Puffer fish are some of the most fascinating animals in the ocean. Not only do they possess an impressive arsenal of defensive tactics, but their skeleton is also quite unique.
The skeleton of puffer fish is made up of wavy fibers that pull taut when inflated. These wavy fibers then harden, forming a tough shell that predators can’t penetrate.
Spines
Puffer fish are some of the strangest fish in the world, with a reduced skeleton, beak-like dentition, and “spines” – spiky skin structures – in certain patches around the body.
These spines are made of nanocrystalline hydroxypatite, protein (collagen), and water; they are modified scales that cover pufferfish in certain patches, similar to how mammals, birds, and other vertebrates get their hair or feathers. The new research, from the University of Florida, US, shows that these spines are a product of genes that are also responsible for the development of other skin appendages in other animals – including zebrafish scales, mouse hair, and chicken feathers.
In addition to being striking ornaments, these spines serve an important protective role in many species. For example, puffer fish have a special defense mechanism that enables them to inflate their bodies, erecting their spines, when they are threatened by predators.
This inflated stance is highly effective in protecting the fish from predators, since the spines are hard and sharp, preventing any attack. The spines can even protect the fish from predators that are larger than it is!
During development, these spines are formed from the mesoderm layer of the skin or dermis. They begin to form between 12 days post fertilization and are able to extend from the pectoral girdle at 46 dpf. The ectodermal units that project from the ventral surface at this stage are called spine primordia and stain strongly with hematoxylin and Alcian Blue, an indicator of developing spines.
Spine primordia are derived from the epidermis and dermis, and they are lined by melanocytes that produce hematoxylin. The formation of these spine primordia involves the same signaling pathways that are involved in the development of all other vertebrate skin appendages.
The morphological diversity of these spines, which range from completely covering the fish’s entire body to leaving the frontal surface bare, is likely due to environmental pressures that affect how the spines develop and change over time. The researchers suggest that this may allow pufferfish to adapt to different environments or climates, and possibly increase their ability to survive in these new situations.
Ribs
The puffer fish, also known as the swellfish, sea porcupine, globefish and tiger puffer, is one of the most bizarre creatures in the oceans. Not only do they have sharp spikes on their bodies, but they are also capable of raising their heads two times their normal size and releasing a poisonous substance called tetrodotoxin that can cause severe damage to their predators’ bodies.
They are able to do these things because of a strange feature in their stomachs that Elizabeth Brainerd, a biologist at Brown University, has discovered.
Pufferfish swallow big gulps of water and air, which expand their bodies into balloons that are two times their normal size. They do this by attaching a sac to their intestines that acts like a muscular valve, shutting off their stomach and esophagus and filling the sac with air from their mouth.
This process is aided by a skin that’s uniquely adapted to ballooning, which includes fibers that straighten out when they inflate. It also contains lots of collagen fibres that allow it to expand up to 40 times its normal size.
In addition, the skeleton of the puffer fish is quite unique in that it does not have any ribs, which would interfere with its ability to expand. This is a good thing, as it means that they can inflate their bodies without breaking any of their bones!
Another interesting part of the skeleton is its skin, which is also adapted for ballooning. When it expands, the skin engulfs the fish’s tail and fins to form a nearly perfect sphere.
It is possible to see this skeletal structure in some zebrafish, though scientists have not yet studied it. Nevertheless, it is believed to be important for their ability to inflate their bodies.
These ribs are made from the same material as other animal skeletons, and they have a number of different genes that help them grow. They include the scleraxis gene, which helps the ribs to mineralize and grow.
This rib structure is also important for the fish’s swimming behavior, as it helps them to keep their heads up when they are in the water and avoid getting tangled up with other fish. The ribs also help to support the body’s weight as they swim.
Vertebrae
Pufferfish have a strange, unusual skeleton. They lack ribs and pelvic fins, and they have fused bones in the cranium and jaw. In addition, they have a unique, beak-like dentition that allows them to swallow large quantities of water in a very short time.
This is called puffing, and it has allowed these animals to grow larger than any other fish on Earth. It also gives them a powerful weapon against predators, such as hermit crabs and sea urchins.
In their puffing behavior, the fish take 35 gulps of water in the course of just 14 seconds. Each gulp pulls in a significant amount of water, which they then use to expand their body and increase their weight.
The pufferfish skeleton has many adaptations that enable this puffing behavior. The shoulder bones on the back of their heads hinge inward, allowing the fish to rotate them and make their snouts larger.
They also have a set of teeth that fuse together, giving them a strong beak-like mouth capable of cracking the shells of snails and sea urchins. They are nocturnal predators, feeding at night.
Researchers have discovered that the spines of a pufferfish are formed through a network of genes that are similar to those that form feathers and hair in birds. The scientists also found that the skin structures of pufferfish were formed through conserved gene interactions that underlie general vertebrate skin ornamentation development.
These findings are a major breakthrough for research into the evolution of the skeletons of these strange creatures. In fact, this discovery could help scientists better understand how the skeletons of other animals develop their remarkable features.
In order to determine how these morphologies developed in the pufferfish, biologists needed to know how the teleost Hox clusters evolved over the course of the ray-fin fish lineage. They did this by performing in situ hybridization on zebrafish and pufferfish embryos. They found that both lineages retained a number of the same original Hox cluster genes, but they did not retain all of them.
Pelvic Appendages
The paired pelvic appendages of fish have a great deal of variation in morphology, function and position. In tetrapods they have evolved into robust weight-bearing hindlimbs necessary for terrestrial locomotion (Clack, 2000; Johanson & Brinkley, 2007).
The evolution of the pelvic fin to pelvic hindlimb transition is an important step in the evolutionary history of the tetrapods. However, it is not known exactly how this transition occurred. Early tetrapods, such as Panderichthys and Ichthyostega, have paired pectoral and pelvic fins that are clearly fish-like, while early tetrapods such as Tiktaalik lack a pelvic fin (Boisvert, 2005; Jarvik, 1980).
Fossil evidence indicates that the earliest tetrapods lost their pelvic fins as part of their transition to a hindlimb-like body plan (Boisvert, 2005; Coates & Cohn, 1998). These fossils indicate that the pelvic fins were adapted for a ‘front-wheel drive’ mode of locomotion that allowed the tetrapod to be propelled forward by body flexion propulsion (Jarvik, 1980; Johanson et al. 2007, 2008).
To explore the developmental mechanism that underlies this transition, researchers from the University of Florida have examined skeletal preparations from wild-type and threespine stickleback and wild-type Takifugu rubripes mice. The researchers found that the expression of Hoxd9 in the pelvic apparatus of anadromous stickleback resembles that of tetrapods, while the fugu mutants lack this expression domain during bud development.
They also found that the fusion of the ilium and ischium of both halves of the pelvis creates a weight-bearing pelvis (Clack, 2000). This was confirmed by comparing skeletal preparations from wild-type and Pitx1 null mice and wild-type and Pitx1 null threespine stickleback.
This suggests that the extinction of a paired pelvic fin may be due to mutations that disrupt the initiation or positioning phase of the development of the pelvic fin. If these mutations became fixed in descendent populations, they could have acted as selective forces that reduce the size of the pelvic fin.