How Coral Bleaches

Healthy Coral

Above you can see a picture of healthy coral that has not been bleached. This picture was taken at Gab Gab Beach on Big Navy Base Guam, on April 21st, 2017.

Now then, last time I posted I talked about how coral is a living organism comprised of smaller organisms. These smaller organisms are called Polyps, and they are tiny animals. This is why you should never touch or stand on coral, because it crushes the tiny animals that it is made of.

This time, I want to talk about how coral bleaches.

First, remember that a coral is like an apartment complex. The building is made of calcium secreted by the coral, and the people who live inside are called polyps.

This is what a polyp looks like.

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As we discussed last time, a coral polyp can eat food from the water that it catches with its tentacles.

However, it is like a Venus Fly Trap: Even though a Venus Fly Trap eats flies, it still needs to absorb sunlight so that it’s chloroplasts can metabolize that light into food for the plant. Without sunlight, it would die.

The specialized chloroplast cells inside of a coral polyp are called zooxanthellae. Even though the coral is perfectly capable of catching food from the water with its tentacles, each polyp still needs its zooxanthellae to metabolize sunlight and produce a reliable food source for the polyp.

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The zooxanthellae are actually all different colors, and they are what gives coral its color. The polyps themselves are clear.

The problem is that the zooxanthellae are sensitive to temperature. If it gets too hot, the zooxanthellae will die. When they die, the polyp expels them into the water, and thus, it loses its color.

This is why coral that is bleached is not yet considered “dead.” The polyp is still alive, but it has lost its zooxanthellae.

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Rare cases have been reported where a coral bleached, but then the polyps were able to catch new algae cells and survive. However, the unfortunate truth is that once a coral bleaches, the polyps are very likely to starve to death and die.

Once the polyps die, the calcium skeleton is all that is left. This skeleton then becomes host to parasitic lifeforms that eventually break it down.

I moved to Guam is August of 2015. In the time that I have been here, I have seen the coral in Tumon Bay bleach and die, as well as the coral inside the reef at Tanguissen Beach.

Gab Gab August 22nd, 2017

However, those are shallow reefs. I understand how they got too hot and their coral began to bleach.

On the other hand, Gab Gab Reef on the Big Navy Base is much deeper. Most of the coral sits at around 20 feet deep, depending on the tide.

And rather than being in an enclosed reef the way the coral in Tumon Bay and Tanguissen is, the coral at Gab Gab is in a very deep harbor.

Gab Gab August 22nd, 2017

I never imagined that the coral at Gab Gab would bleach.

At least, not before I left here in September of 2018. And yet, the images that you are seeing now are from Gab Gab, and they were taken just a few months after the picture of healthy coral at the top of this post.

This is what bleached coral looks like. This is what a reef looks like when it is dying.

Gab Gab August 22nd, 2017

The reef at Gab Gab was truly amazing. It was filled with an incredible diversity of life, and a wide variety of corals. And now, before my eyes, it is dying.

I can’t really tell you the emotional toll of watching corals die.

I know that these are structures that took generations of polyps to build. Many are hundreds of years old. Gab Gab has so many huge, ancient corals that are each home to hundreds of fish.

Gab Gab August 22nd, 2017

It is an environment that took nature so many lifetimes to create. And yet, it is dying in the span of one Navy Man’s tour of duty on Guam. (My husband Rich.)

By the time the military sends my husband to a new port, I will have witnessed the loss of a huge percentage of the coral reefs on Guam. Just four years… and yet, I am seeing so much death.

There is no eulogy solemn enough or beautiful enough to pay tribute to the coral reefs of Guam. They were some of the most amazing things that I have ever seen, and I wish that I could express to you what a huge loss this is to every single one of us.

I am actually having trouble typing this because of the tears. And, if you were watching our oceans dry each day in real time, I have no doubt that you would feel the same.

There is no data yet on how big the 2017 world bleaching event will be. However, I would like to submit these photos to the world and to NOAA as evidence of the death in my small corner of the ocean.

Coral is Alive

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I don’t usually use this blog to educate people about science, but I am afraid that I feel it is necessary. So let’s take a moment to learn about coral, and what it is made out of.

Coral comprised of thousands of tiny animals called polyps.

Polyps are too small to see with the naked eye, but under a microscope you can see that a piece of coral is not a single entity. Rather; it is like an apartment complex.

A cooperative colony of polyps that all live together comprises what we think of as “a coral.”

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Inside the coral polyps are something called zooxanthellae.

Zooxanthellae are specialized chloroplasts that are able to take sunlight and metabolize it into food for the coral, the same way a chloroplast in vegetation absorbs sunlight and metabolizes it into food for its its plant host.

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Some corals also have tentacles that they stick out in order to try to catch bits of food that go by. However, all coral are dependent on their zooxanthellae for long-term survival.

You can think of a carnivorous plant like a Venus fly trap: it eats flies, but it will still die without the nutrients its chloroplasts make from sunlight.

This is why you must never touch coral. When you touch it or step on it, you are smashing hundreds of tiny polyps, and this does permanent damage to a coral. In fact, certain kinds of sunscreen are poisonous to coral, so you may even kill the entire colony just by being clumsy.

The lesson? Coral is a living organism made of many smaller living organism, and it’s not okay to touch it.

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