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Episode 304: Corals are critical to our oceans

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The incredible colours and diversity of the rainforests of the ocean, coral reefs

When you think of coral reefs, you might immediately think of documentaries showing scuba divers gliding over vast colourful shoals of fish flitting between vivid corals. But what you may not realise is that corals are living organisms, and reefs are constantly growing and changing.

The earliest coral reefs first appeared 540 million years ago, making them some of the most ancient organisms covered in this podcast, with only jellyfish thought to be older. The oldest coral fossils from the Palaeozoic era were found in Scotland and China and are the very first scleratinians, or hard corals, although they were solitary organisms and did not build reefs like their ancestors today. As time progressed into the Mesozoic era, tropical seas expanded, and corals became more widespread and complex, forming reefs through the interactions of microbes and chemical sediments in the ocean.

A piece of fossilized coral (http://www.catnapin.com/)

Coral reefs are often called the rainforests of the ocean, as they support incredible biodiversity, with 25% of marine fish relying on reefs for shelter, food and reproduction. Generally, reefs are found in a band around the tropics, in clear waters between 23 and 29 degrees Celsius. There are 3 main types of reefs:

  • Fringing reef: generally grow seawards away from the shore.
  • Atoll: this is a type of fringing reef that grows upwards from a volcanic island reef that has sunk, in an oval shape.
  • Barrier reef: these reefs don’t grow from the shore but are separated from the land by an expanse of water.

Diagrams of the different types of reef. (image from www.howitworksdaily.com)

There are also deep-water corals, found at depths between 50 and 3000 metres that tend to be more solitary, growing on sea mounts and continental shelves, where they provide a safe haven for a range of deep ocean species. These deep species may not be what springs to mind when you think of corals, but there are at least 3000 species listed, with more being discovered every year, in a dazzling array of colours and shapes. There are also some of the most ancient living species in the ocean, with one colony of black coral off the coast of Hawaii determined to be 4625 years old.

The marine invertebrates that are the architects of our modern reefs are small soft bodied polyps that belong to the phylum Cnidaria, which is home to other simple animals like sea fans, sponges and jellyfish. Coral polyps are very basic animals, with simple neurons to help them process sensory information and an oral disc underneath their tentacles to catch food and pass it down into the gastro cavity. These polyps settle into large colonies of genetically identical individuals, where they remove calcium carbonate from the seawater to form a calcareous skeleton. It is these hard shells that form the intricate and colourful coral reefs millions of people scuba dive and snorkel over. Although scientists can’t decide how long the polyps live, with estimates ranging wildly from 2 – 4000 years, the hard limestone reefs they leave behind can last for tens of thousands of years; perhaps the most famous, The Great Barrier Reef, is suspected to be over 20,000 years old.

The anatomy and structure of the coral polyps (image from geoforcxc.com)

You scratch my back, I’ll make you some oxygen: the role of symbiosis in coral reefs

Coral reefs provide us with a great example of symbiosis, a close and long-term beneficial interaction between two organisms. Most reef building corals contain photosynthetic cells algae cells called zooxanthellae (zoo-UH-zan-thuh-lay) that have a very special relationship with corals. The zooxanthellae take advantage of the hard shell of the coral for protection and use the compounds the polyps produce for photosynthesis to make energy. In return, the zooxanthellae supply oxygen and products like glucose and amino acids, which the coral uses to make proteins, fats and carbohydrates. This nutrient recycling relationship is absolutely vital in tropical waters, which may be lovely for us to enjoy a dip in but are incredibly nutrient poor for any animals living in them.

The microscopic algae that are so essential to coral reefs (image from http://www.ecoevolab.com/research/symbionts/)

You can also thank zooxanthellae for the amazing colours of coral reefs. Their pigments colour the corals they call home, and all the varied pigments can absorb different types of light. This is why corals closer to the surface of the water tend to be redder in colour, as red light doesn’t have enough energy to penetrate deeper into saltwater, in comparison to blue light, which can reach deeper water. Anyone who has dived on a coral reef can attest to the fact that everything you first see is red and if you get deep enough, everything becomes blue.

Not only do corals and zooxanthellae have a vital symbiosis, but corals are also essential for the health of our oceans and for humanity. Coastal communities in some of the poorest countries in the world rely on reefs for food with an estimated 25% of small-scale fisheries dependant on coral reef ecosystems for fish stocks. Some of these communities rely on reefs economically by selling fish for the thriving recreational aquarium trade, although sadly this trade is poorly regulated and generally unsustainable. Recreation and tourism generates 35.8 billion US dollars annually, and can provide more sustainable livelihoods for local people, provided it is carried out responsibly. Reefs are also essential for protection against storms and other extreme weather events; they can dissipate up to 97% of energy that would otherwise batter shores, protecting 197 million people and saving $4 billion in damages each year.

Is it getting warm in here? The big coral conundrums

Coral reefs are arguably some of the most sensitive ecosystems in the world and there are big concerns over their future. Overfishing is a major problem for coral reefs, with the indiscriminate removal of fish species upsetting the careful balance reefs exist in. The loss of herbivores, like parrotfish, can be especially detrimental as algal blooms that are normally removed by such fish overgrow, hindering healthy functioning of the reef and preventing new corals from attaching and growing. There is particular concern over parrotfish, which are perfectly adapted for scraping and excavating algae from coral with their tightly packed teeth and parrot like beak. Certain species are also responsible for the soft white sand that is emblematic of a tropical getaway, as parrot fish digestion breaks down fragments of coral into sand. The overfishing of herbivores like the parrot fish is causing coral reefs in the Caribbean and Pacific oceans to be suffocated by great mats of seaweed. Many locals will also supplement their livelihood by catching fish for the aquarium trade, in which fish are often kept in poor conditions and risk spreading diseases across the world.

The incredibly important coral reef engineer, the parrotfish (Image from www.nature.org)

Not only is the amount of fishing a problem, but often the strategies used are highly destructive. Cyanide is frequently used, which will poison anything it encounters and can drift on currents to reefs and other ecosystems. Dynamite is another method used, which can completely destroy whole sections of reef and kill not just the fish intended for plates, but also turtles, crustaceans, invertebrates and of course the corals themselves, which have taken hundreds of years to develop. Often these fishermen are resorting to increasingly desperate and destructive measures to compete with huge European and Chinese trawlers that are fishing the waters bare.

Although tourism can be an excellent sustainable alternative to fishing, some of the recreation and tourism practiced is unregulated and unsustainable. Swimmers are often not educated about the importance of keeping away from coral to prevent damage, or are irresponsible and don’t care, and often boats will drop anchors on top of the reefs, damaging them. In addition to all this, hotels and recreational boats can pollute the water around reefs, upsetting the important balance needed around reefs. Coastal development of new resorts and other buildings require dredging of the coast, leading to more pollution and sedimentation which clouds the water and prevents adequate light reaching the zooxanthellae living in the corals. Often when coastal regions are developed, vast areas of mangroves are destroyed which leads to more sediment run off to reefs. This pollution is suspected to be part of the reason behind increasing rates of coral disease such as black band disease, white pox and stony coral tissue loss disease, to name just a few. First observed in the 1970s, little progress has been made in diagnosis or treatment of these diseases. Studies of the Great Barrier Reef have revealed that warmer summers led to increased incidences of coral disease, which is particularly concerning given increasing sea temperatures.

Acropora coral afflicted with black band disease (image from news.wgcu.org)

Invasive species can often completely devastate the naïve ecosystems they are introduced to, and nowhere is this truer than the case of the Crown of Thorns Starfish. This spiky starfish is a voracious reef predator, chowing down on vast tracts of coral reefs, devastating these ecosystems. Whereas other coral feeders generally cause only minor tissue damage in small areas, the Crown of Thorns can kill entire corals, and if there are enough of them, whole colonies. They can breed rapidly and reach such high densities so quickly that almost entire reefs can be depleted. There are little natural predators of these prickly characters; their hard spines don’t make for an easy meal. Certain species of shrimp and fish will eat the starfish larvae, but once they mature it is very difficult to get rid of them.

The highly destructive crown of thorns starfish (image from ourmarinespecies.com)

Ocean acidification is the result of increased carbon dioxide levels in the atmosphere being taken up by the oceans, causing a decrease in water PH. Gradually, this affects the chemical balance in the water, and there is less calcium carbonate available in the water for corals to take up to build their amazing structures. This can at least stunt the growth of reefs and at worst cause mass die offs of coral colonies.

All these issues put corals in a precarious position, but the warming climate is a big worry for their future. Corals are particularly vulnerable to ocean warming thanks to a phenomenon known as bleaching. When ocean temperatures increase, the symbiotic relationship between the coral and their trusty little zooxanthellae breaks down. Under stress, the corals expel the algae from their tissues, and with them go the vibrant colours and healthy functioning of the coral reef. This leaves behind ghostly white bleached corals that rapidly lose function. If the zooxanthellae don’t recolonise the reef within a few months, individuals’ corals can die, and eventually the whole colony and then the whole reef, if the unfavourable conditions persist long enough. It isn’t just temperature increases that can cause bleaching, but also infections caused by intensive tourism in one area, damage from fishing with trawling and chemicals and from pollution. There have been several mass bleaching events in 1982, 1987 and 1992 that first alerted scientists to the fragility of corals in the human age.

Ghostly bleached coral, where environmental stressors have caused zooxanthellae to abandon their coral host (image from www.theguardian.com)

Conservation optimism

The future for coral does look bleak, but there have been massive steps forward in coral conservation, particularly with the rise of new molecular techniques.

A coral nursery (image from www.livescience.com)

Scientists across the tropics, such as those working for the Coral Restoration Foundation, are growing new corals, both in lab based and ocean nurseries. Charles Darwin was the first to understand that corals could be transplanted and grown. He noticed that corals that were loose and rolling around on the sea floor would quickly turn white if left, but could be saved if they were secured to a piece of bamboo that was hammered into the ground. This basic coral culture has been improved to the point that labs can grow virtually any kind of coral for transplantation back into beleaguered reefs. If reefs have been damaged by storms or human activity, this is the easiest way to repair them, by taking the fragmented corals and attaching them back onto the reef for growth, or to an artificial reef or lab for transplantation at a later point. Newer research is looking at the role of 3D printing to create artificial corals that will be placed on mounds of concrete in the ocean. Floating coral polyps will be able to adhere to these structures and this could cut down the time needed to create a new reef. Facilities like the National Sea Simulator, a $25 million lab in Australia, has dozens of seawater tanks where conditions can be set to exactly mimic temperatures, currents and nutrients in the ocean. Here, scientists are growing and researching corals and other creatures that are essential to reef functioning, hoping to create stocks that can rescue devastated reefs in the future.

Coral being grown at the National Sea Simulator (image from https://aquanerd.com)

Given that many of the stressors corals are exposed to affect them at a cellular level, researchers are turning to molecular methods to tackle the issues of coral disease and bleaching. Scientists are screening the genomes of both coral and their partner zooxanthellae to find natural gene variants that code for better heat and disease tolerance. Coral and algae genomes can then be edited with CRISPR technology to include more advantageous genes, and these genetically manipulated corals can be grown in nurseries and transplanted into reefs across the tropics to help them cope with increasing anthropogenic stressors. Although this seems like a cut and dried solution, there are a lot of complications, not least the sheer number of coral species. Scientists are also concerned that editing certain species of coral may give them fitness advantages once they are moved to the wild, and this could lead to one species taking over and dominating the ecosystem. Until more research is done and discussions had about the impacts of genetically modified organisms, this technology, as impressive as it is, will remain predominantly in labs.

Science has a lot of answers when it comes to saving coral but work with local communities can have enormous impacts too. Policy interventions like the implementation of Marine Protected Areas and No Take Zones can keep coral reefs safe from the worst of destructive, unsustainable fishing and irresponsible tourism. Tackling plastic and other land-based pollution that runs off into the ocean is also essential in the battle to save coral reefs. Many organisations provide guidance for the tourism sector, and you can read some helpful guidelines here if you’re planning a trip to take a deep in tropical waters at https://coral.org/en/coral-reefs-101/why-care-about-reefs/tourism.

Outreach to local communities involves education about the fragility and importance of coral reefs, providing alternative, more sustainable ways to fish and utilise resources. Organisations like the Zoological Society of London have even set up guard posts around coral reefs in the Philippines, providing local people with reliable employment and the reefs with round the clock protection from illegal fishing. Improving the tourism and fishing industries is dually beneficial, providing longer term, more sustainable employment and keeping people enjoying and utilising the reef resources in a way that protects these amazing ecosystems for years to come.

The NOAA Coral Reef Conservation Programme is a task force set up by conservationists and scientists from the National Oceanic and Atmospheric Administration, dedicated to the protection and restoration of corals across the tropics. You can check out their research and conservation action at https://coralreef.noaa.gov/.

Awesome videos!


Cornwall, W. (2019) ‘Researchers embrace a radical idea: engineering coral to cope with climate change.’ Science

Doney, S.C. Busch, D. Shallin, Cooley, S.R. and Kroeker, K.J. (2020) ‘The impacts of ocean acidification on Marine ecosystems and reliant human communities.’ Annual Review of Environment and Resources.

Drew, H. Jordan-Dahlgren, E. Merkel, S. Rosenberg, E. Raymundo, L. Smith, G. Weil, E. and Willis, B. (2007) ‘Coral disease, environmental drivers and the balance between coral and microbial associates.’

McClellan, K. and John, B (2008) ‘Coral degradation through destructive fishing practices.’

Mullock, M. (2009) ‘Saving rainforests of the sea: An analysis of international efforts to conserve coral reefs.’ Duke Environmental Law and Policy Platform.

Pollock, F.J. Morris, P.J. Willis, B.L. and Bourne, D.G. (2011) ‘The urgent need for robust coral disease diagnostics.’ PLoS Pathogens

Richardson, L.L. (1998) ‘Coral diseases: what is really known?’ Trends in Ecology and Evolution.

Strain, E.M.A. Edgar, G.J. Ceccarelli, D. Stuart-Smith, R.D. Hosack, G.R. and Thomson, R.J. (2019) ‘A global assessment of the direct and indirect benefits of marine protected areas for coral reef conservation.’ Diversity and Distributions

West, J.M. and Salm, R.V. (2003) ‘Resistance and resilience to coral bleaching: implications for coral reef conservation and management.’ Conservation Biology.

Woodhead, A.J. Hicks, C.C. Norstrom, A.V. Williams, G.J. and Graham, N.A.J. (2019) ‘Coral reef ecosystem services in the Anthropocene.’ Functional Ecology

Zooxanthellae…What’s That? Oceanservice.noaa.gov



November 09, 2022
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