Corals are related to sea anemones, and they all share the same simple structure, the polyp. The polyp is like a tin can open at just one end. The open end has a mouth surrounded by a ring of tentacles. This tentacle have stinging cells, called nematocysts. That allows the coral polyp to capture small organisms that swim too close. Inside the body of the polyp are digestive and reproductive tissues. Corals differ from sea anemones in their production of a mineral skeleton.

coral Polyps

Shallow water corals that live in warm water often have another source of food, the zooxanthellae. These single-celled algae photosynthesize and pass some of the food they make from the sun’s energy to their hosts. And in exchange, the coral animal gives nutrients to the algae. It is this relationship that allows shallow-water corals to grow fast enough. To build the enormous structures we call reefs. The zooxanthellae also provide much of the green, brown, and reddish colours that corals have. The less common purple, blue, and mauve colours found in some corals the coral makes itself.

In this blog, Pritish Kumar discusses Corals that are found across the world’s oceans.

Coral Diversity

In the so-called true stony corals, which compose most tropical reefs. Each polyp sits in a cup made of calcium carbonate. Stony corals are the most important reef builders. But organpipe corals, precious red corals, and blue corals also have stony skeletons. There are also corals that use more flexible materials or tiny stiff rods to build their skeletons. The seafans and sea rods, the rubbery soft corals, and the black corals.

The family tree of the animals we call corals is complicated. And some groups are more closely related to each other than are others. All but the fire corals (named for their strong sting) are anthozoans. Which are divided into two main groups. The hexacorals (including the true stony corals and black corals, as well as the sea anemones) have smooth tentacles. Often in multiples of six, and the octocorals (soft corals, seafans, organpipe corals and blue corals) have eight tentacles. Each of which has tiny branches running along the sides. All corals are in the phylum Cnidaria, the same as jellyfish.


Corals have multiple reproductive strategies. They can be male or female or both, and can reproduce either asexually or sexually. Asexual reproduction is important for increasing the size of the colony, and sexual reproduction increases genetic diversity and starts new colonies that can be far from the parents.


Asexual reproduction results in polyps or colonies that are clones of each other. This can occur through either budding or fragmentation. Budding is when a coral polyp reaches a certain size and divides, producing a genetically identical new polyp. Corals do this throughout their lifetime. Sometimes a part of a colony breaks off and forms a new colony. This is called fragmentation, which can occur as a result of a disturbance such as a storm or being hit by fishing equipment.


In sexual reproduction, eggs are fertilized by sperm, usually from another colony, and develop into a free-swimming larva. There are two types of sexual reproduction in corals, external and internal. Depending on the species and type of fertilization, the larvae settle on a suitable substrate and become polyps after a few days or weeks, although some can settle within a few hours!

Most stony corals are broadcast spawners and fertilization occurs outside the body (external fertilization). Colonies release huge numbers of eggs and sperm that are often glued into bundles (one bundle per polyp) that float towards the surface.

Spawning often occurs just once a year and in some places is synchronized for all individuals of the same species in an area. This type of mass spawning usually occurs at night and is quite a spectacle. Some corals brood their eggs in the body of the polyp and release sperm into the water. As the sperm sink, polyps containing eggs take them in and fertilization occurs inside the body (internal fertilization). Brooders often reproduce several times a year on a lunar cycle.

From Corals to Reefs

Individual coral polyps within a reef are typically very small—usually less than half an inch (or ~1.5 cm) in diameter. The largest polyps are found in mushroom corals, which can be more than 5 inches across. But because corals are colonial, the size of a colony can be much larger. Big mounds can be the size of a small car, and a single branching colony can cover an entire reef.

Reefs, which are usually made up of many colonies, are much bigger still. The largest coral reef is the Great Barrier Reef, which spans 1,600 miles (2,600 km) off the east coast of Australia. It is so large that it can be seen from space!

Coral Growth

Reefs form when corals grow in shallow water close to the shore of continents or smaller islands. The majority of coral reefs are called fringe reefs because they fringe the coastline of a nearby landmass. But when a coral reef grows around a volcanic island something interesting occurs. Over millions of years, the volcano gradually sinks, as the corals continue to grow, both upward towards the surface and out towards the open ocean.

Over time, a lagoon forms between the corals and the sinking island and a barrier reef forms around the lagoon. Eventually, the volcano is completely submerged and only the ring of corals remains. This is called an atoll. Waves may eventually pile sand and coral debris on top of the growing corals in the atoll, creating a strip of land. Many of the Marshall Islands, a system of islands in the Pacific Ocean and home to the Marshallese, are atolls.

It takes a long time to grow a big coral colony or a coral reef, because each coral grows slowly. The fastest corals expand at more than 6 inches (15 cm) per year. But most grow less than an inch per year. Reefs themselves grow even more slowly because after the corals die.

They break into smaller pieces and become compacted. Individual colonies can often live decades to centuries, and some deep-sea colonies have lived more than 4000 years. One way we know this is because corals lay down annual rings, just as trees do. These skeletons can tell us about what conditions were like hundreds or thousands of years ago. The Great Barrier Reef as it exists today began growing about 20,000 years ago.