Sharks are species of fish that can be found in all of the oceans of the world. If you go by the broadest possible definition they have been around for over four hundred million years, making them one of the oldest species on Earth.
Clearly, sharks are great adapters, winning the evolutionary game time and time again. They have developed a range of adaptations that has given several shark species a reputation as biological killing machines and feared apex predators.
Some attributes that are characteristic of sharks are five to seven gill slits on the side of their heads, and pectoral fins that are not fused to the head. But did you know that sharks have skeletons that differ from that of most species, too?
In this article we’ll discover what shark skeletons are made of, and why this adaptation is important. But first, a quick look at skeletons in general.
How Do Shark Skeletons Differ From Other Species?
When most people think of the word skeleton, they think of the bony structural frame of a body- usually in the form of a human skeleton. But every species has a different skeleton shape, and there are different types of skeletons, too.
Some creatures, as in many invertebrates, have an external exoskeleton that protects the soft tissues within. Others, like earthworms, have hydro-skeletons, which are flexible skeletons supported by fluid pressure.
Vertebrates, like you and me, have an endoskeleton. In other words, we have a sturdy internal support structure, which in the case of humans and most vertebrates is made of bone.
Sharks, being vertebrates, also have an endoskeleton. So shark skeletons are made of bone too, right?
Do Sharks Have Bones?
Well no, actually.
Unlike the vast majority of vertebrates, which includes all mammals, birds, reptiles, amphibians, and fish, sharks do not have skeletons made of bone. You might think that makes sharks relatively unique, but in fact vertebrates make up just 5% of all known animals.
Sharks belong to a unique class of vertebrates known as Chondrichthyes, which have skeletons but not bones. So what are shark skeletons made of?
What are Shark Skeletons Made Of?
Sharks are a group of cartilaginous fish in the Elasmobranch subclass of Chondrichthyes. This means that they have skeletons that are made entirely of cartilage and connective tissue rather than bone.
You might wonder how cartilage alone is enough to support a shark’s body. Well in key parts of the shark skeleton, it isn’t. The backbone, for instance, is the key part of the shark skeleton, holding everything together. When a shark swims powerfully, it puts a lot of pressure on the backbone. Think of shark jaws, too. Sharks have to bite and hold with lots of pressure to catch their prey. So how can they do that without any bones?
Well, the backbone and jaw of a shark are made up of calcified cartilage. This is when calcium builds up around the cartilage, hardening it. This calcified cartilage ends up resembling bone, but is in fact just hardened cartilage. This gives the shark the necessary support in key parts of the skeleton whilst retaining the advantages of having a bone-free body.
Why Doesn’t a Shark Have Any Bones?
Naturally, sharks have adaptations that make them better suited to their environment. If you think of it this way, you can start to understand why sharks have no bones.
Firstly, many species of sharks are apex predators at the top of the food chain. Therefore, they need to be able to catch their prey to survive. This means being both quicker and more agile than the creatures they are chasing, and this is where the cartilage comes in.
Cartilage is about half the density of bone, so naturally, the shark will be able to propel itself through the water quicker. Think of it like race cars. The lighter, the faster! Cartilage is also more flexible than bone, allowing a pursuing shark to outmaneuver its prey.
Having a cartilage skeleton is also more energy efficient. It takes the shark less energy to travel at the same speed as a comparatively sized boned creature, preserving energy and improving the shark’s stamina.
Another reason sharks don’t have a skeleton is that, unlike bony fish, sharks don’t have a swim bladder to keep them buoyant. Therefore, if a shark had a skeleton made of bone, it would struggle to float and sink to the bottom of the ocean.
Unlike humans and most vertebrates, a shark skeleton is not made out of bone. Instead, a shark’s skeleton is entirely composed of cartilage and connective tissue.
This has given the shark several evolutionary advantages that have allowed it to catch prey easier and to out-compete its competitors.