Jonathan Cooke

If you weren’t aware of just how badly coral reefs have been faring due to global warming, it was announced in late November that just over two-thirds of the Great Barrier Reef has been killed off due to warming seas. That is over 435 miles of coral that has died, three times the distance between Sheffield and London. Coral reefs account for the largest areas of biodiversity in the ocean and are considered some of the most diverse habitats on the globe.


Image Credit: Wikimedia

Losing these areas of biodiversity is a disaster for many marine species. Coral reefs act as both a nursery and a shelter for smaller fish, helping to shield them from the ocean’s larger predators. Indeed, many species are specialized to living on or around reefs, creatures such as everyone’s favourite clownfish, Nemo. Without their symbiotic partners, the anemones, these clownfish face likely extinction.

The main problem facing coral reefs is that the rate at which they are dying is massively outpacing the rate at which they grow. The gradual heating up of our oceans is intolerable to many species and causes them to instinctively eject the algae, or zooxanthellae, with which they have formed symbiotic relationships. These algae are responsible for the brilliant and vivid colours we are so familiar with, and provide the coral with nutrients, via photosynthesis, which are key to their survival.

‘Coral bleaching’ is the direct result of the ejection of these algae and whilst it does not spell the immediate death of the coral itself, it leaves them considerably more stressed and prone to suffer from disease. As such, mass coral bleaching is seen as a warning sign of coral’s imminent death.

Can anything be done to counteract such effects however? Well that really depends on which reef we are talking about. Most coral species are ‘endemic’, typically found within one particular reef system. You are unlikely to find many shared species between the Great Barrier Reef and the Florida Keys reef, for example. This means our approach to fixing the reefs must be adapted in each instance to better tackle the issues each coral species faces.

However, some techniques can be transplanted. Ruth Gates, a biologist over in Hawaii institute of Marine Biology, has become somewhat of a coral gardener, growing and cultivating baby corals in controlled environments away from the sea. Since polyps (organisms that build the reefs) grow at tediously slow rate, growing mere millimetres over the course of years, we must find another, quicker way to replace the destroyed coral. It turns out that breaking the polyps up increases the rate at which they grow, as they work quickly to replace the damage.

This finding has fuelled our ability to seed the water with millions upon millions of ‘microfragments’, which can be planted on the surface of dead or dying coral and use this as a structure from which to grow. This technique works for other types of reefs as well; in Britain, many have been using the shells of farmed oysters as bases for young oysters to grow from.  Having a rooted skeleton saves the new coral the energy it would expend rooting itself to the ground, which it can then invest in growth.

Planting juvenile corals is not the only method of reseeding an area and it does have its difficulties. Growing coral takes a long time as previously mentioned; you could be waiting almost half a decade before the corals are ready to be seeded. Even then there’s no guarantee they will survive. Instead, some scientists have taken to using larvae to try and reseed corals.

Coral larvae is free-floating in nature due to their method of reproduction. Adult corals simply release either eggs or sperm into the sea at one or two points throughout the year in mass spawning events. Most coral species do this at the same time, which could lead to some confusing parentage! The eggs are then fertilized in the water column and the larvae develop there until they are big enough to anchor themselves to the seabed. Now, for a sedentary animal this makes sense; releasing your children to be carried by the current limits the chances they will settle down next to you and compete for resources.

However, this method would be ineffective for reseeding efforts as you never know how many would actually settle. To combat this, the teams trapped the coral larvae underneath mesh enclosures so that they would stay at the sites that needed to be rebuilt. Whilst many of the coral settlers will die, enough will survive to sexual maturity to begin the next lifecycle of the reef; in the world of coral breeding, it’s a numbers game.

Now, these methods are all well and good but they miss one key problem: all these corals will be brother and sister. They’ll share all the same strengths, but also all the same weaknesses. As such, any environmental triggers that induce bleaching will cause them all to bleach, landing us back where we started. However, by collecting larvae from nearby surviving reefs, as well as breeding different genetic varieties of coral, we can make sure that we build a genetically diverse reef that can survive into the future.  

Rebuilding the reefs we have destroyed will by no means be an easy task, but it is one we must accomplish. The Great Barrier Reef existed for almost half a million years before we arrived; it is our duty to ensure it is there after we are gone.

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