Are sharks vertebrates or invertebrates? Even though they don’t have any bones in their bodies, sharks are classified as vertebrates.
They belong to the class Vertebrata, a subphylum that includes five different classes – amphibians, birds, fish, mammals, and reptiles. Although the shark’s skeleton is made of cartilage, rather than bone, it has a cartilaginous backbone which qualifies sharks as vertebrates.
As with all other vertebrates, sharks also belong to the phylum Chordata. Chordates possess, at some point during their lifecycle, the same four primary characteristics. These are:
Notochord – a cartilaginous skeletal rod that supports the body. This is present in all embryonic chordates and some adult species.
Dorsal Hollow Nerve Cord – growing out of the embryo’s cells, the dorsal hollow nerve cord develops into a vertebrate’s brain and spinal cord
Pharyngeal Slits – these openings are located behind the mouth and allow for water to exit the mouth during feeding. These are particularly important for filter-feeders like the whale shark.
Post-anal Tail – an elongation of the body behind the anus which assists with locomotion, balance, and communication.
Why Are Sharks Vertebrates if they have no Bones?
Sharks are considered vertebrates even though they have no bones to speak of. Their vertebral column is made up of cartilaginous tissues, not bones, but acts in a similar manner.
More flexible than bone, cartilage is the same clear, gristly stuff in your ears and nose tip. This elasticity makes sharks fast swimmers that can maneuver quickly.
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Replace those cartilaginous tissues with bone, and the shark would no longer be the fast and efficient apex predator we know today, nor would it grow so big.
Unlike bone, cartilage doesn’t stop developing, so sharks continue to grow throughout their lives. That explains why some large sharks can reach lengths of over 30, or even 60 feet!
While cartilage has its advantages over bone, it’s not as strong, so doesn’t offer the same level of protection. Sharks have developed a thicker skin to compensate for that problem.
Having a cartilaginous skeleton may give sharks speed and maneuverability, but it also makes them more susceptible to injury. On the plus side, cartilage heals much faster than bone, so sharks can recover quickly even from serious injuries.
Another benefit of cartilage is that it’s flexible enough to allow the shark to open its mouth much wider than it would be with bone, increasing its bite force.
Having cartilage instead of bone also improves the shark’s buoyancy. Most fish have an internal gas-filled organ known as a swim bladder.
This enables them to control their buoyancy and maintain their depth in the water without expending huge amounts of energy.
Although sharks are fish, they lack this adaptation and rely on their lightweight cartilaginous skeletons to aid buoyancy. Without that, and their buoyant livers, sharks would sink.
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Function of the Shark’s Vertebrae
The primary role of a shark’s vertebrae is to protect the spinal cord and internal organs against injury. It also provides balance and structural support to the body while enabling flexible motion.
The shark’s vertebra also helps propel the shark through the water and assists it when braking.
In a study of dogfish sharks, researchers found that the shark stores energy within its vertebrae by “compressing the bulky centra structures that stack together to build the spinal column.”
The elasticity of the shark’s vertebra means it can “provide more propulsion as the animal increases speed while increasing braking power” as the shark slows down.
This ability is, researchers say, due to the “vertebral column’s unique combination of material properties.”
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Is a Shark a Mammal or a Fish?
The shark is a relatively unique creature and different shark species feature different characteristics from both mammals and fish.
Taking each feature, we’re going to scrutinize the shark’s anatomy and behavior to see whether it should fit into the class Mammalia or carry on hanging out with other cartilaginous fish.
Mammals are Warm-Blooded
Most sharks are cold-blooded, like other fish, but a few have some endothermic capabilities.
Using a unique system known as a counter-current heat exchange, shortfin and longfin makos, porbeagle, salmon, and great white shark can maintain a body temperature higher than the surrounding water.
Mammals Give Birth to Live Young
Around 60% of shark species give birth to live young. Of these, at least 16 species are viviparous, meaning that their pups live off the mother’s placenta until their ready to be born.
Some examples of viviparous sharks include larger shark species like whale sharks and hammerheads and small sharks like the whitecheek and sliteye.
Mammals Possess Different Types of Teeth
Mammals are heterodont, with different types of teeth for different purposes.
The human, for instance, has incisors, molars, and canines. While some sharks, like the whale shark, are homodont with a mouth full of small and hook-shaped that vary only in size.
On the other hand, the longfin mako is a heterodont shark species. It has two different types of teeth, each of which has a distinct purpose.
The grasping teeth are used to anchor its prey, while the shark uses its cutting teeth to tear off strips of flesh.
Sharks and Mammals Belong to Different Classes
Despite these characteristics, sharks belong to the fish family, primarily because they lack any mammary glands so don’t produce milk for their young. Sharks neither sweat nor have fur or hair, which mammals all do.
Sharks breathe through their gills, rather than lungs, which is another prerequisite for entrance into the class Mammalia.
So, if a shark is a vertebrate but not a mammal, what are they?
Like other cartilaginous fish, sharks belong to the class Chondrichthyes.
They are joined in this class by other deep-sea species, including chimeras, rays, and skates. Bony fish belong to a separate class known as Osteichthyes.
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Each member of the Chondrichthyes has certain anatomical similarities, including:
- A pair of powerful jaws
- Cartilaginous endoskeleton strengthened with the deposits of calcium salts
- Skin covered with small tooth-like structures known as placoid scales or denticles
- A brain and spinal cord that are protected by a vertebral column
- Sensory cells in the lateral line that detect motion, water pressure, and vibrations
All sharks also have a heterocercal tail, or caudal fin, which means that one lobe is an extension of the vertebral column and is longer than the other.
Different shark species have adapted this feature to suit their hunting needs, body size, and strength.
The thresher shark, for instance, uses its long caudal fin to stun its prey, while others rely on it almost exclusively for creating thrust “directly through the center of gravity of the fish.”
Conclusion
Sharks are vertebrates without bones, replacing the bony skeleton with a cartilaginous structure that is more buoyant and flexible than bone.
Although some species of sharks are endothermic and give birth to live young, called pups, they still don’t manage to meet the stringent requirements to enter the class Mammalian.
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Regarded as fish, they belong to the class Chondrichthyes, which can be traced back hundreds of millions of years.
They extract oxygen using gills, rather than lungs, lack swim bladders and have powerful jaws that are, due to their cartilaginous structure, capable of greater bite force.
As always, sharks remain something of an enigma, not quite fitting into one class or another. As they’ve evolved, it seems that sharks have taken the best of both worlds to become the dominant predator species of the ocean.
British-born Dan has been a scuba instructor and guide in Egypt's Red Sea since 2010.
Dan loves inspiring safe, fun, and environmentally responsible diving and particularly enjoys the opportunity to dive with sharks or investigate local shipwrecks.
When not spending time underwater, Dan can usually be found biking and hiking in Sharm's desert surroundings.