Like all animals, corals need to eat food. They use their tentacles to catch tiny floating animals called zooplankton and then bring them to their mouths to eat. Since they only feed at night, you won’t see their polyps during the day. But at night, feeding makes their colony look fuzzy.

Corals live in tropical areas, but there aren’t many nutrients in tropical waters, which are kind of like deserts. That means there isn’t a lot of zooplankton to eat. And yet, corals grow really big. How is this possible, you ask? They have a trick—special algae called Zooxanthellae that live inside their cells. Zooxanthellae are plants that use photosynthesis to make food, and corals take this food and use it to grow. In return corals give the Zooxanthellae a safe place to live. This symbiotic, or close, relationship helps both of them, so it’s mutualistic.