Everything you need to know about coral reefs (2024)

Coral reefs are the reason many of us love to go diving, but despite many years of research, there’s still an awful lot we don’t know about them. One thing, however, is certain – coral is vital to the planet’s health. Here’s the basics

What is coral?

Although it was once thought of as a type of rock, corals are a diverse group of (mostly) colonial animals, members of the phylum Cnidaria (pronounced ‘nye-DARE-ih-ah’, with a silent ‘c’), a group of animals defined by the presence of cnidocytes (also known as cnidoblasts or nematocytes), a single-celled structure containing a venomous sting used for both defence and capturing prey. Coral is closely related to other cnidarians such as hydroids, anemones and jellyfish.

Corals are Anthozoans, a sub-phylum of Cnidarians which also includes anemones, of which more than 6,000 species have been described to date. Anthozoans are sessile (not moving) polyps which remain permanently anchored to their base, whose larvae (when produced) are dispersed into the water to drift on the current and find their new forever homes. The name ‘Anthozoa’ comes from the Greek words ánthos, meaning ‘flower’, and zóa, meaning ‘animals’, so, quite literally, ‘flower animals’.

Coral classification

We tend to think of corals as being divided into ‘soft’ corals and ‘hard’ – or ‘stony’ – corals, although the actual taxonomic classification is somewhat more complex. Anthozoans are sub-divided into Hexacorallia – the polyps of which always have a number of tentacles, which is a multiple of six, and Octocorallia, which always have exactly eight tentacles. A third class, Ceriantharia, includes two groups of anemones that like to live in tubes.

Stony corals, Scleractinia – sometimes also called ‘reef-building’ or ‘true’ corals – are contained within the Hexacorallia class, as are black corals, (Antipatharia), sea anemones (Acitniaria) – which are definitely not particularly stony – and corallimorphs, or ‘false corals’ (Corallimorpharia).

Soft corals are all members of the Octocorallia class, further subdivided into the order Alcyonacea, and sometimes lumped in together with sea pens (Pennatulacea). Rather like squishy anemones are found in the same class as stony corals, Octocorallia also contains blue corals (Helioporacea), which form hard skeletons very similar to their reef-building, Scleractinian cousins.

How old are corals?

The earliest record of Anthozoans is found in the Pre-Cambrian period at least 550 million years in the past. Corals that contribute to the building of reefs first appear in the fossil record around 20 million years later, in the early Cambrian, although the symbiotic reef-building corals we know today emerged much later.

It has long been thought that reef-building corals had only evolved within the last 20 million years, although more recent research suggests it was more like 160 million years, meaning T-Rex and its Jurassic-era buddies would have seen reefs very similar to what we see today, if they went for a paddle.

Either way, this makes coral and its ancestors older than dinosaurs (245mya), trees (360mya) and sharks (450mya).

Although coral as a type of animal has survived for more than half a billion years, many of today’s reefs are less than 10,000 years old, due to rapid sea-level rises following the last glacial maximum, which ended suddenly around 11,700 years ago.

How do corals build reefs?

Reef-building stony corals – hermatypic Scleractinia – are what build the reefs that everything else lives on. Corals secrete aragonite – a form of calcium carbonate (CaCO3) – as part of their metabolic cycle, but the process is not well understood.

For many years, it was thought that corals ‘precipitate’ the aragonite by pumping seawater through their bodies to extract the chemical compounds that make up the carbonate molecules. However, more recent research suggests they take a tiny lump of pre-existing carbonate around which the coral’s secretions crystallise, helping to speed up the reef formation process.

• Amorphous calcium carbonate particles form coral skeletons
• Biological control of aragonite formation in stony corals:

What are the different types of coral reef?

Coral doesn’t set out to build reefs in a particular style. As reef-building coral requires shallow water so it can be near to the sunlight, reefs initially grow close to the shoreline, as defined by the contours of the land and the movement of water and nutrients across its surface – these are known as fringing reefs.

As the land recedes through flooding and erosion, the hard coral reef, less affected by either, becomes a barrier reef some distance out to sea, where it serves as protection from wave action and storms.

The third form of coral reef is atolls, ring-shaped reefs with a shallow interior known as a lagoon, which probably grew around volcanic islands that have since crumbled into the deep ocean.

How do corals feed?

Corals are filter-feeders, extracting plankton and other nutrients from the water as it drifts across the polyp colonies, using their stinging cells to immobilise any tiny animals and small fish that drift into its tentacles.

Many coral species, both soft and stony, are also symbionts with a form of algae known as zooxanthellae, an umbrella term for a wide range of single-celled organisms known as dinoflagellates, a type of plankton so called for the tiny, whip-like ‘tail’ – flagella – which propels it through the water.

Zooxanthellae – which are also symbionts with certain species of anemone, jellyfish, sponges, nudibranchs and clams – are capable of photosynthesis, providing energy in the form of sugars to the coral in which they live, which in return provides the zooxanthellae with a source of carbon dioxide and nutrients as part of its metabolic process.

What gives coral its colour?

Coral polyps are naturally transparent and take their colour from the photosynthetic algae which live within them. Like other plants, zooxanthellae contain chlorophyll, which gives corals their greenish-brownish appearance, rather like leaves on trees.

Some corals produce additional, fluorescent proteins which give them a much brighter, more colourful appearance, some of which occur in wavelengths of light invisible to the human eye and require an ultraviolet lamp to see – a spectacular thing to see if you ever have the chance to go fluorescent night-diving

The symbiotic relationship through which most reef-building coral acquires its colour is threatened when the ambient conditions are changed, however, which leads to coral the coral ejecting its zooxanthellae, giving the appearance that the reef has been ‘bleached’ (see below)

How deep can coral grow?

Coral that relies on its symbiotic relationship with zooxanthellae – including reef-building coral – can only grow where there is enough light for photosynthesis to occur, so from the surface to between 30m-100m, depending on available sunlight and water clarity.

Black corals, which are not zooxanthellae symbionts, including familiar species such as wire-corals or ‘sea-whips’, are often found in much deeper water than typical tropical corals. Some deep-water and cold-water corals which also do not rely on the availability of sunlight – such as the Christmas tree coral pictured above – have been observed as deep as 3,000m

How do corals reproduce?

Some corals exist in colonies of either solely male or solely female polyps, releasing their gametes into the current in the hope that they meet somewhere and make a larva, which will eventually settle and grow into a wee polyp all of its own.

Most stony corals, however, are hermaphrodites, releasing their sperm and eggs simultaneously in mass spawning events, the end result of which is a host of free-swimming ‘planula’ larvae that make their way through the depths of the ocean before settling into their forever home.

Many species can also reproduce asexually, dividing like amoebae into genetically identical replicas of themselves.

What is coral bleaching?

Environmental stressors, including changes in the temperature of the surrounding seawater, available light, chemical pollution, and disease can cause coral polyps to eject their symbiotic zooxanthellae.

As the zooxanthellae also provide the coral with its colour, their removal returns the coral polyps to their natural transparent state, revealing the (mostly) white calcium carbonate exoskeleton of the reef-building corals below, giving the coral the appearance of having been ‘bleached’.

Deprived of a major source of nutrients, affected coral polyps will slowly begin to starve. The length of time a bleached coral can survive is dependent on how reliant it is on its symbionts as a source of food. Large coral heads feeding on plankton drifting through the water can survive for several months; smaller, branching corals will die in as little as ten days.

Bleaching is a natural process and, if the stressor is relieved early enough, the coral will reacquire its zooxanthellae and return to normal. Repeated, extended bleaching events, however, do not allow time for the coral to recover, and a series of mass bleaching events in recent years have led to mass global coral mortality.

Why is coral important?

Coral reefs form less than 0.1 per cent of all the world’s oceans, but provide a home to more than 25 per cent of all marine species – including at least 4,000 species of fish – and a hunting ground for others.

The exact number of species living on coral reefs is unknown, but some estimates place it in the millions. Out of the water, more than half a billion people rely on reefs for their livelihood, and estimates circulating on the Interwebs placing the global worth of coral reefs at $375 billion dollars are almost certainly several hundred billion dollars – if not an order of magnitude – short.

Furthermore, rather like their land-based rainforest counterparts, treatments for a range of diseases have been created from reef-dwelling organisms, and scientists think there may be many more waiting to be discovered.

What are the threats to coral reefs?

Warming waters brought on by a changing climate are undeniably the main cause of mass bleaching events, but mass coral mortality has been brought about through a range of damaging human activities.

Coastal construction causes a huge amount of material to be suspended in the water, removing the sunlight coral needs to survive, before settling and blanketing the coral completely, causing it to suffocate and starve.

Chemical pollution from industry and tourism, including toxic chemicals found in sunscreens and runoff from agriculture also has a severely detrimental effect, as has decades of dynamite and cyanide fishing along some tropical reefs.

Overfishing and the removal of habitat can lead to voracious coral predation by other animals, such as the infamous crown-of-thorns starfish, or a growth in algae which can out-compete and smother coral reefs.

Even tourism itself has caused a huge amount of damage, through careless use of boats and anchors, the poor conduct of divers, snorkellers and other marine sports operators, together with the harvesting of coral for souvenirs and trinkets.

What is the future of coral reefs?

Much of the media dwells on reports that coral reefs are under an imminent threat of extinction, but coral has evolved through half a billion years and five mass extinctions, and recent studies have shown that it may be proving to be rather more resilient than we might at first have thought.

Despite seeing a massive loss of coral during a series of continued mass bleaching events during the first two decades of the 21st Century, in 2022, scientists monitoring Australia’s Great Barrier Reef reported the highest level of coral cover along the reef’s extent since records began in 1985.

Studies in the Red Sea have also found that coral, under the right conditions, is able to adapt to much higher temperatures, leading to the hope that more adaptable species may be interbred with those suffering in other locations.

Coral restoration initiatives are also creating an impact, with the largest mass-restoration project in the world currently underway in Indonesia, and eco-resorts like Wakatobi leading the way in protecting coral reefs used by divers; even the use of AI-assisted technology to help rebuild broken reefs is being developed.

Nevertheless, while coral will almost certainly outlive the human race, a mass loss of coral over the next few decades would be devastating to both the entirety of the marine ecosystem, and the peoples that rely on the reefs for their livelihoods, which is – technically – all of us.

It is therefore imperative we do all that we can to ensure their survival.

More great reads:

• Major study shows Maldives reefs survive bleaching
• Highest Great Barrier Reef coral cover in 36 years
• Great Barrier Reef hit by fourth mass bleaching in seven years
• Life Returns – coral reef restoration in Mustique
• Study shows a noisy coral reef means it is returning to health