Puffer fishes are a family of incredibly diverse tropical fishes with extreme anti-predator adaptations, like inflating their stomachs to expand their body size and being covered in stiff, inflexible bone spikes. Researchers are now unraveling how these features evolved, as described in a study published July 25 in iScience.
Scientists discovered that these spines are actually modified scales, and they arose in much the same way as other types of skin appendages.
Spines
Pufferfish spines, which appear as a sort of bony armor, are actually composed of a mix of nanocrystalline hydroxyapatite, protein (collagen), and water—the same stuff that scales are made from. But unlike scales, these spikes aren’t connected to one another. Instead, they are arranged in a dense, spiky matrix that sits under the skin, spreading outward when the puffer fish is inflated.
These spines are the first line of defense against predators. Although they are relatively soft, the tips contain a potent neurotoxin called tetrodotoxin, which can sicken or even kill an attacker. The spines also help disguise the fish’s mucous odor, which is produced when it exhales air from its gills to inflate.
Because of the dangers posed by its spines, the puffer fish is a slow runner that relies on a combination of pectoral, dorsal, and anal fin motion to move around. To compensate, it has an unusual tail fin, which can be used to deliver a sudden burst of speed, like a dolphin’s flipper.
This speed burst can be used to evade prey, or to chase down predators that are too slow to catch up to it. Besides its spiny body armor, the puffer fish’s other natural defenses include stretchable skin and a distensible stomach that can increase in size 50-100 times. Unlike other fish, the pufferfish does not have pleural ribs or pelvis to act as impediments to this process of “puffing.”
Biologists aren’t entirely sure how the pufferfish’s amazing ability to inflate evolved. However, Peter Wainwright, a Florida State University biologist, has been able to use genetic techniques to trace the evolutionary tree of the pufferfish and their relatives. He has found that the fish’s spiky skeleton isn’t an out-of-the-blue innovation, but rather a slight variation on how their relatives, such as triggerfish, blow up their own bodies.
Ribs
Pufferfish are able to inflate their bodies up to three times their normal size by rapidly pumping water into their stomachs. Inflation allows them to escape predators by looking much larger than they are. It also makes them harder to eat, as their internal organs are protected by layers of sharp spines. Pufferfish are able to expand in this way thanks to a unique combination of structural features. They lack rib bones, which would only get in the way of their expansion, and they have fused bones in their cranium and jaws. In addition, they have a special type of bone that isn’t connected to the rest of their skeleton at all, and instead forms spiky spines around certain areas of their body.
Scientists have been trying to figure out how pufferfish inflate for a long time, but they’ve only just discovered how it works. The key is in their stomachs, which act like giant water balloons. They are lined with a tissue that folds like accordion pleats to allow them to expand, and they have special muscles that make it possible. Pufferfish are also able to swallow a lot of water at once by having hinged shoulder bones, which let them rotate their heads back and increase the size of their mouth cavities.
As they inflate, the stomachs of puffer fish engulf their head, dorsal fin, and pectoral fins, making them look like a perfect sphere. They also have a very unique type of skin, which is made of wavy fibers that straighten out when the fish expands. This helps to create a uniform shape and makes the fish less likely to be punctured by its predators.
Another aspect of pufferfish skeletons that scientists are still working on is how they’re able to inflate without their ribs breaking. They’ve been able to discover that their ribs are actually a kind of cartilage, which is much more flexible than the bones of other fish. They also have a very different type of bone, which has been found to be similar to the hardened cartilage of birds and mammals. Scientists think that the rib-like bones might help the fish to move more quickly and precisely in the water, though they’re not sure exactly why.
Pelvic Fins
Pufferfishes (and their relatives) belong to the large order Tetraodontiformes. This diverse group contains 447 species in ten families and lives in marine habitats ranging from coasts and reefs to open pelagic waters and the deep ocean. Pufferfishes are best known for their ability to inflate their bodies several times larger than their normal size as a defensive mechanism against predators.
Their inflating ability is made possible by stretchable skin and a distensible stomach. Pufferfish lack pleural ribs and pelvis, as these bones would be an impediment to their inflating action. Their stomach is also not a digestive organ, as it has lost its capability to digest food and instead absorbs water from the surrounding environment.
They also have a specialized caudal fin that can be used for swimming and scouting at depths of up to 2,000 ft (609 m). Its shape is a hybrid between a typical fish caudal fin and an invertebrate urchin, allowing it to generate powerful thrust with little energy expenditure. In fact, it can provide a fifth of the thrust of a pufferfish’s body weight.
The caudal fin is also important for locomotion, combining pectoral, dorsal and anal fins to provide an overall efficient motion. It can be used to accelerate or decelerate, and to assist with yawing maneuvers. Like the pectoral fins, it is often tucked against the body during moderate- and high-speed steady swimming to reduce drag.
While early studies that amputated the pelvic fins found no change in swimming behavior concluded that they have limited or no locomotory function, recent experiments indicate that they do play a role. In particular, they can alter flow conditions along the ventral surface of the fish by oscillating in a contralateral pattern. This alternation can help stabilize the fish, as well as influence the hydrodynamic function of downstream fins during yawing maneuvers.
The elongated pelvic fin of the three-tooth pufferfish is unique in the fish world and can flare up into a fanlike configuration called a dewlap. The three-tooth pufferfish, Xiphophorus helleri, which is found in the Indian and Pacific oceans, uses this fin to intimidate prey and potential predators. The dewlap is adorned with eyespots, which serve to attract prey and warn off potential predators.
Muscles
Pufferfish are among the most striking creatures in the sea, especially when inflated. They can expand three times their normal size by swallowing water, and they’re able to do this because their stomachs are shaped like accordions. In fact, Science Insider reports that puffer fish puff up not by absorbing air, but by pumping excessive amounts of water into their stomachs, which then fill up with tiny accordion-like folds. The fish’s mouth and esophagus have special muscles that allow them to do this, and the stomach is protected by a special lining that keeps the water from leaking out until it is ready to deflate.
Puffers also have a powerful, beak-like mouth that’s capable of cracking open the shells of snails, hermit crabs, and sea urchins. Their strong jaws help them to hunt for a variety of foods, including octopus and shrimp.
Unlike most fish, pufferfish have very few vertebrae. They have just 36 bones, compared to the more than 500 in an average teleost. They also lack ribs and pelvic appendages, as evidenced by alizarin red staining of their skeletons. Pufferfish and their relatives have reduced skeletal complexity because they lack the genetic changes that would result in the addition of extra bones.
The evolution of the pufferfish’s simplified skeleton may be related to the loss of certain Hox gene clusters. Medaka, a distant relative of the pufferfish, has two orthologs of the Hoxa and Hoxb clusters but only one of the Hoxd genes (Fig. 3). The gene that encodes Hoxd13a became a pseudogene after the divergence of medaka and pufferfish lineages, giving rise to pufferfish.
Another important component of the puffer fish’s defenses is its rapid burst of speed, which allows it to escape predators who are slow to react to sudden movement. This speed is combined with a quick movement of the pectoral and dorsal fins, allowing the fish to move quickly while fending off attackers.
The spines that stick out from the puffer fish’s body only appear when it is inflated, and they are used to scare off predators. The fish can also release a dangerous toxin that causes numbness and prickling on the lips, tongue, and torso, vomiting, low blood pressure, and paralysis. The toxin is absorbed through the skin, and it is possible for humans to ingest it by eating improperly prepared pufferfish soup, fugu chiri, or raw puffer meat, sashimi fugu.