Outforia Quicktake: Key Takeaways
- Sharks do not have bones but instead have skeletons made up of cartilage, a soft and flexible tissue.
- They belong to a class of fish called Chondrichthyes, which also includes rays, skates, and sawfish.
- Cartilage in sharks allows them to float and swim, as they lack swim bladders that control buoyancy.
- The flexibility and elasticity of cartilage enable sharks to make sharp turns, open their mouths wider, and reach high speeds in the water.
- Sharks may have evolved from ancestors with a skeleton made of bone, not cartilage, as evidenced by a 2005 fossil discovery in Australia.
Swimming beneath the waves of every ocean is a class of predators that have haunted the nightmares of frequent beachgoers— sharks.
Sharks come in various shapes and sizes, from the tiny Lantern Shark to the giant Whale Shark. That aside, do sharks have bones? No matter the size or shape, there is one thing every shark has in common: they don’t have bones.
So, if they don’t have bones, what do they have? Do they gain anything from not having bones, or is it a hindrance? In this article, we’ll answer these questions and more.
First, let’s discuss what sharks have instead of bones.
Sharks are Chondrichthyes. This means they belong to a class of fish with cartilaginous skeletal structures. They are in the subclass of Chondrichthyes called elasmobranchs. This subclass includes rays, skates, and sawfish.
To better understand sharks and other Chondrichthyes, let’s look at the function of cartilage in mammals first.
The Function of Cartilage in Mammals
Cartilage forms the skeletal structure of mammals when they are in utero or still in the womb. Cartilage provides the framework for future bone development. Because of its flexibility and elasticity, it also makes exiting the birth canal easier.
There are three kinds of cartilage, each with its specific functions.
- Elastic cartilage: Found in the ear, nose, and trachea, elastic cartilage provides flexibility and elasticity to organs and body structures like the voicebox or outer ear.
Elasticity means that cartilage can be stretched and strained but will return to its normal shape. Try tugging on your earlobe, and you’ll see what I mean.
- Fibrous cartilage: Found in between vertebrae, in the knees, and other joints. Fibrous cartilage helps reduce friction in joints and stops bones from grinding together.
- Hyaline cartilage: Found in the ribs, nose, larynx, trachea, and other places. Hyaline cartilage cushions joints and coats bony services to allow smooth movement and minimal friction.
As mammals grow and develop, their cartilaginous skeleton is replaced with hardened bone, a process known as ossification.
Mammals need hardened bones to protect their organs and provide a solid structure to their frame. Without solid bones, mammals wouldn’t last long in the world. But sharks are a different story.
Cartilage in Sharks
Now that we understand the functions of cartilage and its purpose in mammals let’s look at the cartilage in sharks and see how it differs from us landlubbers.
First things first, just because sharks don’t have bones doesn’t mean they don’t have a skeleton. They do – it’s just made entirely of cartilage, and it still provides a frame for their bodies.
The primary function of a shark’s cartilaginous skeleton is to allow sharks to float and swim. Sharks, unlike bony fish, lack swim bladders that control buoyancy. Instead, they rely on their large, oily livers to keep them afloat.
Shark livers have a different function than human livers. Human livers, amongst other things, remove toxins from the body and process nutrients and other compounds. On the other hand, the primary function of a shark’s liver is to store oil.
Sharks can use their liver’s oil reserve as sustenance in place of food. They can go for long periods of time without eating. When their oil reserve gets too low, it means it’s time to hunt again.
Furthermore, shark livers are huge. They can make up 25% of a shark’s total body weight. One Basking Shark was found with a liver that weighed over 2,000 lbs (940 kg)! For comparison, human livers usually only weigh 3 lbs (1.3 kg).
A shark’s massive liver, along with their light skeleton, keeps them afloat.
If sharks had bony skeletons, they would not be able to swim as fast, or they would sink. Alternatively, they would need an even larger liver which would weigh them down even more.
Other than keeping sharks afloat, there are other benefits to having a cartilaginous skeleton.
- Flexibility: The added flexibility of cartilage lets sharks turn in ways other bony fish cannot. Flexibility is especially useful for catching prey and evading predators, as they can quickly make sharp turns in the blink of an eye.
- Elasticity: Because cartilage is elastic, sharks can open their mouths much wider than if they had bones. Once again, this is useful for catching prey and creates a greater bite force.
- Speed: Without the added weight of bones, sharks can reach incredible speeds in the water. The Shortfin Mako Shark, for example, can reach speeds up to 46 mph (74 km/h).
Sharks also have very sensitive noses that they use as sense organs to explore their environment.
Finally, cartilage is again helpful because if a shark bumps into something, the cartilage absorbs the shock and bounces right back into place.
Disadvantages of a Cartilage Skeleton
With every advantage comes a disadvantage, which is equally true for sharks. Doubtless, their lack of solid bone helps them in many ways. But there are some disadvantages to having a cartilage skeleton as well.
- Not as much protection: Sharks lack a ribcage, so their organs aren’t contained within a protective structure. The lack of a ribcage can cause problems if a shark gets beached because their organs can rupture under the pressure of its own weight.
Sharks do, however, have small sections of calcified cartilage in areas that need extra protection, such as the skull. Hardened cartilage is not bone but a more rigid, less flexible form of cartilage.
- Cannot repair as efficiently as bone: Cartilage cannot repair itself like bone. If it is damaged, the healing process is slow to nonexistent. However, sharks make up for this in their incredible ability to heal cuts and lacerations to their skin quickly.
Ultimately, despite the disadvantages, a cartilaginous skeleton enables sharks to live a life suited to their environment. If the cons outweigh the pros, sharks would have ditched their cartilaginous structures long ago for something different in the course of evolution.
And speaking of evolution…
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Did Sharks Ever Have Bones Before?
However, cartilage does not fossilize as well as bone, so few fully fossilized shark specimens are available for research. That’s not to say cartilage does not fossilize at all. Denser parts of the shark, like the jaw, spine, and snout, can fossilize.
By far, most of the fossil evidence we have of sharks comes from their teeth, scales, or skin imprints.
Scientists believed that sharks represented a very early stage in vertebrate evolution because there is no record of fossilized shark bone.
In other words, some organisms evolved to have bones while sharks, for one reason or another, held onto their primitive cartilage frames. But new research shows this might not be the case.
In 2005, paleontologist John Long found a fossilized shark in Gogo, Australia. After examining the fossilized cartilage, researchers found remnants of bone cells. This finding implies that sharks may have evolved from ancestors with a skeleton made of bone, not cartilage.
This means that sharks evolved to lose their bones, unlike other creatures that gradually evolved to have bones! This evolution towards being boneless probably came about through natural selection to aid sharks in catching prey.
For example, ancient sharks with bones may have struggled to catch prey compared to sharks with a slightly more cartilaginous frame.
Over time, sharks with more cartilage won out over sharks with bone, and this trait was passed on through generations until no bone-building genes remained in offspring.
Other Animals with Cartilage Skeletons
As previously mentioned, sharks aren’t the only animals without bones. They belong to a subclass of Chondrichthyes that includes rays, skates, and sawfish, all of which are close relatives of sharks.
Let’s take a quick look at these other cartilaginous creatures.
Stingrays evolved around 150 million years ago. From looking at them, you probably wouldn’t guess they’re related to sharks. They’re flat, lack the vicious jaws of a shark, and have long, skinny tales.
Stingray teeth are made of the same material shark teeth are made of. But unlike the razor-sharp teeth of their cousins, stingray teeth are flat and dull, used to crush rather than cut and tear.
Despite their name, stingrays don’t really have stingers. Instead, they have a sharp barb on their tail coated in venom. They will thrash their tails when disturbed and cut into the aggressor with their barb. The venom left behind can be fatal if the cut isn’t bad enough. Some stingrays even have enough force to pierce through wooden boats with their barbs!
It should be noted that stingrays are often docile creatures who avoid humans at all costs. There is no such thing as a stingray attack. Instead, there are accidents that occur when a stingray is stepped on, swum over, or otherwise threatened.
Skates are very similar in appearance to stingrays, both in looks and behavior. For example, both are disc-shaped, have their mouth and gills on their undersides, and have tails (although skate tails are usually shorter).
Similarly, both are bottom feeders, eat the same things, and often burrow under the sand for camouflage.
However, there is one significant difference– skates don’t have venomous barbs on their tails!
That’s right; skates are utterly harmless to humans.
Unlike most sharks and stingrays, skates do not give live birth. Instead, they are oviparous, meaning they lay eggs. Their egg sacs are black pouches that often wash up on beaches. In some places, they are humorously called “Mermaid’s purses.”
Interestingly, some skates have mild electrical organs, similar to the electric eel, but far less powerful. These pose no threat to humans and are believed to be used in communication between skates.
Perhaps one of the strangest looking ocean dwellers, sawfish are somewhere between looking like a ray and a shark. They have flattened bodies like rays but a shark’s length and dorsal fins.
Oh yeah, they also have saws for noses!
Sawfish technically belong to a family of rays. Some species can live up to 30 years and reach astonishing lengths up to 23 feet (7.0 m)!
Their sawlike noses, properly called rostrums, gave them the nickname “carpenter fish” and are used for hunting.
Rostrums can act like a shovel, digging through sand to expose potential prey. However, it is also used as a blade. The sawfish will thrash about in schools of fish, killing or stunning prey with its tooth-covered nose.
Unfortunately, sawfish are critically endangered due to overharvesting and entanglement in fishing nets. In fact, they’ve vanished from half the world’s coastal waters and other areas where they once thrived.
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Shark Cartilage in Medicine
Medicinal shark cartilage gained popularity in the 1970s. It was thought to prevent cancer. The reasoning behind this claim was that sharks don’t get cancer, so consuming shark cartilage might help prevent cancer in humans.
Except, this claim is wrong. Sharks do get cancer, and there is zero evidence that shark cartilage can prevent cancer in humans.
Shark cartilage is still sold today as a dietary supplement in capsule form. Some companies even claim shark cartilage helps with arthritis, psoriasis, joint pain, inflammation, and even wound healing.
However, there is no scientific proof to support these claims.
In a paper titled “Shark Cartilage, Cancer, and the Growing Threat of Pseudoscience” one researcher described the sale of shark cartilage as a triumph in pseudoscience and marketing schemes.
They also note that “the promotion of crude shark cartilage extracts as a cure for cancer has contributed to at least two significant negative outcomes: a dramatic decline in shark populations and a diversion of patients from effective cancer treatments.”
In fact, shark cartilage has been found to contain toxins that may lead to diseases like Alzheimer’s or Lou Gehrig’s disease.
In other words, there is little evidence in support of shark cartilage as an effective medicinal substance, and growing evidence suggesting it is not in any way beneficial and may even be harmful.
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Do Sharks Have Bones FAQ
Can a shark bite through bone?
Yes, some species of sharks have a strong enough bite force to crush and bite through bone.
For example, the Great White Shark (Carcharodon carcharias) is the world’s largest predatory fish and has a bite force of over 1.8 tonnes. For comparison, that’s 20 times stronger than the human bite force and half the estimated bite force of a T. Rex (whose bite force was around 3.1 tonnes).
If you need more convincing that a shark can bite through bone, then take the story of Bethany Hamilton for an example of what a large shark can do.
Bethany Hamilton is a surfer from Hawaii whose arm was bitten off by a Tiger Shark (Galeocerdo cuvier) when she was thirteen. Likely mistaking her for a seal, the shark bit her arm clean off right below the shoulder. She was rushed to the hospital and fully recovered despite losing 60% of her blood.
Bethany still surfs today. If you’d like to know more about her story, check out this article: Bethany Hamilton: The Surfer Who Lost Her Arm To A Shark, Then Came Back
Can shark skin cut you?
Yes, shark skin can cut you, or it can at least rub off some skin. Shark skin looks as if you could run your hand along it easily.
However, shark skin is actually made up of tiny, sharp scales. These scales are called dermal denticles and are found on sharks and rays alike. In actuality, they aren’t scales at all, but are modified, enamel-covered teeth! Dermal means skin or relating to the skin, and denticle means a small tooth. So, dermal denticle means “skin tooth.”
Shark skin is often described as feeling like sandpaper. If you’ve ever rubbed your hand across sandpaper, you’ll know that, while it doesn’t rip your skin off, it’s certainly not a pleasant feeling.
So, why do sharks have dermal denticles anyway? There are two main reasons. First, the scales point to the shark’s tail and reduce drag when swimming through water. The second reason for their thick, scaly skin is added protection, especially against parasites.
Do sharks fall asleep?
Yes… kind of.
Sharks don’t sleep like humans sleep. However, they can zone out for periods to rest and recharge. Sharks lack eyelids, so their eyes are always open even when resting. They are also constantly monitoring their surroundings.
Many sharks need to stay moving to filter oxygen through their gills. These sharks are called “obligate ram ventilators.” They draw water in through their mouths and force it through their gills. However, this is only true for some sharks.
Some sharks can remain stationary because they have unique structures called “spiracles.” Spiracles act as automatic oxygen filters for certain sharks, like the White Tipped Reef Shark.
In short, sharks have no actual sleeping schedules. Instead, they have wakeful periods and rest periods.