Coral

Corals are amazing sea animals that live in groups called colonies. They are very small, but when many 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 long wondered if corals were plants or animals. Ancient thinkers like Aristotle and Theophrastus described corals in different ways. It wasn’t until the 1700s that scientists used microscopes to show 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, and Octocorallia, which includes soft corals and blue coral.

Systematic

Corals come in two main types: hard coral and soft coral. Hard coral builds reefs using a calcium carbonate base. Soft coral 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.

Anatomy

Corals are groups of very small animals called polyps that stick to hard surfaces. Each polyp is tiny, but together they build large structures. Hard corals make a skeleton from calcium carbonate that keeps them safe. Over time, these skeletons grow and form coral reefs.

Polyps have a body shaped like a sac with a mouth in the middle surrounded by tentacles. The tentacles catch food, such as tiny animals and plants, floating in the water. Some corals live with tiny algae called zooxanthellae inside their tissues. These algae help the corals make food from sunlight, which helps the corals grow strong and healthy.

Ecology

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

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. This can make them weak and change their color.

Reproduction

Corals can have babies in two ways: sexually and asexually.

When corals reproduce sexually, they release eggs and sperm into the water. The eggs and sperm meet and form tiny baby corals called planulae. These baby corals find a spot to stick and start a new group of corals.

Corals can also reproduce asexually. They can make new coral pieces by budding or splitting apart. Budding makes a smaller coral piece from an existing one. Splitting divides a coral piece into two parts. Both of these ways help corals grow and spread without needing another coral.

Coral microbiomes

See also: Marine microbiome

Corals live with tiny algae and other small organisms. These tiny partners help keep the corals healthy. If the water gets too warm or changes, this partnership can break. Then, corals can lose their color in a process called bleaching.

Scientists study these tiny partners to learn how corals stay alive and healthy. They found that bacteria, viruses, and other small creatures all help corals grow and survive. One special type of bacteria, called Endozoicomonas, seems very 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. These corals make hard skeletons from calcium carbonate, forming the shape 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 each year. Coral reefs are important homes for many sea creatures, including thousands of fish and other animals.

Evolution

Long ago, Earth had creatures like today's corals that built reefs. Their remains are found in sedimentary rocks. Fossils of algae, sponges, and many sea creatures are often found with coral fossils. This helps scientists date rock layers.

Reef-builders like corals first appeared in the Cambrian period, about 535 million years ago. Fossils were rare at first but became more common in the Ordovician period. Types like Heliolitida, rugose, and tabulate corals spread widely. Tabulate corals lived in limestone and shale from the Ordovician but vanished after extinctions. They returned in the Silurian period, forming cushion or branching shapes of calcite. Rugose corals grew more common by the middle Silurian and thrived in the Devonian period.

Both rugose and tabulate corals disappeared in a big extinction at the end of the Permian period, about 250 million years ago. New corals evolved later, in the Triassic period.

Today’s stony corals, called Scleractinia, appeared in the Middle Triassic. Unlike older corals that used calcite, these use a different material called aragonite. Their fossils are rare in Triassic rocks but become more common in the Jurassic and later times.

Status

Main article: Environmental issues with coral reefs

Coral reefs are in danger because of many threats. Human actions like pollution, overfishing, and damage from building harm these special places. Climate change also causes problems by making the ocean warmer and more acidic. This can make corals lose their color, a process called bleaching.

Scientists study these threats to learn how they affect corals. Even small changes in temperature can be risky for some corals. But corals can also adapt a little, which gives hope for protecting them. Many countries have rules to help save reefs, and we all need to keep working to keep them safe.

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.

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. Projects like Green Fins help protect reefs by encouraging responsible tourism.

Coral is also used in jewelry. In medicine, compounds from corals show promise for treating diseases. Coral reefs 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