With their characteristic inflated body, puffer fish (also known as blowfish, fugu or toadies) are one of nature’s most intimidating creatures. These tetraodontiform fish have four large fused teeth to crunch through the tough shells of mollusks and crustaceans, their natural prey.
Pufferfish have specialized muscles in their mouths that allow them to inhale water, pump it into the stomach and expand up to three times their normal size. This expansion requires that the fish lack rib bones that would get in the way.
Pufferfish are covered in spines that lay flat along their bodies most of the time. But when they feel threatened, these nonvenomous spines, which are actually modified scales, stand on end to deter predators. How did this strange skin ornament evolve?
Fossils aren’t available, so biologists have had to piece together the evolutionary story mainly by studying living pufferfish. But they’ve recently found that the evolution of spines is much less of a radical leap than it first appeared—and that, on a fundamental level, it’s similar to how other vertebrates get their feathers and hair.
Rather than ejecting air, pufferfish fill themselves by inhaling water, which expands their stomachs to three times their normal size. To prevent the fish from rupturing during this transformation, it has wavy fibers that wrap around its skeleton—and which, when pulled taut, cause the spines to flip up.
The spines are composed of nanocrystalline hydroxyapatite, protein(collagen), and water—the same materials that scales are made of. But unlike other scales, which originate during development from the mesoderm layer of the skin or dermis, the spines appear to have evolved from specialized cells located under the surface of the skin.
These cells are called skeletal progenitor cells, and they are also the source of other hardened body parts, like the jaws, fins, and skulls. When the skeleton of a puffer fish is inflated, the progenitor cells form spines and other bony appendages in a process known as ossification.
The ossification process is fairly quick—it takes about 24 hours for the spines to fully develop. But when the fish is deflated, the ossification cells quickly revert to their unmodified state—and the spines come tumbling down.
A study published in the journal iScience this month found that the genes that control these changes are very similar to the ones that produce the development of hair and feathers. The research team also discovered that when they blocked genes that are classic markers of spiky skin appendage formation, the number of spines and the location of their occurrence on the fish was significantly reduced.
Puffer fish, also called blowfish or fugu, have a unique natural defense system: they can inflate their highly elastic stomachs to turn into balls several times their normal size. This “inflatability” is thought to be a countermeasure against predators that see puffer fish as easy prey due to their slow, clumsy swimming style.
Pufflefish rely on other antipredator adaptations as well, including being covered with stiff, inflexible armored plates or releasing a variety of body toxins that can poison any creature that tries to eat them. Almost all pufferfish species are brightly colored—a strategy that’s thought to warn any would-be attackers that their flesh is at best unpalatable and at worst lethal.
In addition to a variety of spines, a puffer fish’s skeleton is remarkably slender and curved, which makes it harder for predators to bite or swallow them whole. Puffer fish also lack ribs and pelvic fins, and have fused bones in their cranium and jaw.
When a puffer fish feels threatened by a predator, it rapidly pumps water into its stomach, which loses all of its digestive function to allow the fish to expand. As the stomach fills up, it becomes stretched out like an accordion and sealed shut with a membrane in its peritoneum. The Long-spine porcupinefish, for example, can expand to 3 ft (1 m) in length.
The enlarged stomach can stretch out to three times its normal width. The stomach’s folds are lined with a special lining that allows it to do this without breaking the ribs, which would be quite dangerous for the fish. The ribs, however, can break apart once the fish deflates, allowing the skeleton to return to its normal shape.
The pufferfish can then move around by combining pectoral, dorsal and anal fin motions with a sudden burst of speed, similar to a man duct-taped to a chair scooting around with his feet. They can also squirt a cloud of gill rakers from their dorsal and anal fins. While a puffer fish is moving in this manner, it can still swim by using its tail fin as a rudder.
The pelvic appendages of puffer fish are rudimentary. They consist of a fused bone, the scapulacoracoid, and four proximal radials that are not fully divided. These connect to the fin rays in a one-to-one arrangement. However, the proximal radials are not cartilaginous. This arrangement is similar to that of the bony fish skeleton.
Pufferfish have evolved to survive as a prey item for larger predators. To do this, their bodies are covered with a thick layer of protective plates. The plates are strong enough to crack the shells of snails, hermit crabs, and sea urchins. They also have sharp, beak-like teeth that can penetrate the defensive body armor of these invertebrates.
In addition to their stretchable skin, pufferfish have a distensible stomach. This allows them to expand their belly size by up to 100 times, a strategy known as “puffing.” They do this to frighten off or disable predators and to distract them while they hide in crevices for protection or to evade predation. Unlike sharks and other vertebrates, pufferfish lack pleural ribs and pelvic fins, which would be impediments to this strategy.
The pectoral fin skeleton of pufferfishes is also unique. It consists of four elongated proximal radials and three free posterior fin rays. In a number of basal species, such as the mormyrid puffer fish (Petocephalus bane) and the goldeye puffer fish (Hiodon alosoides) in Hiodontiformes, the proximal radials have decreased to two or three, but they retain the mesocoracoid.
Other derived species, such as the tub gurnard (Chelidonichthys lucerna) in Lophiidae and the Atlantic pufferfish (Pneumocephalus diablo) in Triglidae, have only two proximal radials. The scapula and coracoid of these species also lack the mesocoracoid. These alterations suggest independent morphological changes in derived pufferfish lineages that have resulted in the development of unique skeletal features.
The bones of a puffer fish aren’t as flexible as those of most fish. That’s probably because they aren’t moving very much! Pufferfish move by combining pectoral, dorsal, and anal fin movements with a tail slapping motion. Their slow movement makes them a tempting prey target, but their natural defenses give them a huge advantage: they can rapidly puff themselves up into a hard-shelled ball three times their normal size.
To do this, a puffer fish first swallows water, then its stomach expands to take in extra volume. In the video below, you can see the process from a fish’s perspective. But how do they know to fill themselves up with water and not air? The fish has special muscles that trigger when it senses predatory threats. The extra space allows the fish to fend off predators by hiding in its shell, and their sharp spines are also an effective deterrent.
But it isn’t as easy as it looks. To puff up, a fish needs to be able to swallow 35 gulps in the span of 14 seconds. Each gulp draws in a lot of water, and to accommodate this pufferfish have some unusual anatomic features. For example, most fish have shoulder bones that anchor firmly to the back of their head, but pufferfish’s are hinged, so they can rotate their shoulders back when gulping.
Their distensible stomach is also an evolutionary adaptation that isn’t fully functional, having lost gill openings and pelvic bones. They also lack pleural ribs and a sternum, which would be impediments to their ability to stretch their bodies. The fish’s skin is able to expand and contract along with the stomach, allowing it to puff up 50-100 times larger (Brainerd 1994).
The spiky bones aren’t connected to each other when the fish isn’t inflated, but they do layer over each other like caltrops. Those spikes aren’t just for show, either! They help protect the fish by absorbing impact energy. The skeletons are so stiff that, even when a fish isn’t inflated, they are very difficult to move.