Coral

Corals are amazing sea animals that live in groups called colonies. They are very small, but when many of them work together, they can build huge structures called coral reefs. These reefs are important homes for many sea creatures and are found in warm, clear oceans around the world, like near the Great Barrier Reef off the coast of Australia.

Each tiny coral animal, called a polyp, has tentacles and a mouth. They make a hard skeleton from calcium carbonate, which helps them stay standing. Corals can grow in two ways: by making copies of themselves or by releasing eggs and sperm to make new polyps.

Most corals get their food and energy from tiny plants called zooxanthellae that live inside their tissues. These plants use sunlight to make food through photosynthesis, which also gives corals their bright colors. However, when the water gets too warm or dirty, corals can lose these plants and turn white in a process called bleaching.

Some corals do not need these plants and can live in very deep, cold water, like the ones found near Scotland or off the coast of Washington state. These deep-water corals show how diverse and tough coral animals can be.

Taxonomy

People have wondered whether corals were plants or animals for a very long time. Ancient thinkers like Aristotle and Theophrastus described corals in different ways, sometimes calling them plants and sometimes minerals. It wasn’t until the 1700s that scientists used microscopes to prove that corals are animals.

Today, we know corals are animals that belong to a group called Cnidaria. They are part of two main sub-groups: Hexacorallia, which includes stony corals with a six-fold symmetry, and Octocorallia, which includes soft corals and blue coral with an eight-fold symmetry.

Systematic

Corals are divided into two main types: hard coral, which builds reefs using a calcium carbonate base, and soft coral, which is flexible. Scientists often study a type of hard coral called Acropora because it has many different species. These corals grow in special ways, with some parts growing quickly and others staying small.

Corals face big challenges from changes in the ocean, like rising temperatures and lower pH levels. These changes can harm corals and even cause some species to disappear. Some corals have special proteins that help them deal with heat, but these aren't always enough to protect them from harmful conditions.

Anatomy

Corals are colonies of tiny animals called polyps that stick to hard surfaces. Each polyp is very small, but together they form large structures. Hard corals make a skeleton from calcium carbonate, which protects them. These skeletons build up over time to create coral reefs.

Polyps have a sac-like body with a mouth in the center surrounded by tentacles. These tentacles catch food like tiny animals and plants floating in the water. Some corals work with tiny algae called zooxanthellae, which live inside their tissues and help them make food from sunlight. This partnership helps corals grow strong and healthy.

Ecology

Corals have tiny parts called polyps that eat small creatures like tiny animals and small fish. They catch their food using special stinging cells on their tentacles that help trap and hold the food.

Many corals live together with tiny green plants called algae. These algae live inside the coral and help it make food using sunlight. The algae also help build the coral's hard skeleton and clean up waste. If the water gets too warm, corals might lose their algae, which can make them weak and change their color.

Reproduction

Corals can reproduce both sexually and asexually. Sexual reproduction involves corals releasing eggs and sperm into the water where they meet and form tiny larvae called planulae. These larvae then find a spot to attach and start a new coral colony.

Asexually, corals can create new polyps by budding or dividing. Budding creates a smaller polyp from an existing one, while division splits a polyp into two parts. Both methods help corals grow and spread without needing a partner.

Coral microbiomes

See also: Marine microbiome

Corals live in partnership with tiny algae and other tiny organisms, which help them stay healthy. When the water gets too warm or changes in other ways, this partnership can break down, and corals can lose their color in a process called bleaching.

Scientists study these tiny partners to understand how corals stay alive and healthy. They have found that bacteria, viruses, and other tiny creatures all play a role in helping corals grow and survive. One special type of bacteria, called Endozoicomonas, seems especially important for corals.

Reefs

See also: Coral reef fish and List of reefs

Many corals help build reefs. They live in shallow, sunny waters and work with tiny plants called zooxanthellae, which give them energy through photosynthesis. These corals create hard skeletons from calcium carbonate, forming the structure of the reef.

There are different kinds of coral reefs, like fringing reefs and barrier reefs, mostly found in warm tropical seas. They grow very slowly, adding just a little bit each year. Coral reefs are important homes for many sea creatures, including thousands of fish and other animals.

Evolution

At certain times in Earth's past, reef-building organisms similar to today's corals were very common. Like modern corals, these ancient creatures built reefs, and their remains can be found in sedimentary rocks. Fossils of algae, sponges, and many other sea creatures such as echinoids, brachiopods, bivalves, gastropods, and trilobites are often found alongside coral fossils. This makes some corals useful for dating layers of rock.

Reef-building organisms similar to modern corals first appeared during the Cambrian period about 535 million years ago. Fossils are rare at first but become more common from the Ordovician period onward, when different types of corals such as Heliolitida, rugose, and tabulate corals spread widely. These early corals often had many tiny partners living inside them. Tabulate corals are found in limestone and shale from the Ordovician, but disappeared after extinction events at the end of that period. They reappeared during the Silurian period, forming cushions or branching shapes made of calcite. Rugose corals became more common by the middle of the Silurian and thrived during the Devonian period, with over 200 different groups. Both rugose and tabulate corals disappeared in a major extinction event at the end of the Permian period around 250 million years ago. New types of corals evolved later, during the Triassic period.

Today’s stony corals, called Scleractinia, appeared in the Middle Triassic. Unlike older corals that built their skeletons from calcite, these modern corals use a different form of calcium carbonate called aragonite. Their fossils are less common in rocks from the Triassic but become more frequent in the Jurassic and later periods.

Status

Main article: Environmental issues with coral reefs

Coral reefs face many threats that put their health at risk. Human activities such as pollution, overfishing, and damage from mining and construction harm these delicate ecosystems. Climate change adds extra stress by raising ocean temperatures and making the water more acidic, which can cause corals to lose their color in a process called bleaching.

Scientists work hard to measure and understand these threats. They study how changes in temperature and water chemistry affect coral growth and health. Even small temperature increases can be dangerous to some coral species. Despite these challenges, corals show some ability to adapt to changing conditions, offering hope for their future protection. Many countries have laws to help protect reefs, but ongoing efforts are needed to ensure their survival.

Protection

Marine Protected Areas, Biosphere reserves, marine parks, national monuments world heritage status, fishery management and habitat protection help keep reefs safe from harm caused by people.

Many places now stop people from taking coral from reefs and teach people near the coast how to protect reefs and learn about them. While local efforts like fixing habitats and protecting animals that eat seaweed can help, big problems like ocean acidification, warming temperatures, and rising sea levels still need attention. Protecting a network of healthy and different reefs gives corals the best chance to adapt to changing climates. Efforts to grow corals in places that are not normally warm enough for them are also underway.

Relation to humans

Local economies near coral reefs benefit from fish and other sea creatures as food. Reefs also support tourism through activities like scuba diving and snorkeling. While these activities can sometimes harm coral, projects like Green Fins help protect reefs by encouraging responsible tourism.

Coral is also used in jewelry and has been valued for centuries, especially red coral, which is rare due to overharvesting. In medicine, compounds from corals show promise for treating diseases like cancer and bone loss. Coral reefs also protect shorelines by absorbing wave energy, helping prevent damage from storms. Many communities depend on reefs for food, income, and protection. People also keep coral in special tanks called reef aquariums and work to grow new coral through aquaculture of coral to restore damaged reefs.

Main article: Precious coral

Main article: Reef aquarium

Main article: Aquaculture of coral