Coral 101: Everything You Need To Know About the Ocean’s Living Architects
Published on 8/15/24
Written by Jessica Colla
This blog post is for anyone who wants to learn more about corals and their complex biology that makes them one of the most important organisms in the ocean!
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A vibrant coral reef ecosystem in the south Pacific.
It’s time for a game of twenty questions!
“Are corals … A) rocks, B) plants, or C) animals?”
If you answered C) animals, you are correct! But this question is tricky … and that’s because corals exhibit characteristics of A, B and C! In this guide, we’ll dive deep into the world of corals, uncovering what they REALLY are.
Let’s dive in! (hehe, get it?)
What is a coral?
The poor coral is one of the most underrated and misunderstood organisms in the ocean. As a professional marine biologist, the number one question I get asked about corals is … WHAT ARE THEY?! So, let’s set the record straight: a coral is an ANIMAL.
The Animal:
Despite their sessile (unmoving), benthic (seafloor dwelling) lifestyle and rocky appearance, corals are – in fact – alive! Not only are they alive, but they are some of the most hard-working animals around, constructing massive underwater structures that can even be seen from space! (I’m talking about the Great Barrier Reef – the largest coral reef ecosystem on the planet).
And let’s be honest, they DO look like giant colorful rocks on the sea floor. But that’s because we aren’t looking at them through the right lens … a microscopic lens! Upon closer inspection, you can see hundreds – or hundreds of thousands – of tiny circular animals clustered together in a colony. Each individual is called a polyp and has a ring of tiny tentacles surrounding a small central mouth.
A close-up look at the coral polyps of a branching coral under the microscope. Check out those feeding tentacles!
Corals are in the Cnidaria phylum and are closely related to sea jellies (i.e. – jellyfish) and sea anemones – they even resemble little anemones when you look closely! And they share some of the same traits too. For example, coral tentacles are covered in stinging cells called nematocysts, much like their cousins. When their tentacles extend, they can capture tiny zooplankton (microscopic organisms) floating through the water column. Once the prey is caught and immobilized, the coral uses its tentacles to push the food into its mouth and down into its stomach … I bet you didn’t imagine corals as stone-cold predators, did you!?
P.S. – this is a fascinating process to watch. Check out Maui Ocean Center’s coral feeding video here, it will completely change the way you look at corals!
Although there are some exceptions, most corals are colonial species. This means that individual polyps live in a large group, which make up the “whole coral.” Each coral colony can contain hundreds, thousands, or even millions of tiny polyps! On a single coral structure, each polyp is an identical clone of the one next to it – making them both individual organisms, and part of the whole! (Confusing, I know). Other corals are solitary, meaning they don’t have identical neighbors. These solitary species are some of the largest coral polyps on the planet – like the mushroom coral (one of my favorites!).
A mushroom coral (fungia sp.) on the sea floor. These coral polyps are the larges in the world - up to the the size of a softball!
The Mineral:
Despite their gelatinous exterior, most corals have rigid internal structures! In stony corals, also known as Scleractinian corals, the polyp secretes a rock-hard, white mineral called calcium carbonate (also known as limestone or aragonite) to make a skeleton. Just like in humans, these skeletons give corals their structure, creating a diverse array of shapes and sizes across the reef. Corals can be tall and branching like a tree, they can be wide and flat like a giant table, they can encrust the seafloor like a blanket, or they can simply look like a giant boulder sitting on the seabed. Just like in a city, the coral “buildings” make a beautifully diverse landscape for the animals that live there.
Corals grow in all shapes and sizes!
Corals grow VERY SLOWLY compared to other marine life. Some coral individuals are thousands of years old, growing less than a centimeter each year. Others grow faster and can reach great heights in less than a century. Depositing layer after layer of reef-building mineral, corals grow outward and upward towards the sea surface. Some of the largest coral reef structures have grown so big that they can be seen from space, like the Great Barrier Reef in Australia!
The Plant:
Okay … but what about all those times you’ve heard about corals’ dependence on sunlight for survival? And that they produce their own food?
Those of us paying attention in biology class know that only plants can produce food with sunlight (a process called photosynthesis). While corals DO have tentacles to catch prey, only around 20-25% of the corals’ nutrition comes from food caught with their tentacles. So where does the other 80% come from??
If you’ve been confused about this for a while … good news! The confusion stops here!
Inside the skin of a coral polyp, there are millions of teeny-tiny, single-celled algae (i.e. – plants!) called zooxanthellae. These zooxanthellae algae photosynthesize and use sunshine to create simple sugars for the coral. It’s essentially like having a greenhouse inside of your body! The coral provides the zooxanthellae with shelter, while the zooxanthellae provide the coral with food – this mutually-beneficial alliance is called a symbiotic relationship and it's essential for the survival of most coral species on the planet!
Microscopic "zooxanthellae" - a single-celled algae that live INSIDE the coral animal!
But are you ready for the REALLY cool part? Not only does zooxanthellae provide the coral with food for survival, but it also gives the coral its vibrant color! How, do you ask? Without the zooxanthellae, the gelatinous tissue of the coral polyp would be nearly transparent, just like their jellyfish cousins! But when the coral’s tissue is full of colorful zooxanthellae, they take on the color of their symbionts. This is why we see all of those crazy cool colors on a healthy coral reef. Blue, green, yellow, pink, red, orange, purple … all of those colors come from the humble zooxanthellae algae!
An organism worth saving!
It's clear these underwater architects are anything but simple. They are stealthy predators, capturing food with stinging tentacles. They form powerhouse partnerships with colorful, symbiotic algae. And they are nature’s ultimate architects, building structures visible from space. Corals are full of surprises!
But corals are facing big threats—from climate change to overfishing to habitat destruction. Curious to learn more? Check out our other posts on coral bleaching and coral conservation to see how we can protect these incredible reef builders! Going on a beach vacation soon?? Check out this blog on how to preserve paradise while you’re there!
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