A Deep Dive Into the World’s Largest Coral Reef

It probably wouldn’t take most of you long to guess which marine environment is one of the seven wonders of the natural world. This same feature is also the world’s largest coral reef and one of the most complex ecosystems on Earth. If the Great Barrier Reef (GBR) comes to mind, then you’d be right! 

This incredible environment contains an astonishing 3,000 individual reef systems as well as coral cays and tropical white sand beaches. Its size is so great that it is the only living thing visible from space! Running parallel to the eastern coast of Australia for 2,000 kilometres and spreading from the low water mark of the coastline to 250 kilometres offshore, this is an enormous ecosystem home to thousands of diverse species. Yet it hasn’t always looked like a tropical paradise. Over tens of thousands of years, the reef is thought to have lived through four glacial and interglacial cycles. In glacial periods, the sea level dropped and left the reefs as exposed, flat-topped limestone hills with rivers weaving between. In interglacial times, such as now, the reefs flourished, grew and filled with colourful marine life whilst new islands appeared due to sea level rise. The incredible biodiversity here is partly due to the age of the reef but also due to the fact that it traverses so many different habitats. The reef stretches from warm shallow waters flooded with sunlight to much colder patches of ocean which extend two kilometres deep from the surface. This provides multiple varied habitats which suit a wide range of species whose lives overlap and interlink to create an incredibly complex ecosystem. Here you can find both resident and migrating species which total 1,500 fish species, 400 different types of coral and almost 250 various seabirds! It is also a site of incredible natural wonders such as migrating whales, annual coral spawning events and the world’s largest green turtle breeding area. Let’s take a close look at just a few of these species.

Whilst molluscs may not sound like the most interesting group of animals, there are actually 4,000 different species of mollusc on the reef, many with interesting characteristics. The giant clam, for example, is the world’s largest bivalve mollusc, typically weighing in at 200 kilograms! They come in a range of brilliant colours which are generated by the algae living inside them; the brighter the colour, the healthier the clam. By day, the clams open up in order to allow this algae living inside them the chance to gain sunlight and photosynthesise. In the past, their ability to open and close their shells led many to believe they could trap and devour unsuspecting divers but in reality, these clams move far too slowly to capture any passing swimmers.

Many visitors take to the water in snorkels to see the astounding array of fish on the reef, and which is more famous than the clownfish. These orange fish with white and black stripes are often seen hiding in anemone, with whom they have a pretty good relationship. The bright colours of the clownfish lure predators into the area where the anemone is able to kill them with its poison. The anemone then shares its scraps of food with the clownfish. It is not known how the clownfish avoids the toxic poison themselves but they escape larger predators by hiding inside the jungles of anemone.

Manta rays are another beautiful sight that many flock to the reef to witness. These gentle giants don’t have the stinging tail that other ray species have and so you can relax and watch them as they glide by on their long journey in search of food. They are often known to play with their food by forming long chains and swimming in circles to create a spiral effect where prey, such as fish larvae and plankton, get caught in their wake and are ready to be swept up by hungry mouths. ‘Manta’ actually translates to cloak or blanket in Spanish and with these creatures having a wingspan of seven metres, it’s easy to see where they got the name! On their travels, they are known to stop off regularly at the spa, or rather what is known as reef cleaning stations. These are areas where species, such as wrasse, live which eat the dead skin and parasites that are living on the bodies of passing animals. With the largest brain to body ratio of any fish, the manta ray is able to map their environment and return to their favourite spa therapist time and time again. 

Of course, the GBR is also home to a much more famous group of fish; sharks. From small wobbegongs to spotted whale sharks and tiger sharks, the reef is a melting pot of these often misunderstood species. Whilst some do pose a risk to humans, the whale shark is a big friendly giant. Despite having 300 teeth, they are actually filter feeders and slowly swim along with their mouths open simply waiting for small prey to float in. They are also fairly unique as the females produce eggs that hatch inside them and when the young are fully developed, she will give birth to around 300 babies!

The list of incredible species living here could go on forever; we haven’t even mentioned the 10,000 whales which arrive at the end of their migration from Antarctica, or the seven species of turtle which nest on the tropical islands. Or even of course the most famous reef creature of all; coral! The GBR is home to bottlebrush coral, brain coral and bubble coral, and that’s just the Bs!

In order to protect this unique ecosystem, the area was designated as a marine park in 1975. They have used the conservation strategy of zoning which sees the reef split into different areas with varying levels of activity allowed to take place in each. In ecologically important areas, only boating and diving is allowed, whereas fishing is still allowed in other zones. The government currently has an action plan for the reef which has set a range of targets to achieve by 2050. These include aims to improve water quality, protect and restore parts of the reef and help encourage action on climate change and marine plastics, both of which are transboundary issues threatening the GBR. They have currently pledged $4 billion to make this happen, yet what are the exact threats that the reef is facing?

Currently, climate change is thought to be the greatest threat looming over the GBR, as well as other reefs around the world. When corals become hot, they get excessively stressed which causes them to expel the algae which live inside them. These algae provide the corals with essential nutrients but also their bright colour. Without them, the reef appears white or bleached whilst they are more open to disease and starvation. Corals are fragile creatures and bleaching can occur if the temperature rises by one degree for a period of four weeks. Whilst corals can recover from stress and excessive heat, they need a good length of time to do so in cooler temperatures and normal conditions. In the last seven years, there have been four mass bleaching events and at this high rate, the corals are struggling to recover and grow. The greenhouse gas emissions which are adding to climate change mean that there is more carbon dioxide in the air too and therefore, the ocean is absorbing more carbon. This is making seawater more acidic which is bad news for corals whose skeletons are made from calcium carbonate. This material is delicate and begins to crumble under acidic conditions, which means that reefs struggle to grow and build on top of each other. Scientists are racing against the impacts of climate change however, as they are mixing the genes of various different coral species to try and create more heat resistant strains. Whilst some are experimenting in this way, others are raising young corals in protected nurseries before planting them on the seabed in areas of damaged reef to aid recovery and new growth. Even probiotics are being considered for corals, which hope to improve their resilience to stress and disease just as they do in animals and humans. 

Scientists are also currently battling against another significant threat to the reef; invasive species, particularly the crown of thorns starfish. This creature preys on almost all species of coral, with an individual eating up to 10 square metres of coral a year, meaning a large invasion can quickly destroy important feeding and breeding habitat for many animals. As the name suggests, these starfish are also covered in spines which are toxic to other marine creatures and therefore threaten native populations. The GBR is currently experiencing its fourth major outbreak since the 1960s and in order to protect the reef, divers are hunting individuals one by one and injecting them with bile salt or vinegar which kills the starfish but leaves the rest of the ecosystem unharmed. 

Human activity and the subsequent pollution is another significant danger for the reef, with Australia having some of the world’s largest mines and coal ports as well as a significant amount of agricultural land backing the waters of the Queensland coast. These activities lead to a range of sources of pollution, from oil spills to fertiliser run off, fishing debris to noise pollution which all place extra pressure on the marine ecosystem. In February of this year however, for the first time in history, the Australian government used environmental law to block the development of a coal mine 10 kilometres from the reef due to the potentially devastating impacts on the reef and water quality. There have also been multiple projects working hard over the last few years to restore the parts of the reef which have already been harmed. For example, Lady Elliot Island was stripped to bare rock after years of guano mining but has now been carefully replanted with native vegetation whilst also restoring nesting habitat for endangered turtles. It is hoped projects such as these will allow islands to become safe havens for nature away from human disturbance. There is still a long way to go before the future of the GBR is secure, yet brilliant initiatives by passionate individuals and now a government which seems to be moving onto the right track can give us all a little bit more hope.

By Neve McCracken-Heywood