What are corals?

Coral make up some of the most diverse ecosystems in the world. Thousands of species rely on corals and the reefs they form for survival. Millions of people all over the world also depend on coral reefs for food, protection, and jobs.

Most corals are made up of hundreds of thousands individual polyps like this one. View a detailed diagram and a description of a polyp's anatomy.

Most of the time, a coral is made up of individual animals called polyps. Each polyp has a stomach that opens at only one end. This opening, called the mouth, is surrounded by a circle of tentacles. The polyp uses these tentacles for defense, to capture small animals for food, and to clear away debris. Food enters the stomach through the mouth. After the food is consumed, waste products are expelled through the same opening.

Most coral polyps feed at night. To capture their food, corals use stinging cells called nematocysts. These cells are located in the coral polyp’s tentacles and outer tissues. If you’ve ever been stung by a jellyfish (a relative of corals), you’ve encountered nematocysts.

Nematocysts are capable of delivering toxins that assist them in capturing prey. A coral's prey ranges in size from nearly microscopic animals called zooplankton to small fish, depending on the size of the coral polyps. In addition to capturing zooplankton and larger animals with their tentacles, many corals also collect fine organic particles in mucous film and strands, which they then draw into their mouths.

Nematocysts are special stinging cells used by coral polyps to capture food. View a diagram of a nematocyst cell’s anatomy and how it works.

When we think about corals, it brings to mind clear, warm tropical seas and reefs filled with colorful fish. In fact, hard, or stony corals — the kind that build reefs — are only one type of coral. There are also soft corals.

Hard corals, also known as stony coral, produce a rigid skeleton made of calcium carbonate (CaCO₃). The calcium carbonate of corals provides a hard outer structure that protects the soft parts of the coral. Hard corals are the primary reef-building corals. Colonial hard corals are made up of hundreds to hundreds of thousands of individual coral polyps that cement themselves together by the calcium carbonate they secrete.

Soft corals do not produce rigid calcium carbonate skeletons and do not form reefs, though they are found in reef ecosystems. Like hard corals, most soft corals are also colonial; what appears to be a single large organism is actually a colony of individual polyps that form a larger structure. Visually, soft coral colonies tend to resemble trees, bushes, fans, whips, and grasses.