Blowfish
Blowfish are known as the porcupines of the sea, meaning the same defense mechanism employed by the land creatures is used in the ocean. The blowfish inflates its entire body to look like its more prominent than it actually is. Once it feels like it is under attack, it blows up in hopes of frightening its predators.
Inflation is accomplished by expanding the elastic stomach. The blowfish swallows a large amount of water in just a few seconds. Some species have even evolved to include spines as a way to look more intimidating to enemies. Another defense mechanism is emitting a substance called tetrodotoxin, which is lethal to several kinds of fish and even humans.