Puffer fish—also known as puffers, balloonfish, bubblefish or blowies—are a diverse group of marine and freshwater species. They have a unique ability to increase their size, or “puff up,” when threatened.
Brainerd says that puffers don’t have rib bones or pelvis, because they would get in the way of their inflation. Instead, they have a distensible stomach that can expand to up to 50 times its normal size.
Spines
Puffer fish are some of the ocean’s strangest animals—they have a reduced skeleton, beak-like dentition and spines instead of scales. But how they evolved these spiky skin ornaments has remained a mystery. A new study published July 25 in iScience has identified the genes responsible for their development. The researchers found that the genes used for forming spines are similar to those involved in generating other skin appendages, such as hair and feathers, in vertebrates.
Pufferfish, also called blowers, balloonfish and puffies, have large teeth that are fused together into a single plate and a beak-like mouth for crushing the shells of mollusks and crustaceans, their natural prey. They also have a distensible stomach, a flexible pouch that allows them to grow up to 40 times their normal size by filling it with air, and lack pleural ribs and pelvises that would be impediments to inflated movement (see image below).
Their unique natural defenses are critical for puffer fish to survive in their aquatic habitats. Adults use a complex series of pectoral, dorsal and anal fin motions to move about, while the tail fin is used primarily as a rudder and can be employed in a sudden burst of speed that gives the fish the appearance of flying. This maneuver, known as a “balloon evasion,” is an effective countermeasure against predators, and it is the puffer fish’s most frequent means of survival.
But while the balloon evasion is a highly effective defense mechanism, it does carry some disadvantages. It reduces the puffer fish’s ability to hide in the water, making it more vulnerable to ambush attacks. It also makes the fish less able to move around or to find food in shallow water.
The scientists who studied the Japanese grass pufferfish, Takifugu niphobles, wanted to learn more about how these defenses work. They began by sequencing the genome of the fish to identify the genes that control development of spines and other body parts.
They then used CRISPR-based gene editing techniques to block particular genes that are classic markers for the formation of skin appendages. The result was that the spines on the pufferfish became smaller and less dense, and they were no longer localized in specific patches over the body to enhance protection.
Ribs
Unlike the scaled skeletons of other fish, pufferfish have no ribs and very few vertebrae. Instead, they have fused bones in their cranium and jaw. This simplified skeleton has been linked to their ability to inflate.
The ancestors of pufferfish evolved this capability after they branched off from the triggerfish lineage. When a pufferfish gets agitated by a predator, it can turn itself into a bona fide spike ball. This inflatability is the main defense of the fish, but it has other natural defenses, too. Pufferfish have specialized muscles that they use to pump water into their stomachs, and they can also seal off their stomachs like a drain plug when they are done eating.
Pufferfish can also squirt water from their mouths, spraying a cloud of foam in the face of the predator. While it is not as effective as puffing up, this can still cause serious harm. In addition, pufferfish can rapidly swivel their oversized fins to get around. This is not a graceful maneuver, but it does slow down a predator that is trying to catch the fish for lunch.
Scientists have studied the morphology of pufferfish in great detail, and they have discovered that these animals have a unique pattern of muscle development. Normally, the muscles of fish are attached to their skeletons through connective tissue called collagen. When pufferfish inflate, the connective tissue tightens up and hardens into a tough shell that is difficult for predators to penetrate. The wavy fibers that make up this stiff shell have spines attached to them, which gives the fish a spikey appearance.
Researchers have been able to see how the spines develop inside pufferfish embryos by using electron microscopy. They have found that the spines are formed from collagen fibers, similar to those that form feathers and hair in other animals. The team has also studied how the spines are activated in pufferfish, and they have found that a particular gene controls their activation.
Although pufferfish are beautiful creatures to look at, they are very dangerous. They contain a poison known as tetrodotoxin, which is extremely lethal to humans. Tetrodotoxin is absorbed through the skin and can cause vomiting, dizziness, prickling of the lips, and numbness throughout the body. If ingested, the poison can kill within 24 hours.
Pelvic Fins
Pufferfish are notorious for their ability to blow themselves up, or “puffer”, to many times their normal size. This happens because of special muscles that allow them to inhale water and pump it into their stomachs, making them expand like an accordion. This puffing is used to fend off predators, but is also a great way to eat, since their stomachs are capable of digesting small fish and shrimp.
The specialized muscles in the head also allow pufferfish to eject their heads from their bodies to help them move around. Interestingly, they can also eject their tails from the body as well. While this might make them less maneuverable, it’s not all bad for them because they use the tail to anchor themselves in the substrate.
When they’re not inflated, a pufferfish is basically a stiff armor of spiky bones that can jiggle around a bit, but are otherwise very immobile. They do have one trick, however, that helps them move around, and that’s to simply wiggle their oversized fins a lot, kind of like a man who has been duct-taped to an office chair scooting around with his feet.
These enlarged appendages are called gobiidae, and they’re very useful for these animals. They can be retracted to the body when not needed, or they can be tucked in a clinging position for a better grip on substrates. The gobiidae are actually derived from the pelvic fins of pufferfish, but they’re modified into adhesive suckers that are used to suction prey into their mouths.
To investigate how this unique feature evolved, we compared gene expression patterns in the genomes of the pufferfish fugu, which lacks a pelvic fin, and an anadromous threespine stickleback, which has a robust pelvic fin. In fugu, genes thought to control pectoral fin development in tetrapods maintained Hoxd9 expression within the pectoral fin bud mesoderm throughout its formation, but this region did not develop a pelvic fin. This suggests that mutations blocking the initiation phase of skeletal development, but not those blocking positioning, were responsible for the loss of the pelvic fin in pufferfish.
Bones
Pufferfish have a very different skeleton than other fish. Their bones are fused in many places, and the spines that form their armor actually replace normal fish scales. This allows them to be a lot more flexible and move faster.
Another amazing thing about puffer fish is their ability to expand three times their size in a matter of seconds. This is made possible by a combination of stretchable skin, a distensible stomach, and special muscles that allow them to pump excessive amounts of water into their stomach and seal it off. When the fish is ready to deflate, special muscles push all of that water back out again.
A puffer fish’s skeleton is also unique because it lacks rib bones. This is because ribs would only get in the way of the fish’s ability to expand. They also have a stomach that isn’t really a stomach, and instead serves as an accordion-like structure that can expand along with the rest of the body. In addition to this, puffer fish have a set of muscles that allow them to suck in air and blow it out again, just like a balloon.
Because they’re able to do so much, scientists are constantly trying to figure out how the puffer fish does it. One theory is that their ribs and pelvic fins have fused together to make the fish more flexible. Another is that their spiky bone armor is actually a set of stiff bones that cover the fish’s body and only become visible when they inflate.
Another interesting fact about puffer fish is that their skeletons are pretty much all made out of bones that are only connected to each other by very tight connections. This means that, when a puffer fish isn’t inflated, the bones stick out like little spikes and can be poked by predators. When a pufferfish is inflated, these bones overlap and connect to create an extremely tough armor that protects the fish from attack.
Another theory about why the skeletons of puffer fish are so rigid is that it is due to reduced complexity in their Hox clusters. These are a group of genes that control the formation of many parts of a fish’s body. Scientists have found that the puffer fish Spheroides nephelus and the Japanese pufferfish Takifugu rubripes have two copies of Hoxa, a single copy of Hoxb, and a portion of a third Hoxd gene.
