Where Do Coral Live? 

Corals are everywhere.  They can be found across the world’s oceans, in both shallow and deep water, but reef-building corals are only found in shallow tropical and subtropical waters. This is because the algae found in their tissues need light for photosynthesis and they prefer water temperatures between 70-85°F (22-29°C).

The largest and probably most well known coral reef is the Great Barrier Reef, off the northeast coast of Australia, in the Coral Sea. The largest living structure on Earth, this reef is 1,553 miles in length and 134,363 square miles in area size. It’s so large that it’s visible from space. Rather than one single structure, it’s made up of around 2900 individual reefs and 900 islands.

There are also deep-sea corals that thrive in cold, dark water at depths all the way down beyond 6,600 feet (2000 m). Because deep-sea corals do not have the same algae to rely on and do not need sunlight or warm water to survive, both stony corals and soft corals can be found in the deep sea, but they grow very slowly.

Also, because they grow so slowly, deep water corals do not form in traditional reefs but instead form into what are called patches, banks, or even thickets or groves, like we call groups of trees or bushes.

The world’s largest known deep-water coral reef complex is the Røst Reef in Norway. It lies west of Røst island, inside the Arctic Circle, where water temperatures stay around 35 ℉ (2 ℃). Discovered in 2000, it lies between 980 and 1,310 ft (300 and 400 metres) below sea level. This reef is about 25 miles (40 kilometres) long and is still largely intact.

Reefs as Ecosystems

So, what do these weird rocky things do other than reproduce however they want to and live in shallow warm water or really deep cold water?   Good question.

Reefs are like underwater cities where all manner of sea creatures congregate to feed, often on the larval coral themselves. Some species of worms, snails, fish, and sea stars also feed on the adult coral.  These animals also hang out around the reefs to breed, reproduce, and hide from bigger predators. 

Some of the other animals that like to call reefs home are sponges, crustaceans, mollusks, fish, sea turtles, sharks, and dolphins, just to name a few. For example, the northwest Hawaiian Island coral reefs provide an of the diversity of life associated with shallow-water reef ecosystems. This area alone supports more than 7,000 species of fishes, invertebrates, plants, sea turtles, birds, and marine mammals.

And like any city, coral reefs also have defense forces that protect the corals.  Nematocysts are one example of this. These are the stinging cells on the end of coral tentacles that are used to sting, capture and kill off small prey. Nematocytes look like double-walled structures that each contain a coiled, venomous thread with a barb at the end, so that when the nematocyte is stimulated either physically or chemically, the thread releases, penetrates its victim’s skin and releases poison.

Some coral colonies have crabs and shrimps that live within their branches and defend their home against coral predators with their pincers. This is an example of mutualism, which is the term for a mutually beneficial relationship. 

What we think of as a classic reef are those found in warm, shallow waters, full of fish, and favored by divers all over the world, but there is much more to these amazing worlds.