Corals Are Animals and 6 More Fun Facts You Didn't Know

Did you know that corals are animals? Yes, really! Even though they don’t swim or scuttle like a lot of the ocean’s inhabitants, coral is indeed a part of the kingdom Animalia and closely related to other invertebrates like jellies and sea anemones. Now, with the opening of our 5,000-gallon Healthy Corals, Healthy Reefs exhibit, you can learn more than ever about these fascinating animals at the New England Aquarium—and watch the exhibit evolve as the reef slowly grows! Let’s dive into some other fun facts about these incredible organisms.

1. Corals are both tiny and massive

Coral reefs, the largest structure of biological origin on Earth, are made up of tiny individual animals called coral polyps, most of which are only a few millimeters wide. These polyps grow into colonies as they build their hard calcium-based skeletons next to each other. Over many years, these colonies of coral polyps create reef structures that can weigh up to several tons! 

2. Each polyp has a mouth and stomach

You might not be able to see it, but a coral polyp has a mouth surrounded by tiny tentacles that it uses to pull in food. These tentacles are equipped with a special type of stinging cell called nematocysts that help corals capture their prey and defend against predators. If you’ve ever been stung by a jellyfish, you’ve experienced nematocysts in action.

Corals’ diet can vary from near-microscopic zooplankton to small fish, depending on the size of the coral. Once in the polyp’s stomach, which takes up most of the polyp, food is digested for nutrients, and waste is expelled. The catch? A polyp’s mouth is the animal’s only opening. Therefore, waste must be expelled back through the mouth!

3. Algae living below the tissue of coral causes its bright coloration

Coral is well known for its bright, colorful appearance. Reefs often resemble underwater gardens, boasting a wide variety of shapes and shades. What many don’t know is that this color is caused by a special type of algae. These algae, zooxanthellae, live below the tissue (similar to skin) of a polyp and share a symbiotic relationship with coral. The zooxanthellae are provided a safe place to live and a source of carbon dioxide, which they use for photosynthesis, the process of using sunlight to produce food molecules. In exchange, zooxanthellae provide the coral with a large amount of the nutrients it produces through photosynthesis. Zooxanthellae provide up to 90 percent of the polyp’s nutrients!

4. Coral reefs are the cities of the sea

Coral reefs aren’t just important for their own sake. Though coral reefs cover less than one percent of the ocean floor, they are home to about a quarter of all marine life and are essential to the survival of many species. They are like vibrant underwater cities, with nurseries for baby fish, resting spots for turtles, and varied habitats for species from lobsters to sharks. Reef residents take care of their neighborhood in return. Sea urchins feed on algae that would otherwise smother the coral, and sea sponges help filter toxins in the water around the reef. Millions of humans all over the world also rely on reefs for food and jobs. 

The Healthy Corals, Healthy Reefs exhibit showcases a variety of the species that rely on healthy coral reefs in the Indo-Pacific region, one of the most diverse ecosystems in the world, including angelfish, butterfly fish, wrasses, surgeonfish, and reef-dwelling invertebrates.

5. There are over 6,000 species of coral

When you are observing the coral in the Aquarium’s exhibits, take note of the wide variety of shapes and sizes you see. This astounding variation only represents some of the 6,000 different species of coral in the world. Species generally fall into two main categories: hard corals, like those that build reefs, and soft coral. Though soft corals do not form reefs, they are part of many reef ecosystems, and like hard corals, they build larger structures from individual polyps. Soft corals have flexible skeletons that allow them to bend with ocean currents and often resemble small trees, bushes, fans, and grasses. Reef-building hard coral species exhibit a wide range of shapes, too, including table-like structures, branches, boulders, and mushroom tops. 

6. We all have the power to help coral

Unfortunately, coral populations have declined precipitously over the past century. It is estimated that global coverage of living corals has been cut in half in that time. Climate change has played a major part in this. When corals become stressed by water temperature increases or nutrient changes, they may expel their zooxanthellae, causing the corals to turn white. This “bleaching” is a sign of an unhealthy reef and further harms the coral by depriving it of the beneficial symbiosis. Coral can recover from bleaching if the environment goes back to normal and algae returns, but if coral remains bleached for an extended period of time, they can’t be revived. When reefs are unhealthy, there is a ripple effect across the entire ecosystem, including the animals that rely on coral losing their shelter or food source.

The good news is we can all play a role in helping protect the coral reefs of the world! First and foremost, if you are observing a reef in the wild while swimming or boating, be mindful not to touch or bump any coral. Additionally, when you head down to the beach, make sure you are using a reef-safe sunscreen, which is free of chemicals that can get into the ocean and harm coral. At home, you can take actions like eating responsibly fished seafood, as overfishing can contribute to the destabilization of a reef’s ecosystem.

Think coral could be one of your new favorite animals? Visit the Aquarium to learn more about coral and their essential place in the ecosystem.