When you think about sharks, the most likely images that spring to mind are probably a dorsal fin breaking the surface and perhaps a large mouth filled with teeth.
Sharks have a fearsome reputation as the most powerful creatures in the ocean, so they must have a tough skeleton underneath their skin, right?
In fact, the answer to the question “do sharks have bones?” is no! Sharks are elasmobranchs, and they have a cartilaginous skeleton.
We will find out what being a cartilaginous fish means and look at how sharks benefit from having skeletal cartilage instead of bone. So, read on and find out why sharks don’t have bones.
Do Sharks Have Bones?
Let’s get it clear from the beginning, there are more than 500 known species of shark, and none of them have a single bone. Sharks are members of a subclass of fish called elasmobranchs characterized by having a skeleton made from cartilage.
What is cartilage? Cartilage is softer and more flexible than bone. It’s the same material that we humans have in our ears, nose, and rib cages, amongst many other places.
While we use it for specialized purposes, cartilage makes up the entirety of a shark’s skeletal structure.
So, if you’ve been asking, “how many bones do sharks have in their body?” the answer is none at all!
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If you’ve ever seen a shark on TV or in the movies, you’ll know that these cartilage fish aren’t soft and squidgy like an octopus or jellyfish.
Despite not having bones, the shark’s skeleton gives them all the rigidity they need to cruise the oceans.
Anywhere you would expect bone on a shark, you’ll find tough cartilage instead. This includes their spines, skull, jaw, and fins. The cartilage gives the shark its skeletal structure and shape and protects its internal organs.
You can’t answer how many bones does a shark have in the sense of cartilage as it is continuous.
The larger the shark, the more cartilage there will be. However, even the colossal whale shark (Rhincodon typus), which can reach over 18 meters / 60 feet in length, doesn’t have any bones.
Why Don’t Sharks Have Bones?
So having discovered that sharks don’t have bones, it’s natural to ask why not. Scientists believe that sharks have evolved cartilage skeletons instead of bones because it is advantageous to survive in their habitats.
Cartilage is about half as dense as bone and provides a shark with a lightweight and flexible structure that allows them to hunt food and evade predators with incredible speed.
On a fundamental level, the shark’s lightweight skeleton allows them to swim. Unlike bony fish, sharks don’t have a swim bladder to provide their buoyancy in the water. Instead, sharks must depend on their large oil-filled livers to stop them from sinking to the bottom.
A heavy boney skeleton would mean that a shark would need to have evolved an even larger liver which would be a severe disadvantage as it would add weight and need more food to keep in condition.
In addition, the flexible nature of cartilage allows sharks to twist and turn in the water. This improved maneuverabilitycan be incredibly useful when catching prey or evading predators.
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The elastic properties of cartilage are also used in the operation of the shark’s jaw. Because the jaw can flex, it can open much wider and apply more force than a similarly sized bone jaw would be able to.
Rather than being made of hard bone and susceptible to damage, the soft cartilage in the shark’s nose protects it from damage and acts as a kind of shock absorber, allowing the shark to use its snout as a sense organ to prod and probe interesting objects.
Sharks have evolved cartilage skeletons to survive best in their specific environments. Sharks are typically the apex predator in any given ocean, suggesting this was an excellent evolutionary choice.
One drawback of not having shark bones is that this is usually where red blood cells, white blood cells, and platelets are made. Cartilage doesn’t have blood vessels or marrow, so the sharks must make these vital blood cells elsewhere.
In a shark, blood cells are made in the spleen, epigonal organ, and in some sharks, in the Leydig’s organ.
Do Sharks Have Bony Skeletons?
We’ve already said that sharks don’t have bones, so that must mean they don’t have a bony skeleton. Well, actually, they do, in a way, thanks to calcified cartilage.
In places where extra strength and protection are needed, the shark has evolved a cartilage skeleton that sacrifices flexibility for increased strength by using calcium.
Any part of the shark’s cartilaginous skeleton that needs additional strength is calcified. The calcium salt deposits make the flexible cartilage much more robust and harder and appear as “bone-like.”
This selective hardening allows the shark to benefit from a lightweight, flexible skeleton while having strength and protection where it’s needed.
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Does a Shark Have a Backbone?
So, do sharks have a spine? Yes, they do. Sharks have vertebrae that make up a backbone that contains and protects the spinal cord and notochord.
However, this “backbone” isn’t made of bone. The spine is, like all the other structures in the shark, made of cartilage.
Thanks to the elastic properties of cartilage, the shark’s spine has a much greater range of movement than it would if it were made of hard bone.
The shark can use the elastic flexibility in its spine to increase the power of its movements. When the shark bends and the vertebrae compress, energy is stored to be released when the shark’s tail springs back powerfully in an action that the shark can use to strike or escape.
Scientists believe that as much as 10% of a shark’s movement energy is thanks to the flexibility of its backbone.
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Do Sharks Have Ribs?
While they do have a backbone, sharks don’t have ribs. What skeleton a shark has is made up of cartilage and connective tissue.
The shark’s body is supported by the water surrounding it in the ocean, so it doesn’t require a rib cage as terrestrial animals do.
In addition, not having ribs allows the shark to be considerably more flexible when it finds food. A shark gulps down its food in huge chunks, so having a body that can bend without the restriction of bone is a benefit to its ability to eat as much as possible when the opportunity arises.
The closest thing to traditional ribs that sharks have are their gill arches. These cartilage “ribs” hold the shark’s gills in place and support the gill filaments that absorb oxygen from the water.
The lack of a rib cage is one of the reasons why sharks quickly suffer fatal damage if they are taken out of the water even before they have succumbed to a lack of oxygen.
Without a protective rib cage and the surrounding water supporting them, the weight of the shark can crush its internal organs and cause it to perish, even if it is returned to the water “alive.”
Is a Shark’s Jaw Made of Bone?
The shark’s jaw is made from cartilage in common with all the other structures.
The flexibility of the jaw cartilage allows the shark’s mouth to open wider than might be possible with bone which is useful when the shark wants to swallow prey like a seal or seal lion whole.
The cartilage jaw isn’t attached rigidly to the skull. It can move from side to side and, in some sharks, even extend forwards to stop food from escaping.
The shark’s jaws are strengthened by calcium salt crystals called tesserae. These blocks enable the jaws to endure vast amounts of physical stress and give the cartilage the same strength as bone.
Large sharks, including the feared predators the tiger shark, the bull shark, and the great white shark, have extra layers of tesserae in their jaws to have even more strength.
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What Do Sharks Have Instead of Bones?
The shark’s skeletal system is made from cartilage hardened to differing degrees by calcium, connective collagen tissues, and muscle.
As we’ve already discussed, the most bone-like cartilage structures are found in the jaw and spine. Here are some other notable areas where cartilage has replaced bones.
Sharks’ fins are made from cartilage. They are flexible but have hardened keratin-based supporting rods called ceratotrichia. These resemble feathers and run through the fin to maintain their shape and transmit power to the water as the shark swims.
The shark’s skull is made from denser cartilage resembling bone.
The brain and eye sockets areas are heavily calcified cartilage for extra strength. However, the shark’s snout is more soft and flexible to absorb impacts.
Are Shark Teeth Made of Bone?
In keeping with the rest of the animal, shark teeth aren’t made of bone. Instead, shark teeth are made from a calcified tissue called dentin. This calcium phosphate in shark’s teeth is even stronger than bone.
Unlike a mammal’s teeth, which are rooted in the jaw bone, a shark’s teeth are planted in their gums. Shark teeth are lost naturally and are constantly replaced. A typical shark can use approximately 35,000 teeth throughout its life.
Fossilized teeth have allowed sharks to study ancient sharks long after the rest of their skeleton has gone.
9 Pros of Having a Cartilaginous Skeleton
So, having discovered that sharks have cartilaginous skeletons, let’s tick off the advantages.
1. Lighter Than Bone
Cartilage is considerably lighter than bone. This means that the shark isn’t wasting energy trying to move a heavy skeleton around.
2. Is More Buoyant
Cartilage is more buoyant than bone, so the shark doesn’t need to use so much energy swimming to stay afloat.
3. Is More Flexible
Cartilaginous skeletons are considerably more flexible than hard bones. A shark’s body can bend and twist to a greater degree and much more quickly and dramatically than it would be able to if it had a bony skeleton.
Sharks use this incredible range and speed of movement during their hunting habits to catch prey and shake it vigorously before swallowing it whole. They can also use their rapid, twisting moves to avoid attacks from predators.
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4. Give the Shark a Thicker Skin
Cartilage isn’t as hard as bone, so this means that sharks have evolved to have thicker shark skin to keep them protected from injuries.
5. Let’s the Shark Swim Faster
The low weight, increased buoyancy, and greater flexibility, helps sharks swim faster than it would be able to with a bony skeleton and conserve energy.
6. Allows Jaws to Flex and Extend
Sharks use their flexible cartilage jaws to open their jaws wider to engulf their prey whole. Some sharks can even extend their jaws at lightning fast speeds to suck food into their bite.
7. Makes for a Harder Bite
The elastic jaw means that the shark can bite down with increased force as the cartilage springs back against the food.
8. Heals Quickly
Compared to bone, cartilage shark skeletons can heal very quickly. This allows the shark to recover comparatively easily from even relatively dramatic injuries.
9. Let’s Sharks Grow to Huge Sizes
Heavy bones would limit the maximum sizes that sharks can reach. Lightweight, flexible cartilaginous tissues have allowed sharks to evolve to their incredible sizes compared to bony fishes.
Are Sharks Vertebrates or Invertebrates?
As we’ve concluded that the answer to “do sharks have bones?” is no, does that mean sharks are invertebrates?
No, because sharks have a backbone, they are classed as a vertebrate species.
To be regarded as a vertebrate, an animal needs to have a backbone. A shark has a spinal column, albeit made from cartilage vertebrae that are hardened by calcium salts. These vertebrae protect the spinal cord in precisely the same way as boney vertebrae do.
Cartilaginous fish, including sharks, are one of the seven traditional classes of the subphylum Vertebrata. The other vertebrate species are mammals, reptiles, birds, amphibians, bony fishes, and jawless fishes.
As we’ve answered do sharks have bones, we’ve seen that they don’t have a single bone in the traditional sense. Instead, a shark’s skeleton has structures made from cartilage that is hardened to different degrees as needed by calcium.
Of the over 500 species of shark, none have shark bones. However, they have a highly evolved cartilaginous skeleton which enables them to float better, swim faster, flex to catch prey and escape predators, bite wider and harder, grow bigger and heal more quickly.
A calcified cartilage shark skeleton is the crucial factor that allows sharks to be what they are, one of the most magnificent creatures on our planet.
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Nicky is a British adventurer and animal lover who spends her time exploring the natural world and writing about her experiences. Whether on horseback, underwater, running, hiking or just standing with a fishing rod in hand, she embraces everything her adopted home of South Africa has to offer.