Why Don’t Sharks Have Bones?

Why Don’t Sharks Have Bones?


Hello! This week is Shark Week on the Discovery
Channel, which some of us here on the internet aren’t huge fans of, so instead of sensationalizing
sharks or just making stuff up about megalodons still existing, we thought we’d talk about
science with the understanding that reality is actually pretty fascinating. Sharks and rays and skates are probably the
most complex organisms that don’t have bones, but does this mean that they’re less developed?
No! Sharks are simply part of a different class of fish. Literally, the class chondrichthyes,
which includes sharks and skates and rays and a very small group of various small little
things called holocephalis. Those are all fish with jaws and scales and chambered hearts
and a skeleton made out of cartilage. [Intro] Bones are awesome. They provide a nice rigid
internal structure, they produce red blood cells and provide firm anchors for our muscles
to attach to so we don’t just slump to the ground in a pile of goo. Indeed bony fishes,
osteichthyes, are the largest class of vertebrates in the world, with over 28,000 species. Now sharks might look like just another fish
to you, but bony fish are actually more closely related to us than they are to sharks. Terrestrial
vertebrates like us only diverge from fish around 300 million years ago while chondrichthyans
and osteichthyans broke off from each other around 400 million years ago. For all that
time, sharks and other chondrichthyans have been evolving parallel to bony fish, and boy
do they have some weird features to show for it. Sharks actually split off from fish before
the development of scales, and yet both sharks and bony fish have scales, or do they? Turns
out, shark scales evolved completely separately and are thus completely different. Sharks
didn’t have the necessary biochemical machinery to make scales. However, they did, pretty
obviously, have all of the systems in place for creating teeth, so their scales, which
cover their body, and each are fed by blood vessels, are covered in, get this, dentin,
the same stuff that makes up the inner layer of your teeth. So sharks don’t really have
scales so much as skin teeth, and indeed shark scales are sometimes referred to as dermal
denticles. Sharks also have red blood cells, but we know
that red blood cells are made inside of bones, right? Well, not 400 million years ago they
weren’t. Sharks produce red blood cells in their spleen as well as their epigonal organ
which surrounds their gonads and in a bunch of tissue around their esophagus called the
Leydig’s organ which is unique to chondrichthyans. So we’ve gone some way in discussing HOW sharks
can survive without bone, but that doesn’t help us understand why. I mean there are lots
of classes of fish that weren’t able to overcome extinction events or that were outclassed
by bony fish, so how come sharks, at the tip top of the food chain, manage to survive despite
their seemingly primitive skeletons? Well, that’s kind of going about it the wrong way.
The question isn’t so much why the top predators of the sea have cartilaginous bones, it’s
how a cartilaginous skeleton helps an animal be an effective predator. Because obviously
it does. If it didn’t, they’d be extinct. Okay, so ecologically we know that predators
have to have wider ranges than prey animals. They also have to be as fast or faster than
their prey, and possibly even more important than that, they have to be maneuverable. So
how does cartilage help? Well, first and most simply, cartilage is lighter and more flexible
than bone allowing sharks to weigh less and bend their bodies at sharper angles than bony
fish. It’s a nice combination that increases both power and maneuverability. But that’s only half the story, and here’s
where we get to the true weirdness of the chondrichthyans. Because unlike pretty much
every other vertebrate, most of their muscles don’t even connect to their skeleton! Now of course some muscles do connect to a
shark’s skeletal tissue, their jaws being the obvious example, but their primary locomotive
muscles connect to a stretchy, helical network made of collagen that’s just beneath their
skin. Arthropods use a similar arrangement. Their muscles connect directly to their exoskeleton
which, if you’ve ever been chomped on by a crab, you know is a pretty efficient arrangement.
By grabbing and pulling on the outside of the animal rather than connecting to the spine,
the muscles have greater leverage. Cooler still, this collagen framework acts
kind of like a rubber band or a spring. As the shark bends itself, the collagen stretches
on one side and energy is stored in that tissue. Then the shark simply has to relax for the
collagen to push itself back into place and even over correct a little into the next push
of its tail. The energy of every push helps fuel the next one. So, there you have it! Sharks — their lineage
may be older than those young bony fish, but that doesn’t stop them from sitting happily
atop the oceanic food chain. Thanks for watching this episode of SciShow
and choosing science over sensation. Some other YouTube channels are talking about the
science of sharks this week as well, including It’sOkToBeSmart and on the Brain Scoop there
will be five full days of shark stuff. Check it out and enjoy and thanks for watching SciShow.

100 Replies to “Why Don’t Sharks Have Bones?”

  1. ah, the modern miracle of science.  turns out we are not related to bony fish in the way we thought.  we are more closely related to placoderms,.

  2. Why when you brings dogs inside from playing outside they have that "dog" smell, but the same can not be said for humans?

  3. Sharks have bones look up shark skeleton! The only bones they have are in the fins, jaw, teeth, vertebrae and ribs!

  4. Are there types of sharks with bones? Cause in museums and stuff there are shark bones hanged on ceilings

  5. This is by far the most “WAIT WHAAAAT?!” Video I’ve ever seen on this channel. I feel like i was lied to my whole life(nobody ever mentioned this before now) and learned so much so fast I’m just astonished. Amazing

  6. I got Stockholm Syndrome for sharks when I did a shark cage dive in Hawaii a few years ago. They didn't want to hurt us at all, but so many had fishing hooks trailing from their mouths and fins. One had clearly been hit by a boat propeller, because it had three jagged scars along it's back. We were more dangerous to them them then they were to us.

  7. I once dissected a shark, them having no big tough bones made it quite easy, although I didn't expect them to have such a big liver.

  8. What would happen if our muscles connected to our skin? Would we have better leverages and be stronger? Or would our skin rip from the immense pressure?

  9. THANK YOU! its so rare lately to find a real shark week documentary and not just a "killer shark eith a taste of prey" show

  10. One thing I'm still confused about. If we diverged from fish like sharks did, but later. And we're obviously not considered fish, why are sharks considered fish still? May be a dumb question but I'm just wondering.

  11. Im terrified of the sight of sharks… But they fascinate me… And I love scishow… This is gonna be a long episode

  12. I never thought about why sharks don't have bones, but now I know and it was a thrill ride of emotions.

  13. Why do people continue to say sharks are at the top of the food chain when we know it's Orca as the one true apex predator.

  14. Seriously, Hank, you've gotta stop doing so much coke before you appear on camera.
    Your voice & mannerisms are getting into the realms of parody lately. (You're more like a marionette than a man!) It can't be good for your health either…
    Why don't you take a break, get yourself cleaned up & let Olivia take over for a while?

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