Why does a blowfish puff up

Elizabeth Brainerd: What they do is they actually take water into their mouths in a big mouthful of water, and then they pump it down into their stomach. Brainerd: And they do that anywhere 10 or 15 times, pump, pump, pump, pump, pump, pump, until they inflate completely, and then they hold it and they'll just be a big, spiny ball.

Narrator: And as you might expect, this requires some pretty sophisticated biology, starting with the stomach. It's made of dozens of tiny folds, kind of like an accordion.

These folds are important because when the stomach fills up with water, it can expand without rupturing. And puffer fish expand a lot. Up to three times their size. That's like if an average human man could inflate his waist to a circumference of 3 meters. But there is a drawback to these amazing skills. Brainerd suspects that puffer fish stomachs have actually lost the ability to digest food, which means their intestines have to do all the work. Brainerd: You know, given the apparent importance of this defense mechanism, they've given up the advantages of having a stomach where some digestion can start.

Narrator: But the stomach? It's just one of many bizarre features inside a puffer. For example, they have specialized muscles that you won't find in most other fish.

Some in their mouth, which pump all that water into their stomach; some in their esophagus, to seal off their stomach like a drain plug once it's full; and some in the base of their bellies, which contract to squeeze out water when they're ready to deflate. But what you won't find inside is even more bizarre. Brainerd: There are a couple characters that are really helpful in their ability to puff up, and one of those is that they don't have any ribs, and another one is they don't have any pelvis.

Amazing Animals. Weird But True! Party Animals. Try This! Explore More. Common Name: Pufferfish. Scientific Name: Tetraodontidae. Type: Fish. Diet: Carnivore. Group Name: School. In the movie "Finding Nemo," the pufferfish Bloat inflates in an instant and awkwardly floats away like a beach ball, but it turns out that the fishes' puff has nothing to do with holding in air, the researchers found. The scientists caught eight black-saddled pufferfish Canthigaster valentini in Australia's Great Barrier Reef, and placed them in sealed tanks.

The researchers stimulated the fish by gently suctioning, which caused them to puff up to about four times their normal size. Then, they measured the amount of oxygen in the tank, to check the rate of oxygen consumption — a gauge of whether the fish were holding their breath.

Pufferfish, the researchers learned, can breathe just fine while inflated. Moreover, the fish breathe with their gills when puffed up, which means they can breathe as they always do, even when they inflate to the size of a football.