Is Coral a Plant or an Animal? What Coral Really Is

What is a coral polyp?

A coral polyp is a small, soft animal shaped like a tube or sac. At the top is a mouth surrounded by a ring of tentacles; at the bottom is a base attached to the skeleton. Inside is a simple stomach where food is digested. The polyp has no brain, but a net of nerves running through its body lets it react to touch and light.

Most reef-building polyps are only a few millimeters across. A single colony can contain thousands to millions of them, all genetically identical and linked by a thin sheet of living tissue that shares food between them. In stony corals, the polyps have tentacles in multiples of six. In soft corals and sea fans, each polyp has exactly eight feathery tentacles.

Like jellyfish, corals carry stinging cells called nematocysts on their tentacles. When a tiny animal brushes against them, the cells fire microscopic harpoons that inject venom, and the tentacles pass the prey to the mouth.

Why does coral look like a plant?

Corals spend their adult lives fixed in one place, they grow in branching, bushy or rounded shapes, and many reef corals need sunlight to thrive. With their tentacles open, polyps can look like tiny flowers. It is no surprise that people long thought corals were plants.

The plant-like part of the story is real, but it belongs to a partner. The tissues of most reef-building corals are packed with single-celled algae called zooxanthellae. The algae carry out photosynthesis, using sunlight to make sugars, and pass most of them to the coral. In shallow, clear water, they can provide up to about 90 percent of a coral’s energy. In return, the coral gives the algae a safe home and nutrients from its waste.

This partnership also gives corals much of their color. When heat stress breaks it, the coral loses its algae and turns white, as explained in what is coral bleaching?

How do we know corals are animals?

Corals have all the features that define animals. Their cells have no rigid cell walls and no chlorophyll of their own. They catch and digest other living things, they move parts of their bodies with simple muscles, and they produce eggs and sperm. Their larvae swim before settling down, much like the larvae of other sea animals.

For a long time, naturalists were unsure. In ancient Greece, Theophrastus described red coral as a kind of stone in one work and as a sea plant in another, and later writers called corals and sponges “zoophytes”, or animal-plants. In the 18th century, the French physician and naturalist Jean-André Peyssonnel studied Mediterranean red coral and argued that its “flowers” were in fact small animals. His work was read to the Royal Society in London in 1752, and the idea was soon widely accepted.

What are coral’s closest relatives?

Corals belong to the phylum Cnidaria, a group of simple animals with stinging cells. Within it, corals share the class Anthozoa with the sea anemones, which are essentially large, single polyps without a hard skeleton. A good example is the magnificent sea anemone, the home of many clownfish.

Jellyfish and hydroids are more distant cousins in the same phylum. Despite their names, jellyfish are not fish, as explained in are jellyfish fish?

Hard corals and soft corals

Hard or stony corals, the order Scleractinia, build skeletons of limestone and are the main reef builders. Examples include brain corals, star corals and the branching staghorn coral of Florida and the Caribbean. There are hundreds of species of reef-building stony corals.

Soft corals and sea fans, the octocorals, have no massive skeleton. Their bodies are stiffened by tiny limestone spikes called sclerites or by a flexible core, so they sway in the current. Black corals have a tough, horn-like core, while the precious red coral of the Mediterranean, itself an octocoral, builds a hard, stony red skeleton.

Not all corals live in warm, shallow seas. Cold-water corals, which have no algae and feed only on plankton, grow in deep, dark water around the world. The reef-builder Desmophyllum pertusum, long known as Lophelia pertusa, forms large reefs off Norway, Scotland and Ireland.

Things that look like coral but are not

Some sea life that looks like coral is not true coral. Coralline algae are real algae that harden their tissues with limestone; they form pink crusts and branches on reefs and help cement them together. Fire coral is a hydrozoan, a relative of corals that builds a limestone skeleton and has a painful sting, but it is not a true coral. Many sponges also look coral-like, but sponges are a completely different group of animals with no tentacles or stinging cells.

How do tiny animals build huge reefs?

Each stony coral polyp takes calcium and carbonate from seawater and lays down a cup of aragonite beneath itself. As the colony grows, new polyps build on top of old skeleton, and when colonies die, their skeletons are cemented together by coralline algae and filled with sand and rubble. Over thousands of years, this slow process creates reefs such as the Great Barrier Reef, which stretches about 2,300 km (1,400 mi) along northeast Australia.

Coral reefs cover well under 1 percent of the ocean floor, yet NOAA estimates they support about a quarter of all marine species. All of that depends on tiny animals, most of them smaller than a grain of rice.