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Kidadl Team

AUGUST 06, 2021

Hard Coral: 21 Facts You Won't Believe!

These hard coral facts will leave you awestruck!

Hard corals, also called stony corals are reef-building corals that can be found on nearly every ocean floor in the world. They form coral colonies that house several different marine species. If you've ever watch Finding Nemo, you'll know that Marlin, Nemo, and Dory call a coral reef their home. Hard corals are, biologically, an order of corals meaning there are several different species that can be called stony corals or hard corals. One of them is the brain coral, which is named so due to its brain-like appearance. Corals are said to grow in the shape of a polyp, which is a broad term used to refer to any irregularly shaped tissue of muscular growth. As a result, the terms corals and coral polyps are used mostly interchangeably. Hard corals are a special type of these coral polyps, which are the main architects of reefs. Due to their ability to create massive reefs, they are often seen as the engineers of the sea. They secrete calcium carbonate or limestone, which is the main building block of a reef. While corals can use their spiky tentacles to trap plankton and small fish, this is in no way the main source of food for corals. Instead, around 90% of the nutrition requirements are provided by algae called zooxanthellae, which reside in the hard corals and perform photosynthesis-just like plants! In return, the algae receive guaranteed shelter and sunlight. When one organism lives inside the body of another and engages in a mutually beneficial relationship with it, it is called an endosymbiotic relationship.

If you enjoy this article on hard corals, don't forget to check out our article on finger corals and soft corals as well!

Hard Coral Interesting Facts

What type of animal is a hard coral?

Hard corals are a type of coral, a marine reef builder.

What class of animal does a hard coral belong to?

Hard corals are biologically classified as Anthozoa, which is a class of marine invertebrates.

How many hard corals are there in the world?

While the exact number of stony corals in the world is practically impossible to know, the fact that coral reefs are so widespread in our waters means that their population is relatively stable. However, it is also well-known that recent increases in water pollution and global temperatures have put the well-being of all coral reefs in the world in grave danger.

Where does a hard coral live?

Stony corals can be found on the ocean or seafloor.

What is a hard coral's habitat?

Stony corals can inhabit virtually any type of water and form coral reefs there.

Who do hard corals live with?

Stony corals live alongside a variety of different marine species from fish (like the angelfish), algae, aquatic plants, anemones, and jellyfish. The settlements around a reef are called coral colonies. Sea anemones like the giant green anemone are also found in the vicinity of reefs.

How long does a hard coral live?

The exact number of years that stony corals can live up to is not known, most coral reefs in the world are around 7,000 years old! However, the same coral polyps that originally made the reef do not live to this day. Instead, new coral polyps take over the engineering process and live for around 400 years.

How do they reproduce?

The reproduction process varies depends on the species of stony corals in question. Some stony corals, including fire corals, and elkhorn corals (a type of Acropora) are hermaphrodites, meaning they can produce both male and female reproductive material and thus reproduce asexually. This is due to the lack of both sexes of a given coral species in a colony. However, another way of getting around this used by corals is the completely male or female colony. While coral polyps in one colony continually inject male reproductive material into the water, the female coral polyp colony does the same with their reproductive material. When the two combine, fertilization occurs and a new coral reef is ready to grow!

What is their conservation status?

Hard coral and soft coral are umbrella terms for a large number of coral species, and many of these are listed on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species as Vulnerable, due to the immense pressure their habitat is under as a result of sea pollution and global warming. The coral reef is the backbone of several marine ecosystems that could disappear immediately after the potential extinction of the beautiful coral reef.

Hard Coral Fun Facts

What do hard corals look like?

The major commonality between every single species of stony coral is that they are able to secrete calcium carbonate and thus have a rocky appearance. As far as physical appearances go, this order of corals includes an unimaginable variety of colors, sizes, and shapes. They usually have a very bright hue of orange, purple, red, or blue. Their striking colors look absolutely magical in the deep blue ocean water. They also usually have tentacles protruding from their reefs, and these are often used to catch small fish and plankton.

These reef-building corals can come in a variety of different colors, sizes, and textures.

How cute are they?

Considering all types of hard coral don't have too many features and look largely like a rock, not a lot of people find them cute. However, that is not to say that they are not visually pleasing, they are extremely beautiful and serene creatures, that perhaps do not meet the criteria for being called cute.

How do they communicate?

Corals can not only secrete calcium carbonate, but also different chemicals into the eater to attract fish so they can be caught and consumed, and also send chemical distress signals when their well-being is threatened.

How big is a hard coral?

Corals grow to only about 5 in (13 cm) as an individual coral polyp. However, the largest coral reef in the world, the Great Barrier Reef, covers an area of around 133,000 square miles (344,000 sq km)! An individual polyp is almost 13 times smaller than the Blacktip Reef Shark, and around 10 times smaller than the average sea anemone!

How fast can a hard coral move?

Neither coral polyps nor the corals' reef can move, and the sight of the humongous Great Barrier Reef moving around as it pleases is definitely the stuff of nightmares.

How much does a hard coral weigh?

While a single coral polyp may not weigh more than 10 oz (300 g), an entire reef can weigh several tons! The calcium carbonate deposits harden quickly and increase the weight of the reef dramatically.

What are the male and female names of the species?

Both male and female stony corals are referred to by the same name.

What would you call a baby hard coral?

Baby stony corals are larvae called planulae and can be found near the surface, basking in the sunlight. Later, they find a flat surface to stick to and form reefs, and eventually entire colonies of algae, aquatic plants, and eventually fish on!

What do they eat?

Corals can catch fish and zooplankton using their tentacles, but their tentacles are not very good at it, and thus not the main food winner for coral. Instead, the main source of their food is the Zooxanthellae that live inside their cells. Zooxanthellae can perform photosynthesis to capture the energy of the Sun and pass it into the body of corals.

Are they poisonous?

While most are not, there are a few types of corals such as the Zoanthus corals that can produce lethal poisons from their tentacles. If you're snorkeling in deep waters, it is best to maintain a decent distance from corals due to this.

Would they make a good pet?

Corals are often kept in aquariums due to the splash of color that they add to any tank. Corals are more of an aquarium decoration than a pet in themselves since they do not really do much.

Did you know...

Around 50 different species of corals can be found in the Florida Keys alone, the only active reef system in the United States.

Hard coral vs. soft coral

Soft corals do not have the ability to produce calcium carbonate, and as a result, are unable to create hard limestone skeletons to protect themselves. This is the reason behind the soft texture as well as the name that soft corals have.

Why is coral so hard?

Calcium Carbonate (CaCO3) or limestone is secreted by hard corals and eventually hardens to rock. It discourages predators from feeding on corals and is pretty much the only defense that the soft coral polyps have against them.

For more relatable content, check out these jellyfish facts and stonefish facts pages.

You can even occupy yourself at home by colouring in one of our free printable Hard coral coloring pages.

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