By Blaise Jones
Sharks do have scales, though they are very different from the scales of bony fish. While it varies from species to species, bony fish have scales typically made out of a mixture of bone, collagen, and keratin. Shark scales, however, are made out of teeth.
Sharks have what are known as dermal denticles, which translates to “tiny skin teeth.” Shark scales are considered teeth both because they grow similarly to how teeth grow, and because they are made of the same materials. A dermal denticle is anchored by a strong, flexible material called collagen to the lowest part of a shark’s skin, the dermis. From there it pushes upwards through the upper layer, the epidermis.
The interior of each denticle is made up of soft tissue known as the pulp cavity. The pulp is surrounded by one of the toughest biological materials out there, dentine . In sharks, their dentine is made up of a crystalline mineral called apatite. After that, the denticles is covered by an even harder material, known as enamel.
This combination of hard and soft materials give shark skin the hardness of granite (it’s a 5 on the Mohls scale of hardness, making it harder than copper, brass, and iron and the toughness of steel, while at the same time keeping it flexible and non-rigid. A shark’s skin is roughly as tough as chain mail armor.
Different Denticles, Different Duties
Sharks have different types of denticles for different areas of their body. On their snout they’re rounded and blunt, forming a more protective layer over their sensitive snouts. On a shark’s stomach they tend to be flat and broad, while on their sides the denticles are long and narrow, both of which make them more streamlined. And just like a shark’s teeth, their denticles never stop growing in. The scales grow in and drop out regularly, and never stop until the death of the shark.
Sharp Streamlined Shark-skin
Besides providing an excellent defense, shark skin is also incredibly streamlined. Each denticle has a series of grooves carved into them. Each groove is closely spaced together, which prevent eddies and vortexes in the water from actually coming into contact with the shark’s scales, greatly reducing drag (5). In recent years scientists have discovered the principal themselves, and you can find similar grooves in the designs of everything from golf balls to military aircraft. In fact, scientists have directly copied to design of shark skin for swim suits, greatly increasing the speed of swimmers. These suits were so efficient that they’ve been banned from the Olympics for providing an unfair advantage.
Shark skin actually makes them silent swimmers. When a human enters the ocean, to them the ocean is a silent void. However, the ocean is actually very loud. Besides the noises made by marine mammals and tidal actions, the movement of fish actually makes tons of noise. However, since shark skin actively repels water, this makes the shark absolutely silent as it swims. Lower a hydrophone next to a bony fish and you’ll hear a sloshing noise. Lower the same hydrophone next to a shark, and you’ll hear nothing.
Different Sharks, Different Skin
While all sharks share the same type of skin and scales, those scales aren’t necessarily the same shape. Like their teeth, appropriately enough, the shape of a shark’s denticle is determined by their lifestyle. The bramble shark has knobbly denticles with a sharp, curved thorn in the center. The goblin shark has long, thorny denticles almost the size of barbed wire. Both of these sharks are slow moving animals and thus their denticles are more focused on defense.
Fast moving, open ocean sharks have smaller, lighter denticles than the denser, more armor-like denticles of bottom dwelling and slower sharks. On top of this, the denticles of fast moving sharks consistently have longitudinal ridges. According to R. Aidan Martin, of elasmo-research.org, an experiment performed in 1986 found that the faster sharks have ridge shapes that matched the predicted morphology that’d optimize bursts of speed, and decreased drag as much a 8 percent.
- “Sharks! The Mysterious Killers” by Downs Matthews
- “The Encyclopedia of Sharks” by Steve Parker