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Constraining . Coral Carnage by Sarah Specker

Despite their inanimate appearance, corals are actually living organisms, biologically classified as marine invertebrates. They are often found living in compact colonies of many identical individual polyps. With its sac-like cylindrical body, usually attached to a firm structure on one end, and a central mouth opening encircled by a crown of tentacles on the other, a polyp looks a lot like a vase held by gloved fingers. The tentacles are covered with stinging nettle cells (nematocysts) that pierce, paralyze and poison potential prey, like copepods or fish larvae. The muscle fibers inside the tentacles allow for transportation of this captured food straight into the body where digestive enzymes do the rest of the work: similar to hands putting food straight into your small intestine. 

Surprisingly, coral reefs occupy less than 1 percent of the ocean floor, yet they provide a home for more than 25 percent of all marine species. Because of this enormous biodiversity as well as their ability to perform photosynthesis, coral reefs have earned themselves the nickname “rainforests of the sea.” But, unfortunately, coral reefs are threatened by multiple factors, including pollution, introduced invasive species (e.g. lionfish in the Atlantic Ocean/Caribbean Sea), overfishing, destructive fishing, oceanic acidification, coastal development, diseases and, of course, global warming. Too much stress makes coral expel its own zooxanthellae in an attempt to alleviate the pressure. Unfortunately, this means that coral robs itself of its main nutrient provider; and if the algae don't return, the coral will permanently lose its colour and starve to death—a process called coral bleaching. 

To fortify and protect its delicate body, some polyps build their own limestone skeleton using calcium and carbonate absorbed from the surrounding water. This hard outer reinforcement, called a calicle, shapes the outside of a single polyp. By connecting to one another, these polyp-calicles first create small colonies that function as a single organism. Through growth and fusion with other colonies, these structures eventually form complete coral reefs.

In the last three decades, several global bleaching events have happened, with the longest one ever recorded occurring between 2014 and 2017, killing coral reefs globally on an unprecedented scale. The culprit? A big increase in sea surface temperatures, attributed to global warming. A rise as “little” as 1 degree Celsius—if lasting—is enough to damage coral irreparably.

But no matter how self-sufficient these colonies are, they would be nowhere if it weren’t for their plantlike partner. Zooxanthellae—single-celled algae that live symbiotically inside the polyp—provide the corals with the majority of their organic carbon nutrients (up to 90 percent!) needed for metabolism, growth and reproduction through photosynthesis. Plus, they give the corals' calicles a nice colour too.

The ability to withstand these stressors and recover from bleaching depends on numerous factors, such as the type of coral, duration of thermal stress, type of zooxanthellae, and health of the coral, to name just a few. Unfortunately, the impacts seem to be too many, too big and too long-lasting nowadays, meaning even the most resilient of reefs are taking a big blow.

A polyp’s simple body structure hasn't changed in more than a half a billion years, making it a living fossil; yet, this unelaborate body shape allows for an unimaginable diversity, proven by the amount of different corals present in our oceans.

The rapid worldwide damage and disappearance of coral reefs—our oceans’ lungs—basically means that the oceans are choking. And scientists expect the worst is yet to come … . Thankfully, our understanding of coral characteristics | 21

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