using the venom in its tentacles.

Sea Cucumbers

Some species of sea cucumbers live on coral reefs in shallow water. They feed on tiny particles of algae and decaying organic matter. Example: During the day, edible sea cucumbers hide under corals or rocks. At night, they scavenge for food in the sand.

Sea Stars

Many species of sea stars live and feed in coral reefs. Example: Blue sea stars can be found living on corals and rocks. The sea stars feed on a variety of reef inhabitants, including worms and other small invertebrates.

Sea Urchins

Sea urchins often eat seaweed growing on coral reefs. This helps keep the growth of seaweed in check. Example: The black sea urchin lives in crevices in coral reefs, where it eats seaweed and seagrass. Reefs with declining black sea urchin populations have suffered from seaweed overgrowth.

Sea Worms

Many sea worms live and find food on coral reefs. Example: Christmas tree worms, named for the distinctive shape of their feathery appendages, often burrow into corals. They feed by filtering microorganisms out of the water.

Sharks

Dozens of shark species use coral reefs as refuges and hunting grounds. Example: Whitetip reef sharks often hide in reef caves and under ledges near reefs. They feed on a variety of reef fish and crustaceans.

Turtles

Some turtle species rest or hunt for food in coral reefs. Example: Hawksbill sea turtles use reefs as hunting grounds for sea sponges, one of their primary food sources.

Ecosystem services are the various ways in which humans benefit from ecosystems. Some ecosystems provide services in the form of food, energy, building materials, and other resources. Ecosystems may also help clean the air and water, protect against erosion and natural disasters, and provide animals that pollinate crops.

Many human activities and industries benefit from services provided by coral reefs. Learn about the ecosystem services provided by hovering over the images below.

Fisheries

Coral reefs support many of the species that we eat as seafood, including fish, shellfish, and seaweed. Reefs are also an important source of income for people in fishing communities and the seafood industry. Example: Red snapper and many other reef fish are commonly caught and sold as seafood.

Tourism

Tourists travel from all over the world to visit countries with coral reefs. The tourism driven by reefs generates significant revenue and creates jobs for locals working in boat and diving centers, hotels, shops, and restaurants. Example: The snorkeler in the image is posing with a humphead wrasse, an endangered fish species found on coral reefs. Humphead wrasses are popular with tourists due to their unique appearance and gentle nature.

Shoreline Protection

Coral reefs are large, hard structures that protect the shoreline from incoming waves. This protection reduces coastal damage from erosion and storms, which helps save lives and preserve shorefront property. Example: Scientists studying the impact of a cyclone near Australia found that beaches without reefs were eroded up to ten times more than beaches with reefs.

Medicine

Some of the species that live on coral reefs produce compounds that are useful for human medicine. Reef organisms have helped scientists find chemicals for fighting cancer, AIDS, and other diseases. Example: Scientists have used compounds from sea sponges found on reefs to develop multiple medications, including the anti-HIV drug AZT.

Nutrient Cycling

Reef microorganisms help supply important nutrients, such as organic nitrogen. These nutrients boost the productivity of the reefs and other ecosystems, allowing them to provide more resources for humans. Example: The image shows colonies of Trichodesmium cyanobacteria, which look like light-brown mats, off the Great Barrier Reef. Trichodesmium converts nitrogen from the atmosphere into forms that can be used by other organisms.

Culture

Coral reefs are part of the cultural sites and values for coastal communities around the world. Some of these communities have developed art, stories, and traditions based on coral reefs and the organisms that live there. Example: The image shows a large coral sculpture in the coastal town of Tabarka, Tunisia. Tabarka celebrates an annual coral festival featuring diving and underwater photography.

You may think of coral reefs as large structures that look like colorful rocks. These structures are actually colonies of many tiny marine organisms called coral polyps.

Although a single polyp may be small (typically only a few millimeters to a centimeter in diameter), coral colonies can grow very large as they gain more and more polyps. Over thousands of years, they may build enormous coral structures that contain hundreds of thousands of polyps and weigh up to several tons. Some coral structures, such as Australia’s Great Barrier Reef, are large enough to be seen from space.

Coral polyps start their lives as larvae that are tiny and free-floating. A coral colony begins when a single larva attaches to a hard, submerged surface. The larva undergoes metamorphosis to become an adult coral polyp, then produces new polyps through a type of asexual reproduction called budding. These polyps in turn produce more polyps of their own. Because all of the polyps in the colony are produced through asexual reproduction, they are all genetically identical.

Additional Information: Larvae (singular: larva) are the young forms of certain animals that must go through metamorphosis to become adults. Other examples of larvae are caterpillars and tadpoles.

Additional Information: free-floating: Coral larvae can move using short, hairlike appendages called cilia. However, they are not strong enough to swim against, and thus often flow with, ocean currents.

Additional Information: Asexual reproduction is a form of reproduction that requires only one parent. It produces offspring that are genetically identical "clones" of their parent.

Corals also use sexual reproduction, a strategy that increases genetic variation among populations. In many coral species, polyps from multiple colonies release eggs and sperm into the water once or twice a year. The sperm can fertilize eggs from different colonies, producing larvae with new combinations of genetic material that can start new colonies.

Additional Information: Sexual Reproduction is a form of reproduction that requires two parents. The parents contribute separate gametes (eggs and sperm) that have some differences in their genetic material (DNA). Together, they produce offspring that have different combinations of the parents' DNA.

Individual polyps don’t move very much and might even look like flowering plants. But a closer examination of the polyps’ cells and other characteristics reveals that they are animals called cnidarians.

Additional Information: Cnidarians are invertebrates that typically live in aquatic environments. Their phylum, Cnidaria, contains corals, jellyfish, hydras, and sea anemones. Many cnidarians, including coral polyps, have stinging tentacles that they use to catch prey.

Animal and plant cells have somewhat different structures. They have some organelles in common, but a few organelles are found only in plants. Swipe the image below by dragging the arrow icon left or right to compare the cells of plants and animals. What similarities and differences do you observe?

Additional Information: Organelles: An organelle is a cellular structure that has a specific function in the cell. Examples of organelles include the nucleus, mitochondria, and chloroplasts.

The diagram below shows the parts of a coral polyp’s body. Hover over each part to learn more. Make sure to find the gastrodermis, which contains important microorganisms you'll learn about later on.

Tentacle: The polyp uses stinging tentacles to catch prey like zooplankton and to defend itself from predators. Mouth: The polyp takes in food and releases waste through its mouth. Gonads: The gonads produce eggs and sperm for sexual reproduction. Stomach: Most of the polyp's body consists of a large stomach where it digests its food. Gastrodermis: The gastrodermis is an inner layer of tissue. It contains symbiotic microorganisms called zooxanthellae. Epidermis: The epidermis is an outer layer of tissue. It contains stinging cells called nematocysts in the tentacles. Exoskeleton: Polyps secrete a cup-shaped exoskeleton below their bodies for protection and support. The exoskeleton is what gives coral reefs their hard structure.

A symbiont is an organism in a symbiosis, a close, long-term relationship between two or more organisms of different species. The symbiosis may benefit the symbiont, harm the symbiont, or neither harm nor benefit the symbiont. The table below compares the effects of three major types of symbioses. (In these examples, the symbiosis involves organisms from only two species.)

Three examples of symbioses are described below. Can you figure out which type of symbiosis each example represents?

Example 1: Flea and Cat

Fleas are tiny insects that live on birds and mammals, such as cats, and feed on their blood. When a flea bites a cat, it can cause a swollen, itchy spot and transmit disease or other parasites, such as tapeworms.

Example 2: Barnacle and Baleen Whale

Barnacles are small marine crustaceans that typically do not move as adults. Certain types of barnacles live on baleen whales. (In this image, the barnacles appear as white bumps on the whale’s tail.) As the whale swims, its barnacles can collect plankton and other food particles from the current. In most cases, the whale is generally unaffected by the barnacles’ presence.

Example 3: Bee and Flowering Plant

Bees visit flowering plants to find nectar and pollen for food. When pollen from the flower sticks to the bee’s body, it can be carried from plant to plant by the bee. This pollinates the plants so that they can produce seeds.

Coral polyps are in a symbiosis with microscopic organisms called zooxanthellae (singular: zooxanthella), which belong to a group of single-celled algae known as dinoflagellates.

Additional Information: Algae are a large group of eukaryotic organisms that perform photosynthesis and typically live in water. Some algae, such as dinoflagellates, are single-celled organisms. Other algae, such as kelp or seaweed, are multicellular.

Zooxanthellae are endosymbionts that live inside cells in a polyp’s gastrodermis, or inner layer of tissue. Much of a coral’s color comes from the zooxanthellae that live inside it.

Additional Information: Endosymbionts: An endosymbiont is a symbiont that lives inside the body or cells of its partner organism. Symbionts that live on the body surface of their partner organism are called ectosymbionts.

Zooxanthellae are autotrophs that perform photosynthesis to produce food in the form of sugars that polyps can use. This food provides as much as 90% of the energy that the coral needs to survive. In exchange, the coral polyps provide the zooxanthellae with nutrients needed for photosynthesis, as well as shelter from predators.

Additional Information: An autotroph is an organism that produces its own food from simple molecules (such as CO 2) using sunlight or chemical energy. Autotrophs are primary producers in ecosystems and include plants, algae, and some bacteria.

Corals rely on the zooxanthellae in their cells for food. But how do corals get the zooxanthellae into their cells in the first place? In some cases, corals get zooxanthellae from their parent polyps. During asexual reproduction, for example, coral polyps develop from buds on their parents. Some of the zooxanthellae in the parent polyp end up in the bud.

In other cases, coral larvae or polyps get zooxanthellae that are floating in the water. For example, when a coral polyp feeds, it takes in food and other small particles, including zooxanthellae. After the zooxanthellae enter the polyp’s gut, they move inside cells in the gastrodermis through endocytosis. During this process, the cell membrane pinches inward to form a vesicle that brings the zooxanthellae into a cell.

Additional Information: Endocytosis is a process for transporting materials into cells. Larger particles, such as zooxanthellae, are transported using a type of endocytosis called phagocytosis. Smaller particles are transported using another type of endocytosis called pinocytosis.

Additional Information: A vesicle is a fluid-filled cellular structure similar to a sac. It can be used to store materials and transport them across the cell membrane.

A labeled cross-sectional illustration of a cell. A line across the middle is labeled Cell membrane. The space above the membrane is labeled as Outside the cell. The space below the membrane is labeled as Cytoplasm. The illustration depicts the process of endocytosis in three steps. Arrows point from one step to the next. In the first step, a large particle labeled "Zooxanthellate" is shown outside the cell. In the second step, the particle is enveloped by part of the cell membrane. In the third step, the particle is contained by an isolated sphere labeled "Vesicle" in the cytoplasm.

Chloroplasts are organelles that perform photosynthesis, a series of chemical reactions that use energy from sunlight to produce food (in the form of carbohydrates) from carbon dioxide and water.

Additional Information: An organelle is a cellular structure that has a specific function in the cell. Examples of organelles include the nucleus, mitochondria, and chloroplasts.

Each zooxanthella has one large chloroplast that wraps around the other organelles inside its cell. Unlike plant chloroplasts, zooxanthellae chloroplasts are shaped like a net or a web. This unusual shape may help the zooxanthellae chloroplasts absorb more sunlight when the zooxanthellae are inside corals.

Each chloroplast contains stacks of thylakoids, membrane-lined discs embedded with protein-pigment complexes called photosystems. The photosystems absorb photons from sunlight and start the process of photosynthesis.

Additional Information: Pigment: In photosynthesis, a pigment is a colored biological substance that captures light energy. One common pigment, chlorophyll, is what makes many plants look green. Although the chloroplasts of zooxanthellae have some chlorophyll, they may not look green because they contain other pigments too. These pigments often give the zooxanthellae, and the corals they live in, a yellowish or brownish color.

Coral bleaching occurs when coral polyps expel, or force out, the zooxanthellae that normally live in their cells. It is caused by stressful environmental conditions — such as abrupt temperature changes, overexposure to sunlight, pollution, and disease — that disrupt the symbiosis between the corals and their zooxanthellae.

This process is called “bleaching” because it causes corals to turn white or pale. Although a “bleached” coral is not necessarily dead, it is severely weakened and cannot receive food from zooxanthellae. If too many polyps in a colony lose their zooxanthellae for too long, the colony may no longer have enough food to survive.

Additional Information: Remember that much of a coral's color comes from the zooxanthellae that live inside it. Without zooxanthellae, the coral's tissue is transparent. After a coral expels its zooxanthellae during bleaching, its white skeleton can be seen through its transparent tissue, which makes the coral appear white.

Additional Information: Remember that as much as 90% of the energy that corals need comes from the food produced by zooxanthellae.

Corals live in a relatively narrow temperature range. Unusually low and high temperatures outside of that range can cause bleaching to occur. For example, scientists have found that corals experience heat stress, stressful heat conditions that can cause bleaching, when the sea surface temperature is just 1°C (2°F) warmer than usual.

Corals are especially vulnerable to prolonged or repeated high temperatures, which cause heat stress to accumulate over time. Scientists can measure the accumulation of heat stress using an index called degree heating weeks (DHW), which represents the intensity and duration of heat stress over the last 12 weeks. Coral bleaching can occur at over 4 DHW, and coral death at over 8 DHW.

Test Your Knowledge

The maps below show changes in coral cover, the proportion of a reef covered in living corals, and DHW on the Great Barrier Reef in Australia. Read the captions below the maps for more information.

Graph A, which is on the left, has a vertical color gradient legend that indicates percent changes in coral cover using different shades of purple. Starting at the bottom of the legend, the shades approximately represent the following percent changes: dark purple negative 60 percent, light purple negative 20 percent, and white 9 percent. From 24 degrees south to 20 degrees south, the coastal part of the ocean is mostly white and light purple. From 20 degrees S to 16 degrees S, it is mostly light purple and medium purple. Above 16 degrees south, it is mostly dark purple with smaller areas of light purple and white. Graph B has a vertical color gradient legend indicating DHW using different shades of red. Starting at the bottom of the legend, the shades approximately represent the following DHW: white 0, light red 4, and dark red 14. From 24 degrees south to 20 degrees south, the coastal part of the ocean is mostly white and light red. From 20 degrees S to 16 degrees S, it is a mix of light red and medium red. Above 16 degrees south, it is mostly dark red with smaller areas of light red.

Which of the following statements are supported by the data above? Select all that apply.

In the areas with the lowest heat stress, coral cover did not change over time. Areas with higher heat stress generally had larger decreases in coral cover. Coral cover decreased in areas with moderate to high heat stress (DHW > 4). Submit Question One Answers Submitted

Please make a selection before submitting. ×

Areas where coral cover decreased are indicated by negative percentages on Map A. Many of these areas correspond to areas where DHW > 4 on Map B. In general, the areas with higher heat stress (higher DHW) experienced larger decreases in coral cover. On the other hand, the areas with the lowest heat stress (lowest DHW) experienced increases in coral cover of up to 9%.

Due to global climate change, the world’s oceans are getting warmer, particularly in coastal areas. Most coral bleaching worldwide is caused by these unusually warm temperatures.

How do warmer temperatures cause coral bleaching? The heat damages the photosystems in the coral’s zooxanthellae. The damaged photosystems produce large amounts of reactive oxygen molecules, which are harmful to cells. Coral polyps must get rid of the damaged zooxanthellae in order to protect themselves. As more and more polyps in a colony get rid of their zooxanthellae, the colony bleaches.

Additional Information: Photosystems: Remember that photosystems are photon-absorbing structures essential to photosynthesis.

Additional Information: Reactive oxygen molecules, also known as reactive oxygen species (ROS), are oxygen-containing molecules that react easily with other molecules. High levels of reactive oxygen molecules can damage a cell by reacting improperly with DNA, proteins, and other important cellular molecules.

Corals may use cellular enzymes to destroy the damaged zooxanthellae inside their tissues. They may also expel the zooxanthellae by exocytosis, a process for removing materials from cells.

Similar to endocytosis, the process of transporting materials into a cell, exocytosis involves the formation of vesicles around the zooxanthellae. The vesicles move to and fuse with the cell membrane to release the zooxanthellae outside the cell.

Additional Information: A vesicle is a fluid-filled cellular structure similar to a sac. It can be used to store materials and transport them across the cell membrane.

A labeled cross-sectional illustration of a cell. A line across the middle is labeled as Cell membrane. The space above the membrane is labeled as Outside the cell. The space below the membrane is labeled as Cytoplasm. The illustration depicts the process of exocytosis in three steps. Arrows point from one step to the next. In the first step, an isolated sphere labeled as "Vesicle" contains a large particle in the cytoplasm. In the second step, the vesicle has fused with the cell membrane and is releasing the particle outside the cell. In the third step, the particle is outside the cell and is labeled as "Zooxanthellae."

Without additional stress, bleached corals may eventually recover and regain their zooxanthellae. But if they cannot regain their zooxanthellae within a few days or weeks, the corals will probably starve to death.

In addition, the longer or more often that bleached corals live in stressful environmental conditions, the weaker they become. Weakened corals are more likely to be killed by disease and competing organisms, such as seaweed. If many corals on a reef die, the ecosystem might need decades to recover and grow new corals.

Additional Information: Seaweed often competes with coral for space and access to light on a reef. When corals are healthy, they coexist with seaweed in a healthy balance. But when corals are weakened from bleaching, seaweed may grow over the corals and smother them. Other organisms in the ecosystem, such as parrotfish and sea urchins, may eat the seaweed and keep its growth in check. But in many cases, these organisms have decreased due to overfishing, disease, or habitat loss, so they cannot prevent seaweed from taking over the reef.

Coral bleaching and other threats have already caused major losses in coral reefs worldwide. Many reefs are at risk, especially in areas with unusually warm ocean temperatures. If these temperatures remain high or even increase — making it harder for corals to recover from or adapt to the heat stress — many colonies will likely bleach and die.

Additional Information: In addition to coral bleaching, corals face many local threats such as pollution, habitat destruction, and ocean acidification. They may also suffer from overfishing of species that help corals or physical damage caused by anchors, divers, and storms. These threats can kill or stress corals, making it harder for reefs to recover from bleaching.

Scientists, environmental groups, government organizations, and many others around the world are working together to conserve the world’s remaining coral reefs. One strategy involves using naturally heat-resistant corals, which are less vulnerable to bleaching under high temperatures. Scientists are testing ways to transplant fragments of heat-resistant corals to damaged reefs, where they will hopefully grow and restore bleached areas.

Can you think of other ways to reduce coral bleaching and its consequences?