Natural History

Bedrock of the Reef

elkhorncoralBedrock of the Reef

Hard corals are the underwater realm’s essential, yet vulnerable building blocks.

Story by Suzanne Gerber ~ Photos by Barbara Shively

It’s not exactly like missing the forest for the trees, but let’s just say that the novice snorkeller or diver pays much more attention to the passing fish and other marine life than their stationary habitats. But once your mind — and eye — gets trained to focus on the exquisite coral reefs and learn how vital they are to that underwater life, to the ocean, and even to humans, you’ll never ignore them again.

Coral reefs have been called the tropical rain forests of the underwater realm, and that’s no exaggeration. For one thing, all marine creatures depend on corals for their very existence, either directly or indirectly for food, energy and protection. Corals’ biochemical processes nourish our beaches, and because they’re impervious to erosion, they help maintain the integrity of the shoreline and protect it from storm damage. Finally, because they are home to so many fish, they directly support 75% of all human life on earth (the percent that relies on fish for sustenance) and indirectly sustain life by providing revenue to fishermen and those involved in the diving and snorkelling industries worldwide. That’s just how they function; we haven’t even mentioned how jaw-droppingly gorgeous the hundreds of species of coral are.

And yet across the globe, our coral reefs are in trouble — big trouble. Corals are living animals, not plants, and while they might look sturdy as oaks, they are incredibly fragile. Over the past few decades, their collective health has been declining at an alarming rate. Some scientists conjecture that if we don’t take positive action now to slow, stop or reverse the negative factors impacting corals, we will lose the majority of our life-sustaining coral reefs over the next 50 years. Doomsayers feel we don’t even have that long.

But first, some good news . . . Corals have been on the planet for an incalculably long time, but they weren’t classified as animals until 1753, when a French scientist recognised that despite their plantlike appearance, they actually are attached to the substrate of the ocean and have self-propelled locomotion. Their phylum, cnidaria (pronounced nigh-DARE-ee-uh), also includes anemones, jellyfish and hydroids, which are often mistaken for corals. While there are hundreds of species and many look like complex plants, their structures are actually quite simple: a cup-shaped body with a single opening (for both incoming and outgoing materials) that’s surrounded by tentacles, which both protect corals and help them procur food.

We tropical-water divers are spoiled: we’re surrounded by glorious coral reefs, which most of us take for granted when we engage in our favorite hobby. And yet the conditions that give rise to corals’ existence are both specific and precarious. Water temperatures must consistently be in the 70 to 85º Fahrenheit range, although short periods of higher or lower temperatures can be tolerated. (This is why we only find coral in the “circumtropical” belt, where waters do stay in this range.) Surprisingly, sustained high temps (over 85º for a few weeks) will cause the corals to release the algae that resides within them, which cause the corals to be bleached of their magnificent color. After just a few weeks in this bleached state, the coral will die. But more on this later.

starcoralwithtinyfishCorals also require very specific qualities in their surrounding water: it must be fairly shallow sea (not fresh) water, with the proper amount of salinity and clarity, to allow sunlight through. The base of the ocean (the substrate) must allow the corals to form “roots,” so colonies can form. Any one of these factors goes off, and coral dies.

When conditions are right, corals flourish. They’re remarkably well-designed and well-programmed — as you would hope the elemental building blocks of reef systems are. For starters, they have an ingenious symbiotic relationship with the aforementioned algae, more properly referred to as zooxanthellae (pronounced zoo-zan-THEL-ee, should this come up at your next cocktail party). These zooxanthellae live inside the coral polyp, which is what attaches to the substrate, and when enough individual polyps congregate, which they do naturally, coral colonies — aka reefs — are born. Corals pull calcium from the water to create a hard skeleton, and when they die, the bony exoskeleton builds on what was there, thus expanding the reef.

Of the hundreds of species, which run the palette gamut from soft pastels to deep orange and scarlet and, occasionally, violet, we can divide hard corals into just a few categories: branching and pillar; encrusting, mound and boulder; brain; leaf, plate and sheet; fleshy; and cup and flower. There’s a whole other world of soft corals, also called gorgonians, but that’s a subject for another story. Within one coral reef, there can easily be 50 different species, but it takes years of experience to differentiate them. (Actually, the corals themselves are clear and colorless. It’s the zooxanthellae that contain the hue.) Many seasoned divers still are clueless as to what types of corals are hosting their beloved fish and critters, and that’s a pity.

Another thrilling aspect to coral life is how radically different they look (and function) by day and by night. When the sun is out, polyps are generally retracted for protection, but come nightfall, the underwater world is transformed into a psychedelic wonderland of bright colors, shapes and textures. The exposed polyps in turn attract different critters, and the dive site you visited just a few hours before becomes virtually unrecognizable. No wonder so many divers jump at the chance for a night dive!

And now the bad news . . . Coral reefs are indisputably imperiled. Their fragility makes them vulnerable to both local pressure and global events. Storm damage, overfishing and reckless behavior by everyone from divers to yachties and cruisers (via anchors and sewage-dumping) to local residents (toxic runoff from development as well as pollution) are bad enough, but scientists the world over concur that by far and away, the number one threat to our precious reefs is global warming, with its subsequent raised water temps and UV radiation from ozone depletion. Specifically, marine biologists look to the health of elkhorn and staghorn corals (collectively classified as Acropora) as markers of overall reef vigor and planetary health.

Once the most prevalent in the Caribbean, these species have experienced a decline of 80 to 98% over the past three pillarcoral2decades, and this terrible pattern shows no sign of abating. This hardly comes as a surprise. Many U.S. and international organizations have been aware of this downward turn, and the Center for Biological Diversity has petitioned the National Oceanic and Atmospheric Association (NOAA) Fisheries Office to declare them endangered species, which would force protective legislation — but to no avail. Tragic as this is, the Caribbean is actually in far better shape than much of the wider Pacific and Indian oceans, where coral mortality rates have been in the 90% range in recent years, according to a spokesperson for the Global Coral Reef Alliance, who calls the situation “an underwater holocaust.”

The (moderately) good news is that with awareness, we can insist on action — from our governments. As individuals we can appeal to our politicians to enact and enforce appropriate legislation. We can refuse to participate in activities that lead to reef destruction (thoughtless land-clearing along coastlines, sewage-dumping, pollution, touching or banging into coral when we snorkel or dive). We can organise and insist on local coastal organizations to be responsible for their waste-water treatment and release of toxic materials into the ocean.

On a completely different — and positive — note, corals, like their terrestrial cousins the tropical rain forests, hold promise for human healing. Certain bioactive compounds have been isolated from corals (and sponges, algae, mollusks and more), and in early studies appear to have anti-bacterial and tumor-fighting properties. A few species, which resemble human bone when implanted, may be viable bone substitutes for people suffering from arthritis and other bone and joint diseases. This research is in its infancy, but may prove invaluable in improving the quality of countless lives — not to mention actually saving lives. But first, we have to save theirs.

New York-based Suzanne Gerber writes about scuba, travel and health for a variety of publications. Book your next dive trip by contacting Suzanne at

Avid underwater photographer Barbara Shively discovered Grand Turk diving in 1997 and has returned every year. It is her passion to share the coral reefs’ beauty through her photographs. A variety of her prints are on sale at Art Provo, located in The Regent Village, Providenciales.

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