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.
I learned a TON more than I expected! I get to butcher large boney fish regularly but have never butchered a shark..... They would be completely different than a large bony fish....