Reef-building corals are colonies of small animals, not underwater plants.

A coral polyp has a mouth surrounded by tentacles, which capture food and move it into a digestive cavity. Its feeding anatomy belongs to an animal, even when the colony resembles a branching shrub.

Stony coral polyps produce skeletons of calcium carbonate. As generations grow on existing skeletons, their accumulated mineral structures help form the framework of a coral reef.

Many shallow-water reef builders host photosynthetic algae that supply food compounds. Those algae are partners inside the coral, not evidence that the coral animal itself is a plant.

A coral head is not simply one large animal shaped like a rock. NOAA describes most familiar coral structures as collections of hundreds or thousands of individual polyps. Each polyp is a soft-bodied animal, and in stony corals it sits in a hard skeleton. The living tissue and the mineral framework are different parts of the structure a diver sees.

The animal identity becomes especially clear during feeding. A polyp extends tentacles around its mouth and uses stinging structures called nematocysts to capture prey. Many coral polyps feed at night, taking zooplankton and other suitable food from the surrounding water. Food enters through the mouth, and undigested waste leaves through that same opening. Corals are related to jellyfish, which also possess stinging structures.

Reef construction happens as living polyps add skeleton and successive generations build upon older material. Calcium carbonate persists after an individual polyp dies, so a reef contains far more than its currently living surface. That accumulated foundation provides physical habitat for other organisms. It is not a mass of plant roots, nor is the whole reef one continuously living animal.

The algae help explain how many corals flourish in clear tropical seas. Living within coral tissues gives the algae protection and access to nutrients, while their photosynthesis supplies the host with useful food compounds. The partnership links an animal's feeding and skeleton building with the energy captured by a photosynthetic organism. Capturing prey remains part of the coral's biology.

Bleaching reveals how important that partnership is. Stress associated with temperature, light, or nutrient changes can cause corals to lose their symbiotic algae, leaving the pale skeleton more visible through the tissue. A white coral is not necessarily dead at that moment. NOAA emphasizes that bleached corals can survive, although they are stressed and more vulnerable to mortality.

The reef-building description also needs a boundary: not every coral forms a massive limestone reef. NOAA distinguishes hard, or stony, corals from soft corals. Likewise, the familiar shallow tropical partnership should not be treated as a description of every coral habitat. The central distinction is between the animal polyp, its skeletal structure, and any photosynthetic partners it hosts.

A coral colony is alive at its surface in a way a bare piece of limestone is not. Polyps occupy the structure and carry out animal functions, while the accumulated skeleton supplies much of the visible framework. Separating living tissue from mineral architecture helps explain both reef growth and reef damage. A large structure can remain after the organisms that produced it have suffered or died.