Puffer fish are among the most striking creatures in the ocean, particularly when they’re inflated. But how did these tropical fish acquire this extreme feature?
The members of the order Tetraodontidae—also called puffer fish, blowfish, balloonfish, bubblefish, and swellfish—have stretchable skin and can expand up to three times their original size. They also lack pelvic bones and ribs, which would get in the way of expansion.
One of the oddest fish in the ocean, pufferfish have a reduced skeleton, beak-like dentition and spines instead of scales in certain patches around the body. The spines are crucial for this fish’s defence, allowing it to inflate itself into a formidable foe for predators. But how and why these spiny skin structures evolved remains a mystery. A team of researchers from the University of Florida, US, has now followed their development in embryos, and found that they form independently of scales. What’s more, they rely on the same genetic network that governs the formation of feathers and hairs in other vertebrates. Their findings are published in the journal iScience.
These spiky skin appendages are also known as ‘puff sacs’ or ‘boilers’ because they help to expand the fish’s volume when a predator approaches. But they also provide a defensive covering that protects the fish from predators when it is not inflated. The spines are anchored in tough wavy fibers that are pulled taut by the muscles inside the pufferfish, making it hard for predators to penetrate.
Fraser and his colleagues have now tracked the evolution of these spinal structures, which vary in their size, shape and coverage, across pufferfish species. They also manipulated the genes that regulate their development to find out how the diverse set-ups of spine locations came about. By blocking genes that are classic markers of skin appendage formation, the team could reduce the number of spines or loosen restrictions on where they appeared on the fish.
The researchers think that different morphological set-ups of spines are important for pufferfish to access different ecological niches, and they hope to use this approach to identify the genes that are responsible for their wide diversity. The results could lead to more accurate predictions of the behaviour of pufferfish, including when they might puff up and where their spines will appear.
Pufferfish belong to the order Tetraodontidae, which also includes tropical fish such as triggerfish and tripodfish. They share many features with their more primitive ancestors, including the ability to inflate themselves by swallowing water. Peter Wainwright, a Florida State University biologist, has suggested that it is likely that such early tetraodontiforms would have used their puffing capacity as an anti-predator strategy, by making themselves a bit larger and tougher to avoid being eaten.
The ribs of a puffer fish are important because they provide the fish with a flexible, lightweight body. This helps it maneuver easily in the water, and it also allows it to change pressure quickly, which is essential when it comes to diving for food or hiding from predators. Ribs are also the main structural component of the fish’s body, and they are what makes it possible for it to inflate itself when threatened.
When a puffer fish feels threatened, it can inflate itself to several times its normal size by swallowing large amounts of water. This makes it much harder for a predator to catch or even touch the fish. This is just one of many defense mechanisms that puffer fish use to protect themselves. They can also hide by changing the color of their skin, and some bottom-dwelling species of puffer fish can even camouflage themselves by nestling into their substrate.
Puffer fish are unique among bony fish, which include all species other than cartilaginous fish like sharks and sturgeon. These fish have bone skeletons and a swim bladder to help them float. They are also able to change their shapes, such as the rounded body of a puffer fish.
Blowfish are a group of fish known as Tetraodontidae, which belong to the order Tetraodontiformes. The name blowfish comes from the fact that these fish can inflate themselves when they feel threatened. The fish are also commonly called puffers, pufferfish, blowies, bubblefish, swellfish, honey toads, sugar toads, or sea squab.
Although the spines of puffer fish are not as thick as those of other bony fish, scientists believe that they still evolved from scales. Gareth Fraser and his colleagues have studied the genes that give rise to these spines in puffer fish embryos, and they found that they are developmentally distinct from scales. They are also similar to the gene networks that give rise to feathers in birds and hair in mammals.
Another way that puffer fish protect themselves is by releasing a poison that can paralyze or even kill other fish. Almost all puffer fish contain tetrodotoxin, which is lethal to any animal that ingests it. This is why it is so dangerous to eat the meat of these fish, which is often served as a delicacy in Japan. Only the most skilled chefs know how to prepare them properly, and even then, a bad cut can result in deadly tetrodotoxin poisoning.
The pelvic appendages of puffer fish are important for both their ability to move on land and for protection. They are also used for breathing, storing gases, and camouflage. The appendages can also be used to help the fish float and even as a holdfast organ. They are not as specialized as the fins, but they are still quite important to the fish’s life.
The shape and size of pelvic appendages vary widely across actinopterygian fishes, from nearly spheroidal in pufferfishes to extremely elongate in snipe eels. Despite their variety, all pelvic appendages have the same basic structure and serve similar functions. The pelvic appendages of a puffer fish are divided into two parts: the girdle and pubis. The girdle is the part of the appendage that attaches to the fish’s back and connects to the vertebrae. The pubis is the section of the appendage that extends forward from the girdle.
Recent studies have investigated the evolution of paired fins in a group of bony fishes called the sarcopterygians (Porpoises, Anguilliformes). Pufferfish are members of this group and have a unique phenotype that is not found in any other fish. The study found that the simplification of the pectoral fins in pufferfishes is not due to changes in gene expression during development. Instead, it was caused by a loss of Hox clusters in the lateral mesoderm.
Using CRISPR-Cas9 technology, the researchers blocked genes that are classic markers of skin appendage development. They then examined the effect on pectoral fin morphogenesis in medaka and zebrafish embryos. In the lineage that gave rise to pufferfishes, Hoxc3 became a pseudogene and Hoxd9a lost its expression in the lateral mesoderm.
The authors speculate that the loss of these genes in zebrafish and pufferfish led to a reduction in the size and complexity of the pectoral fins, which was subsequently amplified by selection for increased size and complexity of the pelvic appendages. This suggests that the loss of the appendages in pufferfishes is not a result of reduced genetic regulation, but rather the result of an evolutionary change in environmental conditions.
Pufferfish are bony fish, and as such have a skeletal structure unlike cartilage-covered fish like sturgeon or an octopus. These bones support the animal’s lungs, allowing it to take in air and gasses through the water. Their skeletons also provide protection and a rigid body for movement. In some pufferfish species, the spines are an important form of defense against predators. These spiky structures are composed of nanocrystalline hydroxyapatite and protein (collagen)—the same materials found in scales, but modified for different purposes. The spines are formed during development from the mesoderm layer of the skin, and they can range in size, shape and location on the body.
Puffer fish are nocturnal predators that feed on sea urchins, snails and hermit crabs, whose hard shells they can crack open with their strong jaws. The toxins in these animals, as well as the bacteria that live within their guts and shells, combine to create neurotoxin that is lethal to any other fish or marine creature that eats them.
Scientists have long speculated that pufferfish evolved their unique anatomy to help them survive in their particular ecological niche. But fossil records are too sparse to provide a clear picture of the evolutionary process. Fortunately, researchers have been able to use genetic techniques to reconstruct the evolution of these fish.
A Florida State University biologist and his colleagues have now traced the development of the puffer fish’s unusual skeleton. Their findings, published July 25 in the journal iScience, show that this remarkable trait was likely the result of a series of small changes to the ancestor of the pufferfish and its closest relatives, triggerfishes and tripodfishes.
The ancestors of pufferfish branched off from these and other fishes about 89 million years ago. They then acquired the ability to inflate by adjusting how their muscles and bones work together. This adaptation allows them to pump themselves up by gulping large amounts of water into their stomachs, which is then pushed out through their mouths.
To understand how these fish developed their spiky armor, the researchers studied the development of their spines in embryos. They discovered that the genes that are responsible for producing the spiny skin of most vertebrates are expressed in the developing spines, as well as those that are responsible for giving mammals their hair and birds their feathers. The scientists have also been able to manipulate the genes in an embryo to alter the number and size of the spines, and to restrict their location on the body.