Why Do Pufferfish Puff Up? How Inflation Works and What It Costs

How does a pufferfish inflate?

A pufferfish inflates by swallowing water in quick gulps. It expands its mouth cavity to draw water in, closes its mouth, and then squeezes the water back through its throat into its stomach. Repeating this pumping action many times in a row fills the stomach within seconds. Out of the water, a frightened puffer may gulp air instead.

Research on the head muscles of pufferfishes and their relatives shows how this ability evolved. All fish in the order Tetraodontiformes studied can “cough”, forcefully expelling water from the mouth, and most of them can blow strong jets of water out of their mouths to uncover prey buried in the sand. Pufferfishes use a slightly modified version of the same muscle pattern, combined with changes to the head that turn the mouth into a pump for filling the stomach. Scientists think inflation developed step by step from coughing to water-blowing to puffing up.

What happens inside a puffed-up pufferfish?

The key organ is the stomach. In the balloonfish, a porcupinefish studied in detail in the 1990s, the stomach is folded over on itself many times and lined with a special stretchy tissue called transitional epithelium, the same type of tissue that lines the human bladder. It has lost its role in digestion and works instead as an inflatable sac. As it fills, it expands into a large space inside the body and pushes outward toward the head, the fins and the tail.

The skin is specially built too. The collagen fibers in a puffer’s skin are wavy when relaxed, so the skin can stretch by about 40 percent with very little force. Once the fibers are pulled straight, the skin suddenly becomes stiff. A tight skin around a ball of water creates a firm, hard-to-bite sphere, and in porcupinefishes it holds the long spines upright.

Pufferfishes also lack pleural ribs, the ribs that surround the body cavity in most fish, and have no pelvis or pelvic fins, so nothing rigid stops the belly from expanding. These features were already present in the ancestors of pufferfishes before inflation evolved, and only later proved useful for it.

Why does puffing up protect a pufferfish?

Pufferfishes are slow swimmers that move mainly by flapping their small fins, so they cannot simply outrun a predator. Their first defense is sharp eyesight and a quick dash for cover. If that fails, inflating changes the contest.

Many predatory fish swallow their prey whole, and the size of their mouth limits what they can eat. A puffer that suddenly doubles or triples in volume may become too big to fit. The stiff, rounded body is also hard to grip, and in species with prickles or spines it becomes painful to bite. Predators that try to swallow an inflating puffer may get it stuck in their mouth or throat.

Many pufferfishes have a third line of defense: tetrodotoxin, a powerful nerve poison stored mainly in their organs and skin. A predator that does manage to eat a toxic puffer may become sick or die. How dangerous this toxin is, including to people, is explained in Are pufferfish poisonous?

Do pufferfish hold their breath when inflated?

For a long time, the standard explanation was that an inflated puffer stops breathing through its gills and survives on oxygen absorbed through its skin. A 2014 study tested this idea on the black-saddled puffer, a small Indo-Pacific species, and found that it was wrong.

While inflated, the puffers kept pumping water over their gills and took up oxygen at about five times their resting rate. Their skin absorbed almost no oxygen. The gills, not the skin, were doing the work.

The study also showed that puffing up is expensive. After the fish deflated, their oxygen use took an average of 5.6 hours to return to normal. The researchers concluded that the frantic activity before and during inflation, including gulping all that water, forces the fish to rely partly on anaerobic energy, which must be paid back later.

Does puffing up harm a pufferfish?

A healthy puffer can inflate and deflate without lasting injury, simply by spitting the water back out. Even so, inflation is a last resort. It burns a great deal of energy, it takes hours to recover from, and an inflated puffer swims very poorly, so it has to wait until the threat goes away.

For these reasons, divers and snorkelers are urged never to touch, chase or tease pufferfishes to make them puff up. A puffer lifted out of the water may swallow air, which may be harder for it to get rid of than water, and handling it also exposes people to the fish’s skin, which in many species contains toxin.

Which fish can puff up?

Every species in the two sister families of inflating fish can puff up: the pufferfishes (Tetraodontidae), with about 190–200 species, and the porcupinefishes and burrfishes (Diodontidae). Familiar examples include the white-spotted puffer of Indo-Pacific reefs, the northern puffer of the US Atlantic coast, and the long-spine porcupinefish, which bristles with spines when inflated.

Their relatives in the same order, such as triggerfishes, filefishes, boxfishes and the ocean sunfish, do not truly inflate, although at least one group of filefishes has evolved a weak version of the behavior. Pufferfishes are the true experts, and their combination of inflation, prickles and toxin makes them some of the best-defended fish on the reef. They also appear in the list of most poisonous animals.