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What is Coral?

Mary McMahon
Updated Mar 05, 2024
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Coral is a marine organism in the class Anthozoa. There are over 2,000 species in this class, many of which form distinctive colonies of genetically identical organisms. Many people refer to these colonies as “coral,” and to the individuals inside the colonies as “polyps.” The organism plays a critical role in the marine environment, with collections of colonies known as reefs hosting around 25% of the known marine organisms, despite the fact that it is present on less than 1% of the ocean floor.

Individual polyps have soft bodies with a hard foot known as a calicle. They settle on the ocean floor, on a sandy or rocky substrate, depending on the species, and then they begin to build hard skeletons from the minerals present in seawater. Some polyps live as solitary individuals, but others reproduce through asexual budding, cloning numerous identical polyps to expand into a “coral head,” a large structure that can host hundreds or thousands of individual polyps, depending on how old it is.

Coral heads can take a variety of forms, depending on the species. Some are very hard, while others are soft, and a number of fantastic shapes from huge platters to sprouting antlers can develop. The landscape of a reef is often quite striking as a result.

In addition to reproducing asexually through budding, corals can also reproduce sexually by spawning. When they spawn, they release clouds of eggs or sperm into the water, with gametes forming in these clouds when they drift together and eggs and sperm fuse. The resulting gametes free-float and develop into new polyps, which take root and repeat the process all over again.

Many of these organisms have a symbiotic relationship with zooxanthellic algae, algae that live inside the coral and photosynthesize, generating energy for the polyps. These algae are responsible for the distinctive colors of coral, with non-zooxanthellic ones being generally white to cream in color. In addition to gathering food and energy from the algae, they are also capable of trapping prey with sweeper tentacles covered in minute stinging cells.

Coral polyps look very similar to sea anemones, which is perhaps not too surprising, since the two are close relatives. These small organisms are very vulnerable to damage and changes in their environment, such as elevated temperatures, increased solar radiation, nutrient runoff, and human interference in the form of harvesting, trawling for fish, or accidental damage inflicted by divers and boats.

Frequently Asked Questions

What exactly is coral?

Coral is a marine invertebrate within the class Anthozoa of the phylum Cnidaria. They typically live in compact colonies of many identical individual polyps. Corals secrete calcium carbonate to form a hard skeleton, which serves as a structure for coral reefs, one of the most diverse ecosystems on the planet.

How do corals reproduce?

Corals can reproduce both sexually and asexually. In sexual reproduction, corals release sperm and eggs into the water, resulting in free-swimming larvae that settle to form new colonies. Asexually, corals can reproduce through budding, where new polyps bud off from parent polyps to expand or start new colonies.

Are corals animals or rocks?

Despite their rock-like appearance, corals are indeed animals. They belong to the same phylum as jellyfish and sea anemones. The confusion often arises because of their hard, durable skeletons, which are rock-like structures composed of calcium carbonate, but the living part of coral is the thin layer of living polyps that cover this skeleton.

What is the significance of coral reefs to the marine ecosystem?

Coral reefs are often referred to as the "rainforests of the sea" due to their incredible biodiversity. They provide habitat and shelter for nearly 25% of all marine species, according to the National Oceanic and Atmospheric Administration. Reefs also protect coastlines from storms and erosion, and they are vital to the world's fisheries and tourism industries.

Why are coral reefs under threat?

Coral reefs are facing numerous threats, including climate change, which leads to coral bleaching and ocean acidification; overfishing, which disrupts ecological balance; and pollution, which can cause disease and death in coral populations. These factors combined have resulted in significant declines in coral health globally.

Can corals recover from bleaching, and how can we help protect coral reefs?

Corals can recover from mild bleaching if stress conditions subside and they are given enough time to recuperate. To help protect coral reefs, we can reduce carbon footprints to combat climate change, practice sustainable fishing, and avoid using harmful chemicals that can reach the ocean. Supporting reef-friendly businesses and conservation initiatives also makes a significant impact.

AllThingsNature is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By anon984741 — On Jan 10, 2015

What are the different colors that a coral my come in and are they still that same color once they have dried after having been harvested? Anyone?

By lighth0se33 — On Jan 20, 2013
I've seen coral jewelry that really doesn't look like it's made of coral. Some pieces are just beads made of coral that have been smoothed, rounded, and carved.

I saw a red rose pendant that I was surprised to learn was made of coral. I've also seen perfectly round beads of coral.

I've also seen necklaces that actually looked like bits of shell strung together. They are flat but have irregular edges and are stacked at angles to each other. This is a really popular type of cheap coral necklace.

I've read that the people who make this kind of jewelry only use the coral that washes up on the beach. I hope this is true, because I would hate to be wearing something that was harvested in a way that could harm the coral reefs.

By giddion — On Jan 20, 2013

I think it's fascinating that coral can take so many different forms. I've seen some that look like bronchial tubes in the human body and others that look like tiny trees and a lot of shapes in between.

It all depends upon factors beneath the surface of the water. Coral is truly shaped by its environment, probably more so than many other kinds of undersea life.

By wavy58 — On Jan 20, 2013

@Perdido – Yes, my friend has living coral in her aquarium. I've seen everything from soft coral to mushroom coral in a tank before.

She has to keep the lighting and the water current at a certain level for the coral to do well. She also feeds the coral brine shrimp.

It's really a spectacular sight. Her aquarium looks like a section of the ocean floor. The color is stunning, and you really feel like you are looking at a television screen playing a show filmed with an underwater camera.

By Perdido — On Jan 19, 2013

I've seen aquarium coral before, but I'm not sure if they are real. Can coral actually survive inside of a home aquarium? If so, what do you feed them?

By Amphibious54 — On Jun 14, 2010

@ Babalaas - You are quite right about the importance of coral reefs. 25% of all marine species live in coral reefs, and many species spawn in reefs and the lagoons they create. Loss of coral reefs can mean putting the 1 billion people who rely on seafood as their main source of protein in jeopardy of starvation. Because corals can be killed by rising water temperatures, subsistence fishing populations may ultimately be at the mercy of the actions of the developing and industrial world. However, there has been some action taken to protect the world's corals. I saw a recent show on National Geographic (I think) that drew attention to the problems of coral reef degradation, and coral bleaching. The show took an in-depth look at farming coral to seed dead or dying reefs. The results looked promising, and reefs were re-grown in only a couple of years. Let's hope that projects like this can help replace coral reefs as fast as they are dying.

By Babalaas — On Jun 14, 2010

Coral reefs are home to so much of this planets biodiversity, yet there are so many threats to the worlds coral reefs. What happens if the world's coral reefs are lost? What can or is being done to protect these natural treasures?

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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