The Arapaima is one of the largest freshwater fish in the world. It can grow to be 10 feet long and weigh 400 pounds. It's not just big. It's also really a unique fish. One reason is because this fish is an air-breather. In the Amazon River, where the Arapaima lives, oxygen levels are often low. But the Arapaima can use a special organ called the labyrinth organ to breathe air and survive in waters with very low oxygen levels.