Puffer fish have a reduced skeleton and spiky “spines” that form in patches around their body. The spines can frighten predators and, if swallowed, can deliver lethal poisons that attack their nervous system.
Biologists have wondered how puffer fish develop their inflated shape. They have now discovered that, surprisingly, the fish’s inflation behavior isn’t a radically new innovation.
The spiky spines on the bodies of puffer fish are a common defense against predators. When the fish is threatened, it can eject air from its mouth and inflate to many times its normal size. This makes it a stiff, unappetizing ball that is difficult for predators to swallow.
Puffer fish also have a unique ability to hide under coral reefs or similar abodes to camouflage themselves from predators. Some species change their main colors to match their surroundings, and other puffers hide their heads in a crevice or other hidden spot.
But what about their skeleton? Despite their weird spiky body armor, the bones of a puffer fish are actually quite normal. They are a bit like scales, except they are made of a different material—nanocrystalline hydroxyapatite and protein (collagen). Like standard scales, they form during development, when the outer layer of skin, known as the dermis, develops from mesoderm cells (see the image below).
The spines on puffer fish are very similar to the external spines on their closely related porcupinefish relatives. But while the spines of the porcupinefish are always visible, the spines on puffer fish are only visible when the fish is inflated. The differences are likely due to different ecological pressures. Puffer fish feed on a variety of items, including mollusks and crustaceans, and their spiky body armor is an effective deterrent against predators.
Pufferfish, along with their close relatives, belong to the order Tetraodontiformes. They include familiar species such as triggerfish, tripod fish and filefish, as well as the infamous puffer fish. The evolutionary tree below illustrates how these fish converged on their remarkable adaptation of reduced skeletons and spiky spines.
Interestingly, the puffer fish’s simplification of their skeleton appears to have occurred after genome duplication. Their Hox cluster complement is similar to medaka, but much simpler than that of zebrafish. In particular, Hoxc3 became a pseudogene in the lineage that gave rise to puffer fish and their closest relatives, and Hoxd13a disappeared altogether. These changes are likely the result of adaptive evolution, allowing the pufferfish to take advantage of new ecological niches by evolving diverse morphological set-ups for their spines.
Puffer fish are among the ocean’s most distinctive creatures, especially when inflated. They have a reduced skeleton, beak-like teeth and “spines” — spiky skin structures — in some patches around the body. Scientists have long wondered how they got this way, but research into puffer fish embryos is helping to solve the mystery.
The first step came when ancestors of puffer fish evolved the ability to expand themselves, called inflating. The fish pumped water into their stomachs, which expanded like accordion folds. They also lost pleural ribs, which would get in the way of expansion. This simplification of their skeleton allowed them to puff up without breaking any bones.
Next, their skin evolved. It is uniquely suited to stretching, with wavy fibers that straighten out when the fish inflates. Some species also have spines anchored in these fibers. When the fish puffs, the spines flip up, giving the fish a hard shell that predators have a hard time penetrating.
Finally, the skeletal muscles that control inflating evolved into a system that is controlled by the hormones epinephrine and norepinephrine. These epinephrine and norepinephrine receptors are located on cells throughout the body, and they act on those cells to trigger inflating and deflating. The skeletal muscles also control the rate at which water is pumped into and out of the stomach.
Puffer fish eat a variety of small invertebrates, including crustaceans and mollusks, such as snails, hermit crabs, and sea urchins. The spiky teeth of some species help them break open the external skeletons of these animals. Puffer fish also rely on their impressive arsenal of defensive tactics to ward off predators, including the lethal concoction of body toxins they inject into their prey.
Puffer fish rely on their ability to inflate themselves when threatened or attacked, but they also use their mouths, fins and pectoral fins to move about the water. They swim by side-to-side sculling movements of their dorsal and anal fins, while their pectoral fins assist with balance and direction. They are primarily ocean fish, with a few freshwater species.
The paired pelvic fins (also known as hindlimbs) of jawed vertebrates have evolved to differ widely in morphology and function. In fish, the pelvic fins are mostly used as minor anchors during swimming and manoeuvring, while in tetrapods the hindlimbs are employed as the major form of locomotion on land. Pufferfish have evolved a simplified skeletal system that is intermediate in both morphology and position with slender pelvic fins and robust weight-bearing hindlimbs.
The pelvic fin skeleton in pufferfish is not as complex as the skeleton of a zebrafish, with only six fused axial segments, and no ribs or pelvic vertebrae. The paired fins are also much shorter than in the zebrafish, with the shortest being located in the anterior portion of the body. Despite the simplicity of the skeleton, pufferfish are highly active and capable of propelling themselves in the water by pushing their abdominal fins in an alternating fashion, while also using their head to steer.
Until recently, it was believed that pelvic fins appeared within gnathostomes after the development of jaws. However, new fossils, re-examination of older fossils and evidence from developmental biology suggest that the paired pelvic fins may have appeared as early as the Silurian period. This evidence consists of the presence of pectoral and pelvic fins in antiarch placoderms, and the occurrence of a pelvic girdle in the tetrapodomorph dinosaur Panderichthys. The pelvic girdle in this fossil is small, club-shaped and distinctly fish-like (Boisvert, 2005).
Other evidence reveals that the earliest tetrapods, such as Ichthyostega and Acanthostega, did not have functional hindlimbs (Pierce, 2012). Specifically, these fossils lack a pelvic ilium and ischium, which are essential to the use of the hindlimb for terrestrial locomotion. A recent study has shown that a conserved gene, Hoxc10, plays an important role in the positioning of pelvic fins. This gene controls the formation of pelvic fin precursor cells, and its expression correlates with the position of pelvic fins in crown group taxa. Murata and colleagues have demonstrated that the ilial girdle of pufferfish is located just posterior to the location of the Hoxc10 expression domain.
Puffer fish have some of the strangest skeletons in the animal kingdom, and they’re not just for show. When the fish isn’t inflated, its spike-like spines lie flat across its body and are anchored in fibers that extend from the skin to resemble caltrops. As the fish puffs up, the wavy fibers pull tight and straighten out, turning those spines into hard armor that predators can’t penetrate.
The same spiky bones that cover the skin also form a layer around the mouth when a pufferfish isn’t inflated. These bones are arranged in a way that’s similar to a fish’s jaw and allow the puffer to swallow prey by bursting its mouth open like a water balloon.
Once a pufferfish is inflated, it takes 35 gulps of water in 14 seconds to get its full volume. This massive pumping is possible because the fish’s stomach is a water balloon and it can expand 100 times its normal size. It’s also possible because pufferfish lack ribs and pelvic fins that would be impediments to the process, and their distendable skin is stretchy and folds in on itself like accordion pleats when they blow up.
Inflating can be deadly for a pufferfish because it causes the animal to inhale a dangerous mix of air and water, and this mixture could block its gills or even cause asphyxiation. To protect themselves, pufferfish have evolved a number of strategies for getting rid of excess water, including releasing it from their lungs and even using a specialized gill structure to pump out extra air.
Puffer fish are found in tropical and warm temperate waters around the world, and their close relatives include the stout, bloated ocean sunfishes (Molidae) and the porcupine-like sun fishes (Diodontidae). Like other members of their families, they have a short stout body, small fins, and large eyes. They swim by side-to-side sculling movements of their dorsal and anal fins, and they have pectoral fins for balance and direction.
Scientists have been debating what explains this secondarily simplified skeleton in pufferfish, and they’ve discovered some interesting clues. To start, they’ve compared genomes from pufferfish (Spheroides nephelus) and from Japanese pufferfish Takifugu rubripes (fugu). The results indicate that the simplified puffer fish skeleton evolved in the lineage that gave rise to both the southern pufferfish and fugu, and it was accompanied by a reduction in Hox cluster complexity.