This odd-looking creature is a fish called a puffer fish. Like all fish, puffer fish are vertebrates. The puffer fish puffs up by rapidly filling its stretchy stomach with water. The reason? Getting bigger makes the fish seem larger to potential predators. Just to be on the safe side, the puffer fish also produces venom. In fact, it’s the second-most-poisonous vertebrate in the world after the poison dart frog. In this chapter, you’ll learn more about vertebrates such as these. You’ll find out that vertebrates are a diverse and fascinating group of animals.