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 Holocephali. 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 diverged 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's Okay To Be Smart and on the Brain Scoop there will be five full days of shark stuff. Check it out and enjoy and thanks for watching SciShow.