By Blaise Jones
Sharks are members of the group Chondrichthyes, which comes from the Greek words for “cartilage” and “fish.” This group of fish all share one major characteristic and that’s their skeletons. All the Chondrichthyes, which includes sharks, stingrays, skates, and chimera (or, less elegantly named, ratfish), possess skeletons made of cartilage. This feature is the main dividing point between sharks and the other fish in the sea, and it provides them with a myriad of advantages.
Buoyant, Bendy, Not-Bones
Cartilage is much, much lighter than bone, and because of this a shark’s skeleton makes up only 8 percent of their total weight. Since sharks do not have swim bladders, they are not naturally buoyant in the water. Their light skeletons reduce the amount of energy they need to spend on swimming. The more energy efficient a shark is, the higher their survivability is.
It’s not only the lightness of the cartilage that makes it a useful adaptation, it’s also how flexible it is. While cartilage is very tough, it is surprisingly pliable. This allows sharks to have a high degree of flexibility and some species can even turn around and bite something that’s grabbing their tail.
However, not every area of the shark’s skeleton is made out of the same type of cartilage. Certain important areas get special types of cartilage. The jaws and backbone of the shark, both of which have to endure heavy wear and tear, are made out of calcified cartilage. This unique cartilage is coated in calcium salts. This makes them stronger and more durable, like bone, while keeping them lightweight and sacrificing some of the flexibility.
Sharks skulls and noses are also made of specialized cartilage. The skull of a shark is made up of a denser cartilage than others, giving it more protection to the brain while still keeping the flexibility. The nose of the shark is made up a soft, spongy cartilage which is designed to absorb impacts and deflect damage, kind of like a built-in airbag.
A Joint Effort
Another benefit having a cartilaginous skeleton is a reduction in joint wear and tear. When bones meet at a joint, they rub against each other, causing them to degrade and wear away. Cartilage doesn’t do this. On top of this, sharks have shock-absorbing bags of fluid between their joints, known as the synovial capsule (5). This fluid packet works to reduce the friction between the joints.
Adding to their flexibility, sharks have strong, elastic straps of tissue that hold the joints stable, while at the same time allowing for the shark to over-extend its joints without damaging them. This allows sharks to bend in ways that’d normalyl damage the joints, and allows them to open their jaws and gills extremely wide when needed (5).
- “Sharks! The Mysterious Killers” by Downs Matthews
- “The Encyclopedia of Sharks” by Steve Parker