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10 Ways Corals Are Really Strange Animals (They Argue)


Corals resemble underwater trees. They just stand there. They do nothing. But if that were true, then this would be a very short list. The truth is much more entertaining. Corals argue and smooch. They grow taller than buildings and follow fish. They even beat up bullies with bodyguards.

Corals also present us with unexpected dangers and mysteries. From halos around reefs to ER evacuations, here are the strangest things that corals do.

10. Corals Make Their Own Sunlight

Corals need sunlight to feed. Since they cannot turn sunlight into sustenance themselves, corals snack on the nutrients that are provided by the sun-loving algae living with them. This logic fell apart with deep-sea corals.

Some nest in places so far down that sunlight cannot reach them. This seemingly nutrient-less existence was not the only mystery. Corals in shallow water fluoresce to protect their algae food factories from the Sun’s radiation. Deep-sea corals also glow. But why? They do not have to deal with any radiation.

In 2017, both mysteries were solved by a coral reef in a laboratory. The reef was studied under authentic deep-sea conditions and revealed that deep corals glow for a different reason than their shallow-water cousins. They become their own Sun. The light allows their algae to photosynthesize and produce the nutrients that feed the corals.

9. Coral Reefs Have Halos

A mystery once surrounded coral reefs (literally). Large circles appeared around reefs. Sometimes, the clusters around a single reef covered hundreds of thousands of square feet, making them visible from space. But when scientists looked at them up close, they found that each circle was a barren patch on the seafloor. Nobody could explain why they hung around coral reefs nor why some clusters grew bigger than others.

In 2019, the mystery was solved. Regrettably, reef halos are not the marine version of crop circles. The answer is more mundane. Fish pick the areas clean of plants and prey until nothing but sand remains. For some reason, they create a circle as they forage.

Paranormal fans might’ve been disappointed but conservationists hailed the discovery as a great way to check the health of a coral reef. You know, fish-wise.

Where predator fish are missing due to overfishing, larger clusters of halos form. The fish they prey upon has less to fear and venture longer distances away from the reef, creating circles as they feed. But where carnivores are present in proper numbers, fish are more cautions and create smaller circles closer to the reef.

8. Corals Might Live For Thousands Of Years

Scientists are unified when it comes to coral conservation. But ask them, “Hey, how long do corals live?” Then the gloves come off. Some researchers insist that certain deep-sea corals croak at 70 years old. Other studies say that the same species can lord over the seas for more than 4,000 years. If corals live for millennia, it will earn them a place among the longest-lived creatures on Earth - and certainly the longest in the ocean.

But dating corals is tricky. Most scientists rely on radiocarbon dating to tell them the age of something. The problem? The method only works when everyone agrees where the fossil or sample got its carbon from. This is where things get Fight Club. The two main studies about coral ageing cannot see eye-to-eye about where corals get their carbon from. Or even how carbon reveals their age.

One study counted the rings in coral structures, working on the same principle that rings can tell the age of trees. It came to the conclusion that corals keel over at 70 and that the other study’s “older” corals were false readings due to the creatures eating ancient carbon in their environment. The other study tested carbon inside corals as well as the carbon in their environment and this gave them the 4,265-year estimate.

Confusing? That’s probably why the true longevity of corals remains a mystery.

7. They Love Fish Pee

Here’s another strange link between corals and large predator fish. Where there are plenty of barracuda (and other toothy meat-eaters), corals tend to flourish. Why? Because big fish come with big bladders - and fish pee is packed with nutrients. More specifically, it’s a phosphorus bonanza. Fish also release nitrogen when they “breathe” through their gills. For corals, this combination of phosphorus and nitrogen is heaven-send because it keeps them healthy.

Scientists wanted to know two things. How important was fish pee to reef growth? Which fish produced the most nutrients? To find out, they embarked on a peculiar study. For four years, researchers caught different species and waited for 30 minutes while the fish did their business inside a plastic bag (it’s not clear whether the scientists bagged a barracuda but kudos if they did). The “before” and “after” pee levels were measured to see which fish dropped the biggest cloud of phosphorus.

The numbers showed something remarkable. Fish provide almost half the nutrients a reef needs to keep its ecosystem alive. It also confirmed what scientists had already suspected. That the bigger the fish, the greater the volume of its bathroom moments. That’s why coral reefs with plenty of large predators are also better “fed” and healthier.

A toothy barracuda.

A toothy barracuda.

6. Coral Babies Follow Noise

Corals are not good parents. When Junior is born, it joins a swarm of other larvae and they are expected to find their own way in the world. The larvae use ocean currents to take them to new places but they do not settle down willy-nilly. They look for real estate with the right temperature, light conditions, and chemicals.

In 2018, it was discovered that the larvae also search for noisy places. This does not mean that they scoot after ship propellors. The creatures make a bee-line for reefs that rumble with the sounds of plenty of fish (they are probably after the pee). How coral larvae detect these fishy frequencies remains unknown.

5. There Is Smooching And Fighting Too

In 2016, the first microscope camera was left on the seafloor inside the Gulf of Eilat in Israel. The instrument was designed to work on its own. More specifically, to watch what corals get up to when nobody’s looking. When scientists looked at the footage, there was a pleasant surprise. The camera had captured two events that had never been seen before.

Corals kiss. They also appear to have turf wars.

Coral organisms are called polyps. These tiny creatures create the tree-like structures and reefs that most people refer to as corals (one can say that polyps are the true living corals). The romance caught in 2016 remains a mysterious thing. Not true kissing (of course), the polyps would lean towards each other with their tentacles, embrace, and smush together.

Nobody knows why they do this. But since polyps smooch after they’ve caught plankton, they are probably sharing their lunch. The so-called “turf wars” occurred when researchers placed different species of corals together. Those from the same group ignored each other but kept poking at other species with their tentacles.

Coral polyps (seen here in light blue).

Coral polyps (seen here in light blue).

4. Coral Zombies Are Real

In 2003, a heatwave killed a quarter of the coral population of Spain's Columbretes Islands. It was the same tragic story that constantly hits the headlines. Climate change had destroyed yet another coral garden. In 2019, biologists visited the dead colonies and discovered something unexpected. Like zombies worth their salt, the corals had risen from the grave.

Logic suggested that the corals were new ones. But a closer look revealed a remarkable survival strategy. It did not save all the animals but the 38 percent that revived after the devastating heatwave was an event never seen before - or even thought to be possible.

Once the heat became too much, the polyps semi-abandoned their home (those tree things), shrunk and knuckled down as best they could. For all the world, the corals appeared to be dead. But those who survived slowly grew back over the years and build new homes.

3. Corals Have Bodyguards On Standby

When the ecosystem of a reef weakens, seaweed takes over. This is bad news for corals. Turtle weed, in particular, is a problem. The moment it touches coral, the weed releases a toxin that causes bleaching. When you are a tiny polyp facing certain death, the best recourse is to pick up your phone and punch in Dial-A-Bodyguard.

Goby fish stay with the same corals their entire lives. They also eat turtle weed. In 2012, scientists wondered why gobies only eat seaweed after the plants had wrapped themselves around a coral tree. The study revealed a remarkable relationship between the fish and polyps. Within 15 minutes of being zapped by turtle weed, corals release chemicals that attract gobies. The fish arrive within 15 minutes (or under) and shred the seaweed.

It appears to be a defence unique to corals. No other reef organism lures a different species closer to deal with an enemy on their behalf.

2. They Grow Taller Than The Empire State Building

One of the most studied places in the ocean is the Great Barrier Reef of Australia. For this reason, a person might be forgiven for no longer expecting any new discoveries. You know, of the giant kind. But in 2020, underwater vehicles found an unknown part of the Reef. Incredibly, it was a coral tower taller than the Empire State Building.

The blade-like structure was over 1,640 feet (500 meters) tall. How did it elude researchers for so long? The coral tower is a “detached reef.” These corals are a part of the Great Barrier Reef system but they stand apart from the main reef. Standing apart, perhaps in a well-hidden place, could have caused it to go unnoticed. Whatever the reason, the ginormous spike is the first of these free-standing reefs to be discovered in 120 years.

1. Coral Almost Wiped Out A Family

The Stevenson family wanted to add live corals to their aquarium. So they purchased an ornamental coral bridge for £50. They had happy thoughts about their spruced-up aquarium and at one point, the couple and their four children went on holiday. When they returned, all the fish were dead.

That might have been a warning…

Due to the mess, the tank had to be cleaned. The family was scrubbing the aquarium when the mother fell ill. Soon, everyone except one of the children was in the emergency ward. Their symptoms were so severe that the entire ward was emptied of other patients and the staff kept the family awake out of fear that they would fall asleep and die.

Frighteningly, they had almost been killed by the coral bridge. It was purchased from a retailer who gave them no warning about what the ornament was capable of. As a result, the Stevenson family took home a coral species that releases palytoxin when it feels threatened (the coral was probably harassed by the fish and disliked being scrubbed by the family). Palytoxin is the second-deadliest toxin in the world.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Jana Louise Smit


Jana Louise Smit (author) from South Africa on December 19, 2020:

Hi Peggy, thanks for the great comment! Corals are indeed unusual and interesting creatures. I'm happy that you liked these odd facts about them. Enjoy your day. :)

Peggy Woods from Houston, Texas on December 18, 2020:

Your article about corals is fascinating! I learned much that I never knew by reading it. Thanks for the education.

Jana Louise Smit (author) from South Africa on December 18, 2020:

Hi Alnajda. Me neither but as I started to research them, it became clear how unusual coral reefs really are. :) Enjoy your day!

Jana Louise Smit (author) from South Africa on December 18, 2020:

Thank you, Bill. They are truly strange and wonderful animals. I'm glad you enjoyed the article. :)

Alnajda Kadi from Tirana Albania on December 16, 2020:

Oh very interesting. I didnt know much about it. :))

Bill De Giulio from Massachusetts on December 16, 2020:

Very interesting. I had no idea that coral leads such a fascinating life. There is certainly much more going on in coral reefs than meets the eye. Thanks for the education.

Sp Greaney from Ireland on December 15, 2020:

I don't know much about coral but the points you made here were very interesting. It seems to be a very unique thing.

I also think it is one of those things that you hear things about but only when there are new discoveries. But there seems to a lot more to it than one would things.