What are the tiny animals that build coral reefs called?

The tiny animals that build coral reefs are called coral polyps. Each polyp is a small, soft-bodied cnidarian related to sea anemones and jellyfish, with a mouth surrounded by stinging tentacles.

Reef-building, or stony, coral polyps secrete calcium carbonate around their lower bodies. This mineral skeleton remains after generations of polyps reproduce by budding, creating colonies that can eventually form enormous reef structures. A coral “head” is therefore not one giant animal but a community of many genetically related polyps.

Polyps also receive much of their energy from microscopic symbiotic algae living in their tissues. They can capture plankton with their tentacles, especially at night, but photosynthesis by these algae supplies much of the food that supports rapid growth in warm, clear, shallow water.

A common mix-up is treating coral as a plant or rock. Coral is an animal while alive; its hard skeleton is the rock-like structure it builds. Sponges and algae can contribute to reef growth, but coral polyps are the principal builders of modern coral reefs.