The skeleton is absolutely vital for survial. This network of hard structures protects delicate tissues and vital organs from injuries, provides support to hold the body in shape, and attaches to musculature, so that the animal can move. In many creatures, including human beings, the skeleton is made-up of dense bone. However, in the aptly-named cartilaginous fishes, the skeleton is instead made of cartilage. This is no mistake of evolution, but a highly advantageous adaptation that make sharks perfectly suited to their ecological niche. So why do sharks have cartilage rather than bone? What does a shark skeleton look like? And why have they evolved this way?
The Evolution of a Cartilaginous Skeleton
When we are talking about fish, scientists seperate different kinds of fishes into two broad groups: Chondichthyes and Osteichthyes. The "Chondrichthyans" (sharks, skates, rays and chimeras) differ from their "Osteichthyan" (aka bony fish) relatives because their skeletons are made from cartilage, not bone (Dean & Summers, 2006 & Inoue et al, 2010).
Chondrichthyan (Ancient Greek) =
χονδρ (khóndros) 'cartilage' &
ἰχθύς (ikhthús) 'fish'
The boney fishes and cartilaginous fishes diverged from each other around 420 million years ago (Inoue et al, 2010).
Within the cartilaginous fishes, scientists usually specialise in studying either the Chimeras (aka "Holocephalii") or the "Elasmobranchii" (sharks, skates and rays) (Dean & Summers, 2006 & Inoue et al, 2010).
Cartilage versus Bone
Human beings and many other animals have a skeleton made of bone. They may seem inanimate, but actually our bones are alive! They are made up living cells called "osteoblasts" and "osteocytes". Found at the surface of the bone, osteoblasts secrete a collagen-rich substance known as "osteoid", which becomes calcified after it has been deposited. The osteoblasts become trepped within this hardened material and convert into ostyocytes; montioring the bones for damage. This structure means our skeletons are strong, yet flexible (so they can flex slightly without breaking), but they are also quite heavy.
Comparatively, the skeletons of the cartilaginous fishes consist of cells called "chondrocytes" (cartilage-producing living cells), within a layer of fibrous material called the "perichondrium", that surrounds an extracellular matrix. The extracellular matrix is "calcified", meaning it contains lots of crystals of calcium phosphate. Thanks to this structure, the skeletons of sharks and their relatives are very strong, yet flexible, but they are much lighter than bone (Dean & Summers, 2006).
Why are Sharks' Skeletons Made of Cartilage?
Cartilage makes for an excellent skeleton because it's strength allows it to protect delicate internal structures, like the vital organs, but it's felixibility means it can bend without breaking. As we mentioned before, though, bone has similar properties. So why cartilage?
It might surprise you to learn that sharks are actually "negatively buoyant". This means that their bodies are heavier and more dense than the surrounding water - so they should sink. Many fish have combated this problem by evolving a gas-filled swim bladder to keep them neutrally buoyant. Sharks and rays are different because they do not have a swim bladder. Instead, they rely on buoyant oils in their liver to stop them sinking. Their fins are also important for creating lift as they swim. They have evolved to have a skeleton made of cartilage, because it makes their bodies lighter - meaning it is easier to maintain their buoyancy in the water.
Not All Cartilage is Created Equal
An interesting detail about shark skeletons, is that some cartilage is more or less mineralised, depending on where in the body you look. More calcified cartilage contains higher quantities of crystals in the extracellular matrix. By containing fewer or more crystals in the extracellular matrix, the cartilage can have different degrees of strength versus flexibility (Dean & Summers, 2006).
The areas supporting the extremities and appendages (known as the "appendicular skeleton"), like the fins, have limited mineralisation. This means these areas are strong, yet bendy. This is perfect for these parts of the body which experience shearing forces as they swim through the water (Dean & Summers, 2006 & Summers & Long, 2006.
In many modern species of sharks, especially heavily mineralised cartilage can be found in the vertebral column and in the jaws. What is unique about this "tesselated cartilage", is that the mineralisation is not continuous along the whole structure. Instead, the calcified surface layer in these bones is made up of plate-like tiles called "tesserae". Compared to a continuously mineralised structure, this plate-like format means the bones are less likely to crack under strain (Dean & Summers, 2006 & Summers & Long, 2006.
The vertebrae in the spine have a highly mineralised, tesselated layer on their surface, covering an unmineralised core. This provides the vertebra strength for their attachment to powerful swimming muscles (Dean & Summers, 2006 & Summers & Long, 2006).
Very heavily mineralised cartilage can also be found in the jaws, making the mouth very strong, with very little give. Tesselated cartilage is also found in the base of the tooth (known as the "peduncle"), where multiple layers of tesserae form powerful attachments for the teeth (Dean & Summers, 2006 & Summers & Long, 2006 & Fratzl et al, 2016).
For large species of sharks, like the great white (Carcharodon carcharias), which are apex predators, it is vital to have a strong bite force to disable large prey like seals and dolphins. Modelling how the cartilage responds to stress and strain from a great bite force has shown that varying degrees of mineralisation at different points within the cranium, allow the whole structure to have great strength, yet just the right amount of flex at cetiain locations, so that the jaw does not break (Fratzl et al, 2016 & Seidel et al, 2021).
The outer layer of the teeth are also sheathed in enamel. This enamel is extremely calcified cartilage with a high fluoride content and well defined structure of crystals in the extracellular matrix. The structure provides the teeth great strength, so they do not break as the shark bites down on its prey (Dean & Summers, 2006).
It is the calcified nature of the teeth and jaw which allow these structures to become fossilised. It is very common to find fossilised shark teeth, but rare to find entire skeletons because the cartilaginous structures with little mineralisation decompose before they can become fossilised.
If you are lucky and you know where to look, you can even find fossilised shark teeth just laying around at the beach!
References
Dean MN & Summers AP (2006). Mineralized cartilage in the skeleton of chondrichthyan fishes. Zoology, 109, 164–168. Access online.
Fratzl P, Kolednik O, Fischerc FD & Dean MN (2016). The mechanics of tessellations – bioinspired strategies for fracture resistance. Chemical Society Reviews, 45:2, 252-267. Access online.
Inoue JG, Miya M, Lam K, Tay B-H, Danks JA, Bell J, Walker TI & Venkatesh B (2010). Evolutionary origin and phylogeny of the modern Holocephalans (Chondrichthyes: Chimaeriformes): A mitogenomic perspective. Molecular Biology and Evolution, 27:11, 2576–2586. Access online.
Seidel R, Jayasankar AK & Dean MN (2021). The multiscale architecture of tessellated cartilage and its relation to function. Journal of Fish Biology, 98, 942–955. Access online.
Summers AP & Long JH (2006). Skin and bone, sinew and gristle: the mechanical behavior of fish skeletal tissues. In: Lauder GV & Shadwick RE (Eds). Fish Biomechanics: Fish Phsyiology, 23, pp. 141–178. Academic Press, San Diego. Access online.
By Sophie A. Maycock for SharkSpeak
