1. What Does "No Bones" Mean for Sharks, Rays, and Chimaeras?
Cartilaginous fish include sharks, rays, and chimaeras. Except for teeth, their entire skeleton is made of cartilage [1].
According to the Hawaii Division of Land and Natural Resources (DLNR), a shark’s skeleton has a shape very similar to that of other fish. The difference is that it is made of cartilage, not bone [2]. It is more accurate to read this as "the material of the frame is cartilage" rather than "there is no frame."
2. Why Cartilage Makes a Shark's Skeleton Light and Flexible
The Natural History Museum in London explains that the density of cartilage is about half that of bone [3]. This means it is lighter when comparing the same size, not that the weight is exactly half. This figure is from a museum explanation, so it is best to treat it as a rough guide.
Cartilage is not just soft. The cartilage in the jaw, spine, and around the brain is reinforced with calcium salts [3]. The structure combines lightness and flexibility with strength where it is needed. The same explanation states that a light, flexible skeleton helps the shark bend its body quickly [3]. However, this is a general explanation for the public.
3. How Liver Oil Helps Sharks Float Without a Swim Bladder
Cartilaginous fish do not have swim bladders. Instead, they store oil (liver oil) that is lighter than water in their livers to adjust buoyancy [1]. In the case of the blue shark, the liver is said to make up about one-fifth, or 20%, of its body weight [5].
The Australian Museum explains that some deep-sea sharks have livers that make up to 30% of their body weight. Using a light oil called squalene, these sharks can achieve an almost neutral state where they neither float nor sink [4]. Note that "30%" is an upper limit for some deep-sea species, not all sharks.
Even so, sharks still sink slightly. The Hawaii DLNR states that all sharks have a property of sinking slightly (negative buoyancy) [2]. Even with light cartilage and an oily liver, they do not float completely. It is closer to the truth to think of them as "harder to sink" rather than "floating due to oil."
4. How Tile-Like Mineral Blocks Reinforce Shark Cartilage
The cartilage of cartilaginous fish is often reinforced with small blocks hardened by apatite, a mineral containing calcium. These calcified parts are called "tesserae," and they can be arranged like tiles [6].
These blocks cover the outside of the core of uncalcified cartilage [7]. This is considered a derived feature specific to cartilaginous fish [7].
"No bones" does not mean "not hard at all." A shark’s body is not entirely soft cartilage. There is a layer of tiles on the outside of the cartilage, and especially hard parts are reinforced. Both "cartilage is flexible" and "there are hard parts" are true.
5. Did Shark Ancestors Have Bones That Were Later Lost?
Reports suggest that the common ancestor of jawed vertebrates, and the group leading to cartilaginous fish, had bony internal skeletons [7]. The view is that shark skeletons did not "start as cartilage" but "changed from ancestors with bones."
A fossil fish called *Minjinia* was found in Mongolia. It lived about 410 million years ago (Early Devonian) and resembles placoderms. Its bones showed a reticulated structure of endochondral bone similar to that of bony fish [8]. This is a clue suggesting that the light skeleton of sharks may have evolved from bony ancestors.
Another review estimates that cartilaginous fish lost dermal bones and other features more than 400 million years ago based on fossil evidence [6]. However, this is an "estimate," not a certainty.
Why return to cartilage? A fossil fish expert at the Natural History Museum in London explains that shark ancestors likely had bony skeletons and returned to cartilage for lightness and flexibility [3]. The benefit of being able to bend quickly is also mentioned, but this is at the level of "said to be a benefit." No research paper in these materials definitively states that they evolved to become lighter.
6. How to Feel Cartilage in Your Own Ear and Nose
You can try this today: gently pinch and bend the rim of your ear or the tip of your nose. This specific tissue feels softer than bone and returns to its shape immediately, giving you a sense of what cartilage might feel like. Next, compare the descriptions of shark and other fish skeletons in a fish encyclopedia at your library or home.
If you go out: At an aquarium, observe how sharks twist their bodies. Do not tap the tank and do not use a flash.