Is a coral an animal, a plant or a rock?

A coral is an animal. It can look like a stone, a shrub or even a flower, but a coral colony is made of many tiny animals called polyps. Each polyp is a soft, sac-like creature only a few millimetres across, with a ring of tentacles around a single mouth. Corals belong to the same animal group as sea anemones and jellyfish, and the hard skeleton people pick up on a beach is something the animals built around themselves.

Each polyp is also a small hunter. At night it spreads its tentacles, which carry coiled stinging cells that pierce, poison and hold prey such as plankton and fish larvae. A polyp has a second food supply as well. Most reef-building corals house microscopic algae called zooxanthellae inside their tissue. The algae use sunlight to make food and hand most of it to the coral, which is why these corals live in clear, shallow, sunny water.

For centuries nobody could say what a coral was. The Roman writer Pliny the Elder said that some sea creatures, such as sea nettles and sponges, were "neither animals nor plants" but had a "third nature". Al-Biruni, the Persian scholar who died in 1048, argued that corals were animals because they respond to touch. Even so, most people went on believing that corals were plants or rocks until the eighteenth century. In 1750 the French naturalist Jean-André Peyssonnel argued in letters that they belonged to the animal kingdom, and William Herschel later looked through a microscope and found the thin cell membranes of an animal.

So the stone is the animals' house, the colour is their lodgers, and the whole colony is a family of identical polyps that share food and keep growing.