Few things in the ocean are more easily recognizable than a pufferfish having a bad moment. When they’re threatened, the fish rapidly gulp down water into their stretchy, distensible stomachs, puffing themselves up to both appear larger than they normally are and to show off the pointy spines that cover their skin. “The final result is a spiny ball that can be three-to-four-times the resting volume of the fish and not easily ingested,” says Georgia McGee, a marine biology student at James Cook University in Australia.
It’s an impressive defense against predators, but as far as scientists thought for years, it had one big drawback. The fish appeared to hold their breath while inflated, keeping their gill flaps closed and neither bringing oxygen in through them nor passing waste out. A pufferfish, then, could presumably only stay puffed for as long as it could hold its breath, and it had better hope that that’s longer than the attention span of whatever predator was bothering it. To compensate, some scientists thought the fish might absorb oxygen through its skin while in spiky beach ball mode.
After reading about this speculation in other studies, as well as watching Finding Nemo, McGee was inspired to see if these ideas held any water. With Timothy Clark, a physiologist who studies fish respiration at the Australian Institute of Marine Science, McGee captured Black-saddled pufferfish near the Great Barrier Reef and brought them back to the lab. The researchers provoked each fish into inflating while sensors in the puffers’ tanks monitored how much oxygen they took from the water through their gills, and a smaller hand-held sensor was pressed to their bodies to measure oxygen intake through the skin.
They found that right when the fish inflated, they actually took in about four times as much oxygen as when they were deflated. That rate went down as the fish stayed puffed (which they did for anywhere from three to 18 minutes), but they kept taking in oxygen and didn’t hold their breath. The fish didn’t breathe like scientists had assumed, and oxygen intake through the skin was “essentially undetectable.” Instead, the fish kept breathing through their gills, which McGee could clearly see moving.
Pufferfish can breathe normally while inflated and don’t have to choose between getting eaten or running out of breath. But that doesn’t mean blowing up into a spiky ball is easy. After the fish deflated, McGee noticed they were “breathing heavily” and took a while, sometimes as long as five hours, to return to their normal respiration rates. Other studies have likewise found a long recovery time after inflation, and that the fish can only inflate so many times in a row before getting fatigued and being unable to puff up again. McGee thinks that chugging enough water to inflate, on top of any escape attempts the fish might make before inflating, is energetically taxing and leaves the fish tuckered out.