The Science of Corals

Why corals? Simply put, these amazing little animals are some of the most beautiful of all creatures in the ocean, and are responsible for the formation of some of the greatest natural structures on the Earth - the coral reefs.

But don’t just take our word for it, watch this brief trailer from Chasing Corals, and then scroll down to see why corals are our Animal of the Month.

At IE we have had the chance to bring students to the corals reefs of Belize, and like them, we were awed by the experience. So, in this article, we will explore a little more about the science of corals. As you will see, corals are incredibly complex organisms who are responsible for one of the most important ecosystems on the planet, the coral reef - the rainforests of the ocean.

WHAT ARE CORALS? 

The word “coral” can be used to describe thousands of species. Corals are animals- not plants, despite their sometimes plantlike appearance- of the phylum Cnidaria, which includes jellyfish and hydrozoans. Within Cnidaria, corals belong to the class Anthozoa, making them closely related to anemones. 

A colony of great star coral, Montastraea cavernosa. Image by Tobacco Caye Marine Station

A single coral colony is made up of up to hundreds of thousands of individual animals, known as polyps, which may be as small as a few millimeters across. A polyp is shaped like a hollow tube with one open end, through which food enters and waste is expelled, and one closed end, which allows the polyp to attach to the sea bed or to other polyps and form a coral colony. 

Polyps may be small, but they have a hidden weapon. The open end of the polyp is surrounded by tentacles equipped with barbed, stinging cells called nematocysts, which are common across all animals of the phylum Cnidaria - a group that includes the jellyfish. These nematocysts can be used both to capture prey and to defend the coral polyp and colonies, including against other competing corals. 

A polyp of Leptastrea purpurea, a species of stony coral. Image by Narissa Spies

Classifying Corals

Classification of corals is, at best, complicated, and always under revision as new evolutionary relationships are discovered. Corals (class Anthozoa) may belong to one of two subclasses: Hexacorallia and Octocorallia. Hexacorallia includes stony corals (order Scleractinia) and black corals (order Antipatharia), and their relatives the sea anemones, corallimorphs, and zoanthids. Octocorallia contains the soft corals (order Alcyonacea) and the blue corals (order Helioporacea). 

An overview of coral classification and some close relatives of the corals. Image by Kayla Windelspecht at Inspire EdVentures

Stony corals (Scleractinia) are also known as “true corals,” and are perhaps the most famous corals due to their relationship with coral reefs. Stony corals are named for their hard external skeleton made of calcium carbonate in the form of aragonite, which provides a rigid framework for the coral colony to build upon. (Calcium carbonate is the same material used by other organisms to make seashells.) In fact, though other forms of coral are present on reefs, coral reefs themselves are made almost entirely of stony coral. Stony corals include well-known reef staples such as brain coral, star coral, and elkhorn coral. 

Grooved brain coral, Diploria labyrinthiformis, is an example of stony coral. Image by Tobacco Caye Marine Station.

Soft corals (Alcyonacea) lack stony corals’ external skeletons. Instead, soft corals contain thin structures called sclerites- also made of calcium carbonate- which provide support for the corals’ tissues. Soft corals form colonies and may be found in or near reefs, but are not true reef-builders like stony corals. Gorgonian corals such as sea fans and sea whips, formerly classified as a separate order, are today considered part of order Alcyonacea. 

A soft coral of the Dendronephthya genus, also known as carnation coral. Photo by Ahmed Abdul Rahman

Black corals (Antipatharia), sometimes known as thorn corals, are more closely related to stony corals than to the “true” soft corals of order Alcyonacea. However, like soft corals and unlike stony corals, black corals do not have external skeletons. Black corals are most often deep-water species, with some being found at depths of nearly 20,000 feet (6,000 meters). 

A species of black coral, Antipathes dendrochristos, also known as Christmas tree coral. Photo by the National Oceanic and Atmospheric Administration.

Blue corals (Helioporacea) are the odd corals of the bunch. Consisting of a single species, Heliopora coerulea, blue coral is found only on shallow-water reefs in the Indian Ocean, with large colonies that can reach up to 6.5 feet (2 meters) in diameter. Like stony corals, blue corals secrete hard skeletons of aragonite, and are in fact the only octocorals (members of the subclass Octocorallia) to have external skeletons. 

A preserved specimen of Heliopora coerulea, showing its distinctive blue color. Image by the Yale Peabody Museum.

Also included in class Anthozoa are corallimorphs, zoanthids, and sea pens. Corallimorphs blur the line between coral and anemone, and are sometimes called mushroom corals or false corals. (Mushroom coral, which lives individually instead of forming colonies, have polyps up to 5 inches (12.7 cm) across.) Zoanthids are unique in forming structures of marine sediment, such as sand, to support the polyp or colony. Sea pens resemble plants or even feathers stuck into the sea bed, and are classified as octocorals. 

A corallimorph from genus Discosoma. Image by user Haplochromis.

WHAT DO CORALS EAT? 

Corals are mainly filter feeders, meaning that instead of hunting their food, they use their tentacles to “filter” it from the ocean currents around them. This food includes very small organisms such as phytoplankton and zooplankton, some of which cannot be seen without a microscope. 

Phytoplankton, a favorite food of coral. Photo by NOAA on Unsplash

Some corals- mainly those in warm, shallow waters- have a symbiotic relationship to zooxanthellae, a type of algae that lives in their tissues. Zooxanthellae are photosynthetic, meaning that they produce energy from sunlight, like terrestrial plants. Some of this energy is taken up by the coral host. Zooxanthellae can also be responsible for coral pigmentation, especially the  green, brown, and red colorations. 

Coral bleaching events, which occur when ocean conditions become too hot or acidic for corals to thrive, involve a coral colony expelling zooxanthellae from their tissues. This causes the corals to appear white or translucent. Though the corals themselves are often not immediately killed, and may recover from small-scale bleaching events, the loss of zooxanthellae can lead to starvation in coral colonies. 

An example of coral bleaching. Photo by Naja Bertolt Jensen on Unsplash

We will explore the causes and impacts of coral bleaching in Part 2 of this article later this month.

HOW DO CORALS REPRODUCE? 

Corals engage in both sexual and asexual reproduction. Asexual reproduction takes two forms, budding or fragmentation, and produces offspring genetically identical to the parent. In budding, a coral polyp simply divides into two, expanding the coral colony. Fragmentation occurs when a piece of the coral colony is dislodged, such as by a storm or other sea organism. If the coral fragment reaches a suitable area, it may become rooted in the sea bed and eventually grow into a new coral colony. 

Sexual reproduction in corals is often more complicated. Many corals can produce both eggs and sperm, which are released into the water column in spawning events. These spawning events are triggered by environmental conditions or even time of year and may occur on a massive scale. Once fertilized, the coral eggs mature into new, unique polyps, which may form their own colonies and begin the reproductive cycle once again. 

Coral spawning in the Flower Garden Great Banks National Marine Sanctuary. Image by the National Marine Sanctuaries.

Want to Know More?

In part 2 of this article we will be taking a closer look at structure and importance of a coral reef, as well as threats to this important ecosystem.

You can also help support coral conservation by purchasing one of the biodiversity decals from Tobacco Caye Marine Station at the Inspire Me! Store. Proceeds go to protect the coral reef ecosystem.

Better yet, take a virtual tour of the reef with your family and friends!