An Introduction to Coral Reefs

Sea turtle and fish at a coral reef in the Maldives | Photo: Tchami, some rights reserved
Sea turtle and fish at a coral reef in the Maldives | Photo: Tchami, some rights reserved

The tropical latitudes of the world’s oceans could be called the desert sand dunes of the ocean. They are barren, relatively sterile places. The water is a pleasure to swim and snorkel in – but because it is so warm, it tends to float on top of the colder water underneath. This inhibits the deep-water upwellings that bring nutrients to the ocean’s surface – the same process that makes waters off the California coast so exorbitantly productive, though frigid and murky.  

But there is one brilliant exception to the general scarcity of life in the tropical oceans: coral reefs. These colorful ecosystems teem with circuses of sea creatures. They host as many one quarter of the ocean’s species, including such photogenic reef icons as moray eels, clownfish and groupers, with food and shelter. Coral reefs, unique to bathtub-warm waters, create wave breaks that protect coastal communities. They provide income for fishermen and food for millions. They support lucrative tourism, often based on snorkeling and scuba diving. The organisms living in coral reefs may even offer scientists clues in their search for a cure for cancer.

Though the U.S. Geological Survey has calculated the value of the world’s coral reefs at $375 billion, it’s probably fairer to say that these rainforests of the sea are priceless.

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Coral reefs are the product of a symbiotic relationship between photosynthetic microorganisms called zooxanthellae and tiny sea anemone cousins called polyps. The zooxanthellae, which live in the tissue of the polyps, capture energy from the sun and pass it on to their hosts. Meanwhile, the polyps secrete calcium carbonate to create shells around their bodies. Over time, this mineral deposition grows upward and outward, with only the very surface of the structure inhabited by living polyps, the interior of the reef made up of eons’ worth of abandoned polyp homes. Some coral reefs are the size of houses. Others are large enough to be seen from space. Australia’s Great Barrier Reef is more than 1,000 miles long and was built by vast colonies of polyps over some 25 million years. The second largest reef in the world is the Belize Barrier Reef, which runs almost 200 miles along the Caribbean nation’s coast. Globally, coral reefs cover roughly 100,000 square miles.

Anatomy of a coral polyp | Image: NOAA
Anatomy of a coral polyp | Image: NOAA

When a coral colony first gets established and begins growing, it must do so from a fixed surface. A rocky bottom in warm, shallow water is a good place to start. Since the zooxanthellae on reef-building corals depend on sunlight, coral reefs will only form in shallow waters, within the photic zone – usually not much more than 100 feet deep. (There are also deep-water corals capable of living in icy cold darkness, but they are not the reef-building corals we’re discussing here.) Over time, the coral basically chases the sunlight like a plant will, the reef’s surface growing upward as fast as about a centimeter per year.

So, imagine, for a moment, this process of coral growing from the seafloor toward the light, creating a huge stony structure covered with life. Pretty straightforward.

Now, if we combine the way in which coral reefs grow with various forces of erosion, tectonic shifting and changes in sea level, things get really interesting. Let’s say the seafloor beneath the reef is sinking, which is often the case near volcanic islands, which press down more and more on the Earth’s crust as they grow. If the seafloor subsides faster than the coral can grow upward, the coral will eventually die, since the symbiosis between polyp and zooxanthellae depends on sunlight. This process produces dead reefs that have been dragged deep underwater – something scientists have studied extensively in Hawaii.  

But if the seafloor sinks slowly enough, the growth rate of the coral can match or outpace the crust’s downward migration. This process creates barrier reefs many miles from but parallel to the coast of a continent. It also creates atolls, placid lagoons surrounded by conspicuous ring-shaped reefs that mark the approximate boundaries of the islands that have gone under. Other than the fact that they often have no source of freshwater other than rainfall, atolls were great places for mariners to settle. At many atolls, coral debris and sand has accumulated along the reef, creating acreage suitable for cultivation and inhabitation – not to mention plenty of rainwater storage tanks. Tuvalu, Tokelau, Rangiroa, and Nauru are all inhabited atolls.

It was atolls, specifically, that grabbed the attention of Charles Darwin in the 1830s. Naturalists had already been pondering coral reefs for decades, proposing a variety of hypotheses on how they formed. Most of these thinkers had been impressed by the same general observations – that coral reefs created broad platforms of many acres, often flat on top and just meters below the surface. It was understood that coral was alive, that it was animals that created the mineral structure of reefs, that they liked sunlight, and that they grew. But why, naturalists wondered, did coral so often grow in that uncanny ring-shape? And why was there dead coral high on coastal mountainsides? Scientists of the time proposed a rather flabby explanation – that islands capped by circular volcanic craters rose upward, pushing coral out of the water in places but more often to just a few meters below the surface, creating shallow reefs. 

Atafu Atoll in the Southern Pacific Ocean| Photo: NASA
Atafu Atoll in the southern Pacific Ocean | Photo: NASA

But Darwin, looking hard at the basic features of atolls, wasn’t convinced by this proposition of rising islands. In his mid-20s at the time, he agreed that there were places where uplift had exposed coral reefs. However, he asked why – and how – it could be that thousands of mountains had pushed upward only to stop – all of them – several feet below the ocean’s surface. Unless one believed that coral polyps had a way of calling for a tectonic elevator ride up to the photic zone, there had to be a better explanation.

And Darwin nailed it: Most ocean islands, he suggested, weren’t rising; they were sinking, and as they did so, the coral growing along their shorelines grew upward, pursuing the sunlight that was their source of life. Limited by physics and physiology, the coral never grew past the surface. This theory perfectly explained the ring formation of atolls, and it explained barrier islands that paralleled continental shorelines many miles from the coast; and it explained why huge anvil-shaped coral massifs with flat tops that covered many square miles never, at any location, broke the surface. The theory was flawless, and Darwin’s contemporaries – some of whom had been backers of previous explanations – slapped the young scientist on the back in congratulation and immediately hailed his theory as fact. Eventually, later scientists would add the rise and fall of sea level to the processes that generated coral growth – or, in many cases, killed it by drowning or by exposure. Darwin, though, gets credit for first seeing the light.

Beyond providing a great thinking exercise for 18th and 19th century naturalists, coral reefs were also the terror of seamen. The English explorer extraordinaire captain James Cook had a run-in 250 years ago with the Great Barrier Reef. He first encountered it as he explored the warm waters off Australia’s east coast on June 11, 1770. More specifically, he sailed his boat, named Endeavour, onto the reef, badly damaging the hull. His crew repaired the vessel, and within days they were sailing again – but they couldn’t go fast. Cook and his men were surrounded by a seemingly endless maze of treacherous crags and wave breaks – and for three months they tiptoed their way carefully through the reef before escaping into open water. Cook may have been lucky, for coral reefs have killed countless other sailors. The Great Barrier Reef alone has gouged and sunken an estimated 1,600 boats, the remains of which lie on or beside the reef. Indeed, there was probably little in the ocean that could so surely keep a sailor on his toes during his watch shift – and which could allow him, at best, troubled sleep as his boat moved through uncharted darkness – as a coral reef.

But today, we’re the ones threatening reefs. More to the point, we are destroying them – rapidly. Around the world, roughly 50 percent of coral reefs have died in just the past few decades. The Great Barrier Reef was even declared dead last year. Outside Magazine writer Rowan Jacobsen wrote a stirring obituary for the reef.

“The Great Barrier Reef of Australia passed away in 2016 after a long illness,” Jacobsen wrote. “It was 25 million years old … Among its many other achievements, the reef was home to one of the world’s largest populations of dugong and the largest breeding ground of green turtles. 

“The reef was born on the eastern coast of the continent of Australia during the Miocene epoch. Its first 24.99 million years were seemingly happy ones, marked by overall growth.”

He goes on to describe health complications related to the planetary takeover by Homo sapiens, the evolutionary event initiating the geologic hour that things began to turn sour for coral reefs.

Bleached coral on the Great Barrier Reef | Photo: Oregon State University, some rights reserved
Bleached coral on the Great Barrier Reef | Photo: Oregon State University, some rights reserved

The Great Barrier Reef isn’t truly dead. The death declaration was something of a media stunt, but it was concocted as a sort of warning. While most of the Great Barrier is still beautiful and lush as coral can be, the reef is not well, with great expanses of it looking as grim, gray and abandoned as a war-torn Balkan city. Most coral reefs around the world, in fact, are suffering. Fishing gear – including but not limited to trawl nets, cyanide and dynamite – causes avoidable yet persistent damage to coral. So do boat anchors, clumsy divers and invasive species. So do pollutants and silt that run off from the mainland. Overfishing of fish species that eat seaweeds is also a problem for coral, since seaweeds compete with coral for resources and can overwhelm a reef if grazing fishes, like parrotfishes, are eliminated or reduced in number.

But the anthropogenic acceleration of carbon dioxide emissions is certainly the greatest universal threat to coral reefs – and the “long illness” Jacobsen referred to in his Outside faux-bituary. CO2 is causing the atmosphere and ocean to warm. The zooxanthellae that live symbiotically with coral polyps do best at temperatures between 73 and 84 degrees Fahrenheit. When coral reefs are exposed to temperatures beyond this range – whether warmer or colder – the essential zooxanthellae-polyp energy exchange is chemically interrupted. The polyps become stressed and eject the zooxanthellae from their own tissue, and the reef turns white – a spooky phenomenon known as “coral bleaching.” Bleaching is not immediately fatal to the coral, and while reefs can recover from bleaching, increasingly they don’t. CO2 emissions also result in higher carbon dioxide content in the ocean. This creates higher levels of carbonic acid, which in turn inhibits coral polyps’ ability to build the calcium carbonate structures that ultimately form coral reefs. If a coral reef’s rate of growth slows enough, erosive forces may outpace regeneration and, in effect, place a clear lifespan and expiration date on the reef.  

A reef in the Maldives | Photo: Tchami, some rights reserved
A reef in the Maldives | Photo: Tchami, some rights reserved

Fortunately, many, many coral reefs are still thriving and remain hubs of sea life and color. They continue to draw tourists and to support, and protect, coastal communities. However, there is no doubt business as usual in the Anthropocene is killing coral reefs, and for the Charles Darwins of the future, there may be a great oceanographic mystery to solve – why corals of the 21st century died almost all at once, and why humans of the time didn’t save them.    

Banner: coral polyps up close. Photo: Nazir Amin, some rights reserved

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