Pufferfish have a strange combination of features, including a reduced skeleton and spiky skin structures in certain patches. The how and why of these spines has been something of a mystery, but scientists now have clues thanks to research on pufferfish development.
These spines are actually modified scales. And like scales, they develop during embryonic development.
Spines
The spiky skin structures that cover the bodies of puffer fishes have long been intriguing to biologists, but how they got there has remained a mystery. In a study published July 25 in iScience, researchers have identified the genes that control the evolution and development of these spines. It turns out the process is pretty similar to how other vertebrates get their hair or feathers.
The scientists began their investigation by tracking the growth of spines in pufferfish embryos. They initially expected to find that the spines formed from scales, but were surprised to discover that they actually form independently of scales. They also found that the development of these spines relies on the same network of genes that shape feathers and hairs in other vertebrates.
These spines are made from nanocrystalline hydroxyapatite, protein(collagen), and water—the same materials as the scales that they replace. However, unlike scales, the spines are not attached to any bones and can be moved around the body by a small muscle that contracts when the fish inflates. When not in an inflated state, the spines lie flat against the fish’s body. When threatened, pufferfishes can inflate with a burst of air, erecting their spines to discourage predators from getting too close.
Pufferfishes are part of the order Tetraodontidae, which also includes the closely related porcupine fishes (Diodontidae). This group is primarily marine and estuarine, but a few species live in tropical Africa and Asia. Tetraodontids are known for their puffed-up appearance and the large numbers of sharp spines that cover their bodies when uninflated.
The researchers next tracked the growth of the spines in the Japanese grass pufferfish (Takifugu niphobles). They found that the genes responsible for the development of spines in this fish are also active in the development of scales, zebrafish hair, mouse fur, and chicken feathers. These findings confirm that the same set of genes shapes all vertebrate skin appendages. “It just blows my mind that regardless of how evolutionarily different the resulting structures are, they all share these common signals during development,” Fraser says.
Bones
Puffer fish, also called blowfish or puffers, are a type of fish that are well-known for their ability to inflate themselves into an almost indestructible ball. This is achieved by their highly elastic stomachs which can rapidly ingest water and air, making the fish three times larger than its normal size. This inflated state is a very effective defense mechanism against predators, as the fish releases tetrodotoxin from its skin, a poison that is lethal to other fish and humans.
Pufler fish skeletons are unique in that they lack the typical bones found in most vertebrates. They instead have a ring-shaped bone in their pelvic fins and fused bones in their cranium and jaw. This reduced skeletal structure is thought to have evolved secondarily from a common ancestor of all fishes. The genus Spheroides is the only modern fish that displays this unusual skeletal structure.
The skeletal remains of these strange fish are incredibly complex. A study published in 2015 found that puffer fish skeletons are composed of over 100 individual bones, many of which have a distinct shape. The study was the first to examine these bones in great detail, using CT scanning to get a closer look at their structures.
Another interesting thing about the puffer fish skeleton is that its spines are actually a form of dermal armor. The spines are similar to beak-like teeth and replace the typical fish scales in certain areas on the body. The study that led to this discovery found that the development of these spiky skin structures was linked to genes commonly expressed in feathers and hairs.
These armored fish can also be used as a camouflage tool, as the colors of some species can change to blend in with their environment. When not inflated, puffer fish can be found tucked away in a crevice or hiding behind coral reefs. While they are able to inflate themselves to protect themselves from predators, the fish can also be eaten as a delicacy in some parts of the world, where it is known as fugu. However, preparing this dish requires extreme care to avoid a lethal dose of body toxins.
Muscles
Puffer fish, also known as blowfish, have an unusual way of protecting themselves from predators: they can inflate their bodies by swallowing water or air. This makes them much bigger and reduces the range of possible predators to those with very large mouths. Several species of pufferfish also have spines on their skin, which makes them even less palatable to would-be predators. In addition, almost all pufferfish contain tetrodotoxin, which can be up to 1,200 times more poisonous than cyanide and is enough to kill 30 adult humans.
Unlike other vertebrates, pufferfish lack ribs and pelvic fins, and many of their bones are fused together in the cranium and jaw. The muscles of a pufferfish are also different, being much more flexible than those of a vertebrate. It has been proposed that this is a secondarily simplified skeletal morphology, resulting from loss of Hox complexes. To test this theory, genomic sequences of the Southern pufferfish Spheroides nephelus and the Japanese pufferfish Takifugu rubripes (fugu) were analyzed. Both species have at least seven Hox clusters, including two copies of the Hoxb and Hoxd clusters and a single copy of the Hoxc cluster.
The fast skeletal muscle tropomyosin protein in pufferfish is highly similar to the mammalian protein, but has several substitutions that affect its myosin activation profile and thermal stability. The reason for these differences is that the mammalian tropomyosin gene has two alternative splicing variants, whereas fish tropomyosin genes are encoded as a single isoform.
Pufferfish skeletal muscles have an extremely high concentration of actin, which is similar to the actin in mammalian muscle and squid mantle. Actin is considered to be a very conservative protein with respect to molecular evolution, and amino acid sequences of actin from different sources are nearly identical. However, the tertiary structure of the actin from the fast skeletal muscle of a pufferfish is very different from that of the mammalian muscle, indicating that this protein has undergone significant structural changes during evolution.
Muscle proteins are an important component of the body. They have a variety of biochemical functions and determine the quality and nutritional value of raw fish and seafood products. Therefore, it is essential to understand their biochemical and physicochemical properties. This is particularly true of fish skeletal muscle proteins, which are a major source of protein in the diets of humans and other animals.
Internal Organs
Blowfishes can inflate themselves up to three times their normal size, which is a pretty daunting defense against a hungry predator. They also sport spines on their skin, so a predator that does manage to snatch one up won’t be feeling very lucky for long. And, of course, most pufferfish species are toxic, containing a poison called tetrodotoxin that can kill people if ingested in large enough doses.
When a puffer fish inflates, it does so by rapidly gulping water and expanding its stomach to a much larger size than it normally has. Interestingly, though, it isn’t absorbing air like a balloon; rather, the fish is simply pumping water into its stomach, which has accordion-like folds that facilitate the expansion. Once the puffer fish has pumped in all that extra water, special muscles in its mouth and esophagus clamp shut, sealing it off and pushing all the water back out when the fish is ready to deflate again.
The fish’s eye sockets also become a bit bigger during this process, allowing them to move their eyes around with more ease. This is important because, as with all blowfishes, the fish is nocturnal, and when it’s dark out, it needs to be able to see where it’s going so it can crack open the tough shells of snails and other mollusks that are the main staple of its diet.
Pufferfish also have special glands in their skin that produce a natural venom that they can use to defend themselves against prey and ward off predators. The poison, which contains the deadly tetrodotoxin, is concentrated in specific organs within the body and is particularly concentrated in their livers and intestines.
Pufferfish are a fascinating example of evolution in action, and you can see how this fish developed its unique adaptations by studying the skeleton of an uninflated puffer fish. The spikes, for example, are actually bones that line up together like caltrops but spread apart and point outward when the fish inflates. In addition, the bony scales aren’t connected to one another; instead, they are fused into a solid armor that is incredibly stiff.