Sharks are beautiful killing machines. I’m from San Diego, but a fear of sharks keeps me from ocean swimming. Sharks are the fastest fish, the cheetahs of the sea. When I’m kayaking with my family, I imagine sharks striking the flimsy boat from out of nowhere. The last thing I needed was to learn more about how sharks are great killers.
But last week, I found myself at a lecture about shark skeletons. (I needed a topic for today’s column.) Like a good horror movie, the lecture was terrifying but thrilling. Dr. Adam Summers, an associate professor the University of Washington, studies sharks. He gave a lecture on their unique construction last Wednesday.
Sharks don’t have bones like humans. They have skeletons made of cartilage – the same stuff that forms your nose and ears. To gain momentum in the water, boney skeletons appear ideal: The stiff bones and flexible joints allow for speed and range of motion. Summers referred to cartilage as a “crappy building material” because it’s soft and flexible. Stiffness is required for speed – soft skeletons would bend too much for a thrashing shark to move. Yet Summers pointed out that mako sharks can move faster than swift boney fish like marlin.
He’s discovered shark bones do have a cartilaginous “gooey core,” but the outside of the cartilage is tiled with mineralized plates of harder cartilage. Picture a soft mozzarella stick with a hard, breaded shell. Sharks have 112 precaudal (no spines or ribs attached) vertebrae “stacked like coins,” Summers said. These vertebrae are flexible because they’re held together by overlapping layers of collagen. Between the harder cartilage and the flexible collagen, shark skeletons allow for speed.
The second secret to shark speed lies in their skin. Sharks have strong, thick skin made up of muscular fibers. Summers calls the skin a “whole body exotendon.” Shark skin acts as a fiber-wound cylinder, which bends when inside pressure increases. With the ability to get faster with a pressure build-up, the shark can out-swim other fish. The marlin, for example, has thin skin, so it lacks this advantage.
“The marlin from a standing start can’t do very well,” said Summers.
Like a good car, the shark can move fast from a standing stop and “as it gets faster, it does better,” Summers added.
When sharks hunt marlin, they strike from an oblique angle. Summers compared this to football, saying sharks take out their prey “like a free safety on a wide receiver.” Sharks like to immobilize their prey by chomping the tail fin. Once that’s ripped off, the fish is a goner.
The problem is pressure on a skeleton wears it out quickly. Unlike human bones, shark cartilage does not heal itself. After so many high-speed chases, they have no way to repair fatigue damage. Again, sharks have found a way around that problem.
The mineralized skeleton tiles, called tesserae, are tied together with fibers that compress under pressure.
“You can resist compression much, much better than you can resist tension,” Summers said.
Humans follow this rule themselves when they choose to lift heavy objects with their knees rather than their backs. Sharks’ gooey bone core is also useful because it is viscous – it absorbs pressure. Once the pressure leaves, the strain is gone.
I’m scared of sharks, but I’ve got to respect the animals. Millions of years of evolution have perfected a creature that humans, a brilliant species, can’t duplicate. We’ve tried growing cartilage, but we’ve failed. We’ve tried using similar kinds of compartmentalized pressure systems in submarines, but it’s really difficult.
Bulletproof vests are a unique field where we lucked into a shark-like technology. A kind of ballistics armor used today in the military has hexagonal tiles (like shark tesserae) over a core of foam aluminum. Bullets and strain energy are absorbed by the foam, much like energy is absorbed by shark cartilage. The vests weren’t made with sharks in mind, but “it looks a lot like the shark material,” Summers said.
Bulletproof vests are great, but humans are still vulnerable with our soft skin and dull teeth. If I’m ever going to brave the oceans deeps, we humans have a lot of catching up to do.
MADELINE MCCURRY-SCHMIDT bans family members from humming the Jaws theme song when she’s at the beach … or a lake … or a swimming pool. It’s not funny, guys! Contact her at memschmidt@ucdavis.edu.
