A diver observes the marine wildlife of a vibrant coral reef.
A diver observes the marine wildlife of a vibrant coral reef. (Photo: Rich Carey/Shutterstock)

Although they only take up a tiny fraction of space within the planet's oceans (about 0.1 percent), coral reefs are some of the most diverse ecosystems. They are home to about 25 percent of all marine species, including fish, crustaceans, mollusks, marine worms and, of course, the cnidarians that are responsible for building the reef's stony coral infrastructure. These intriguing cnidarians create these expansive reefs by settling upon seabeds and secreting calcium carbonate to serve as exoskeletons for their otherwise fragile bodies.

There are three main types of coral reefs — barriers, fringes and atolls. Barrier reefs, which tend to be the largest type of reef, are separated from the mainland by deep lagoons or channels. Fringe reefs are the most common type of reef in the world, and they are usually directly connected to the shore. Atoll reefs are ring-shaped coral formations that encircle a lagoon or sand cay.

Despite their status as vital hotspots for marine biodiversity, coral ecosystems around the world are dying due to a variety of man-made threats — most notably the rise of ocean temperatures and acidification caused by global climate change. As a result of these drastic environmental shifts, scientists have been noticing a troubling increase in mass coral bleaching events, in which the polyps turn a ghostly white:

The tragic beauty of bleached coral.
This is what bleached coral looks like. (Photo: Ethan Daniels/Shutterstock)

One of the most visceral ways to contextualize this pressing ecological issue is to compare images of healthy coral and bleached coral. In the image below, we see just how drastic the changes are to the hues of staghorn and table coral as they undergo a bleaching event:

A sobering comparison between healthy coral and bleached coral.
Healthy coral versus bleached coral (Photo: Ethan Daniels/Shutterstock)

So what's happening to these corals when they start to bleach? NOAA explains:

Coral colonies are made up of hundreds or even thousands of genetically identical individuals called polyps. These polyps have microscopic algae called zooxanthellae living within their tissues. The zooxanthellae work like an internal symbiotic vegetable garden, carrying out photosynthesis and providing nutrients which help reef-building corals create reef structures. When a coral bleaches, it loses its zooxanthellae, and will die within a matter of weeks unless the zooxanthellae can be replaced. The term bleaching is used because the dazzling colors of living corals are due to the colors of zooxanthellae in coral tissue, and when zooxanthellae are lost, corals appear white, or "bleached."

Because the relationship between the zooxanthellae and coral is symbiotic, it becomes difficult for the coral to survive once the algae have been expelled.

A boulder coral undergoing a bleaching event in Raja Ampat, Indonesia.
A boulder coral undergoing a bleaching event in Raja Ampat, Indonesia. (Photo: Ethan Daniels/Shutterstock)

Scientists believe the expulsion of the zooxanthellae is caused by any number of environmental stresses that interfere with the coral's ability to provide photosynthetic nutrients to the algae. This can include drastic changes in water temperature, ocean acidification caused by CO2 pollution, sea level changes, salinity fluctuations, bacterial infections and even the pollution of non-biodegradable ingredients of some sunscreens.

To better understand this mechanism, researchers at the Queensland University of Technology in Australia recently recorded a fascinating time-lapse video that shows just how remarkably violent one of these bleaching events can be. To make the video (below), they placed a coral specimen in an aquarium, which was gradually warmed up to facilitate the expulsion.

In many ways, the bleaching of corals is akin to dead canaries in a coal mine — an ominous warning of what's to come if appropriate actions aren't taken to address the issue of climate change.

To get a better sense of what's at stake for these important habitats and the creatures that depend on them, here's a brief look at five impressive coral ecosystems that are worth fighting for.

1. Great Barrier Reef

Great Barrier Reef of Australia
Great Barrier Reef of Australia (Photo: JC Photo/Shutterstock)

2016 coral bleaching map of Great Barrier Reef

Stretching 1,400 miles off the coast of Queensland, Australia, the Great Barrier Reef is the world's largest coral reef system. It can even be seen from outer space.

Despite being a UNESCO World Heritage Site, and one of seven natural wonders of the world, more than half of the reef's stony coral cover has died in the past 30 years. Bleaching is an ongoing threat, but some years are worse than others — and 2016 has been especially bad, as seen in this map by the ARC Centre of Excellence For Coral Reef Studies. Northern regions were hit hardest, losing an average 67 percent of shallow-water corals in just nine months. The 2016 bleaching has surpassed even the brutal bleaching of 1998 and 2002, and marks "the most severe mass bleaching event on record," according to a report by the Australian Institute of Marine Science, which cites record-breaking ocean temperatures.

In addition to mass coral bleaching events, the reef is also threatened by runoff pollution and an increasing population of crown-of-thorn sea stars, which feast on coral polyps.

2. Red Sea's coral reefs

A snorkeler swimming amongst the Red Sea's coral reef ecosystem.
A snorkeler swimming amongst the Red Sea's coral reef ecosystem. (Photo: John_Walker/Shutterstock)

Sandwiched between Africa and Asia, the Red Sea possess a rich, storied history and lays claim to being one of the saltiest bodies of water in the world. But did you know it's also home to 1,240 miles of fringing coral reef? These reefs, which are composed largely of acropora and porites polyps, are estimated to be between 5,000 and 7,000 years old.

3. Belize Barrier Reef

Belize Barrier Reef
Belize Barrier Reef (Photo: Wollertz/Shutterstock)

The Belize Barrier Reef is just one chunk of the larger Mesoamerican Barrier Reef System, which stretches from the tip of Mexico's Yucatan peninsula to the shores of Nicaragua.

Designated as a World Heritage Site in 1996, Belize's 190-mile section of reef is the country's top tourist destination, which is why the continued survival of the reef is not only important for marine wildlife but also the livelihood of the local economy. One of the most striking features within this reef is the Great Blue Hole, a 407-foot-deep submarine sinkhole that's a popular destination for free divers and scuba divers.

4. Indonesia's Raja Ampat Islands

Coral reefs thriving within a lagoon in Raja Ampat, Indonesia.
Coral reefs thriving within a lagoon in Raja Ampat, Indonesia. (Photo: Ethan Daniels/Shutterstock)

Indonesia is home to about 17 percent of all the planet's coral reef ecosystems, including ones found within the beautiful Raja Ampat Islands (above). Despite the sheer volume, Indonesian reefs are among the most threatened and woefully unprotected. In addition to illegal blast fishing and cyanide fishing, the country is a leader in coral exports — about 500 tons of coral are harvested and sold every year.

5. The Maldives

Coral reefs surrounding an island in the Maldives.
Coral reefs surrounding an island in the Maldives. (Photo: ArtTomCat/Shutterstock)

Famous for its minimalist white-sand beaches and luxurious overwater bungalows, the Maldives is one of the most sigh-worthy countries in the world. It's also one of the most fragile. Comprised of 26 atolls that boast a diverse array of marine ecosystems, this scenic archipelago country is under frequent siege by the effects of climate change. In 1998, two-thirds of the Maldives' coral reefs were killed by that year's El Niño, which caused a staggering 9 degree Fahrenheit rise in ocean temperatures.

With the effects of climate change growing more evident each year, you have to wonder — what's next for the lowest lying country in the world and its all-important reefs?