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Coral Reefs


Coral reefs are one of the world's most diverse ecosystems. Its structure is built of coral, and many fish and invertebrates live within it.


Reef building corals live in warm, shallow tropical ocean waters. Coral is actually an animal that has a symbiotic relationship* with a microscopic algae, called zooxantellae, that lives in its tissues. Coral provides a home and nutrients for the zooxantellae, and zooxanthellae (a photosynthetic algae*) provide the coral with food and oxygen. The zooxanthellae are actually what give corals their beautiful coloring.

Coral itself is made up of a colony* of many small polyps. Stony (or hard) coral have polyps that live on the surface of a calcium carbonate skeleton. The skeleton is the hard structure that is left behind even if the coral dies. Soft corals don't have such a skeleton, and move around with the appearance of a plant.

coral polyp.jpg
An up-close image of the polyps
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Coral reefs cover a small amount (0.1%) of the ocean floor, yet they support 25% of all the species in the ocean.

Animals gather at coral reefs in great abundance. The structures coral creates offers a great place for small fish to hide, or young fish to grow up where they have some protection from predators. Coral reefs are also a good place for species to meet up and breed. Some fish always live on the reef, others may live out their early lives there. Where many small fish gather, larger fish are also attracted because of the opportunity to feed. A whole food chain can thus exist on coral reefs. In fact, even humans depend on coral reefs to get food. The fish that rely on coral reefs feed more than a billion people around the world.

Many animals also know they can visit the reefs if they need a good cleaning. Small fish and shrimp offer "cleaning stations" on the reef, and larger fish, sea turtles, or manta rays will seek them out. The cleaning fish and shrimp eat the parasites and dead skin from their clients (sometimes even out of their mouths!), which keeps their clients healthy and stress-free.

Cleaning stations:
Making sand:

Parrot fish chew algae off of dead coral, and the little bits of calcium carbonate (the corals skeleton) that is eaten, get ground up and pooped out. This actually creates the sand for some white sand beaches like the ones in Hawai'i. 

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Protection of the shoreline:

The physical structure of reefs also protects beaches and the people who live on shorelines.  By dispersing, or breaking up, the force of waves, reefs reduce the wave's energy by 97%! The height of the waves also reduces by 84%. This means that in coastal areas that have offshore reefs, if large waves from storms approach, the reefs can protect the shoreline from being destroyed. 

Some animals (besides coral) that are found on reefs:

Manta rays:

Manta rays are the largest ray in the world, growing up to about 25 feet wide! They are filter feeders, swimming with open mouths to eat zooplankton. Mantas are highly intelligent and have excellent long-term memories. Mantas visit coral reefs for their cleaning stations. Since mantas travel to other parts of the ocean, they bring nutrients from those areas, and deposit them on the reef through their feces, which helps promote coral growth. Manta rays live up to 50 years. They are slow to reproduce, giving birth to about one pup every couple of years. Their population is rapidly declining due to fishing for their meat.


Octopus, such as the Caribbean Reef Octopus seem demonstrating camouflage in this video, can live on coral reefs. Octopuses are highly intelligent. They are good at problem solving and mazes, have great memories, and use tools. Octopuses are masters at camouflage, changing the color, reflectivity, and texture of their skin to hunt and avoid predators. Sometimes they will even make themselves appear to be a rock, and very slowly move along the ocean floor. Octopuses have 8 arms, and the underside of each has suction cups which have chemoreceptors that allow the octopus to taste what it touches. Most neurons exist in the arms instead of the brain, which means each arm can independently move and sense things without input from the brain. Octopuses also have 3 hearts and blue blood, which makes them very interesting animals!


Moray eel:

These eels tend to lie in wait, depending on their sense of smell to ambush prey, such as small fish, octopus, and crustaceans. They rhythmically open and close their mouth, which makes them appear threatening, but they are really doing this to move water through their gills to breathe. While groupers are known predators of moray eels, they sometimes team up to hunt. The grouper gets the eel to follow it and has the eel flush out prey from small spaces. Sometimes the grouper gets the fish, and sometimes the eel does.


Groupers can get up to three feet long. They hunt smaller fish, octopus, and crustaceans, sucking them into their large mouth, and swallowing them whole. Groupers are intelligent fish, forming partnerships with eels and octopuses to hunt. In certain species of grouper, all fish mature as females and some will change into males later in life.


Hawksbill sea turtle:

These turtles eat sponges that grow on coral reefs. Without the turtles eating them, the sponges overgrow and outcompete corals. Hawksbills allow the reef-building corals to grow, which supports a healthy and diverse reef.

Like all sea turtles, hawksbill turtles lay their eggs on land. Warm sand creates more female hatchlings, and cool sand hatches more male turtles. As climate change warms the beaches, more females are being born.


Grey reef shark:

This is one of the most common sharks living in Indo-Pacific reefs. They will chase other sharks out of their favorite reef spots, but will tolerate others of their own species coming into their home range. Grey reef sharks rarely venture into open water, but when they do they have been known to spend time with pods of dolphins or large fish like sailfish. They feed on fish and squid. Grey reef sharks are curious, and often approach divers to inspect them, but swim off once their interest is satisfied. 

Threats to Coral Reefs

Even though so much life depends on coral reefs (including humans), they are at risk of disappearing in our lifetimes.

Even though coral reefs are so important, they are also very fragile. Coral is sensitive to warming oceans and acidified water, two results of climate change.  These changes in the water cause the coral to bleach- which means the zooxanthellae leaves the coral, which turns the coral white. In this state, the coral may starve, or become weakened and vulnerable to disease. Sometimes the coral can recover and the zooxanthellae returns, but often the coral dies.

Coral also breaks easily when boat anchors accidentally drop on them, or divers accidentally touch them. When coral begins dying, all the life on the reef that coral supports begins to suffer as well. If small fish cannot find a place to live in the coral, then bigger fish do not have enough to eat.

Overfishing is another threat to coral reefs. As fish disappear, all life on the coral reef begins to decline. Because all life on coral reefs depend on other organisms on the reef for food, protected habitats, or breeding sites, everything suffers together. Humans also feel these losses. When coral reefs are damaged or die, we can no longer swim out to admire their beauty. Our food sources become depleted, and our shorelines become damaged and eroded from the impact of waves.

How you can help protect coral reefs
  • When you're at the beach, wear rash guards to protect yourself from the sun, or use reef-safe sunscreen. The chemicals in most sunscreens damage corals.

  • Don't buy souvenirs that are made from coral. Coral grows very slowly, and is so important to the ecosystem.

  • If you swim, snorkel, or dive around coral, be very careful not to touch it.

  • If you eat seafood, only buy sustainable seafood.

  • Keep your beach clean. Whenever you leave the beach, try to pick up 5 pieces of trash (in addition to yours of course!). Because waterways lead to the beach, keeping litter and pollution off the ground inland is important too. When the rains come, it will get washed into the ocean.


*Photosynthetic: The process by which plants use sunlight to create energy that can be used to fuel its activities.

*Algae: An aquatic (water) plant that lacks roots, stems, or leaves. A common algae is seaweed.

*Symbiotic relationship: A long term association between two organisms. There are three kinds of symbiotic relationships, but the one described here is mutualistic, meaning that both organisms benefit from the relationship.

*Colony: Many polyps connected to each other and acting like a single organism.

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