Today’s Wonder of the Day was inspired by Sofia from , . Sofia Wonders, “Is coral reef plant or animal?” Thanks for WONDERing with us, Sofia!

When you think of paradise, what comes to mind? Although the answer is sure to be different for everyone, many people likely share a vision of paradise that includes warm, sunny days spent on pristine tropical beaches.

If your idea of paradise includes sandy beaches and crystal-clear waters, then you would likely spend hours each day swimming and diving in your lush aquatic surroundings. In addition to playing amongst fishes all the colors of the rainbow, you'd probably also enjoy the sight of long stretches of coral reefs.

As you swim past and examine the corals up close, you might begin to WONDER exactly what they are. They appear to be alive, yet they also look a lot like rocks. The way they're rooted to the ocean floor also makes them resemble plants. So are they animals, plants, or rocks?

Scientists will tell you that corals are indeed invertebrate animals. They belong to the colorful group of animals known as Cnidaria, which also includes jellyfish and sea anemones. These interesting creatures consist of a simple stomach and a single mouth surrounded by stinging tentacles.

Corals cannot make their own food like plants. Instead corals possess tiny arms that look like tentacles. They use these to capture food in the water around them.

Unlike most other animals, corals can't be recognized by their faces or any distinct body parts. In fact, a structure that we would refer to as a piece of coral is usually made up of hundreds or even thousands of tiny coral creatures known as polyps.

Each polyp has a soft body. To survive in its aquatic environment, each polyp extracts calcium from seawater and converts it to a solution of calcium carbonate that it secretes around itself. The solution hardens to form a limestone outer skeleton. As the skeletons of thousands and thousands of polyps attach to each other, they form coral reefs and take on a distinctive, rock-like appearance.

Corals are sessile. That means they attach themselves permanently to the ocean floor. Taking root to the ocean floor isn't the only thing that makes them seem like plants, though.

Corals share a special symbiotic relationship with plant-like algae called zooxanthellae that live within their tissues. The microscopic algae process a coral's metabolic waste to use during photosynthesis. In turn, the algae remove waste and produce oxygen and food that corals need to thrive.

Scientists estimate this relationship between corals and zooxanthellae has existed for over 25 million years. In fact, they believe it's the reason coral reefs are the largest living structures on Earth, rivaling old-growth forests in their longevity.

Wonder What's Next?

Join us in Wonderopolis tomorrow for a look at things on the other side!