Cartilaginous Fish
Sharks are fish with a lot of specialized adaptations not found in other fish. What makes them so different is that they split from the rest of fish before mineralized bones became a thing. They; along with rays, skates, chimeras, and lampreys; still have skeletons made up of lightweight, flexible cartilage. Your body still remembers how to make cartilage, it's in your nose, your ears, your joints, your windpipe, your ribs, and it's the cushion between the bones in your spine.
Cartilage is light, flexible, and easy for the body to repair. As a result, many sharks are highly manuverable mouth missiles.
Ampullae of Lorenzini
What's the point of being highly manuverable when your eyes are on the side of your head and your mouth is on the bottom? How do sharks even know where their food is when they're chasing it? They have an extra sense! Mostly around their snout, they have sensory organs called Ampullae of Lorenzini. Ampullae of Lorenzini are pores which can sense electromagnetic fields. It just so happens that everytime a nerve fires or a muscle contracts, an electromagnetic field is generated. Now imagine a fleeing fish! Sharks can sense, not life, but the natural consequence of life.
Ampullae of Lorenzini are like blackheads full of conductive jelly. They transmit the electric current from the surrounding water directly to a nerve at the bottom.
With enough of these things you could generate a fuzzy sense of wireless touch, but only for animals or underwater cables.
It's also thought that maybe sharks use this organ to tell where they are by sensing the Earth's magnetic poles.
Dermal Denticles
Sharks don't have scales like other fish. Sharks are covered in dermal denticles! Dermal denticle literally means little skin tooth. Just like our teeth, they have a fleshy core with nerves and blood vessels, a hard case of dentine, and an enamel coating.
The denticles are different between different species but also between differnt parts of the shark. The shark experiences different drag forces all over and maybe needs more protection in some places, so they have a bunch of differences all over! I think they're really pretty, but I don't really like the idea of that many teeth rubbing together.
Infinite Teeth
When you know the whole shark is covered in teeth, it only makes sense. Sharks continuously generate new teeth deep in their jaws and slowly push them to the front. Even filter feeding sharks have teeth, they're just really small.
Sharks Don't Drink Water
It's not very well known that bony saltwater fish have to drink water. The ocean is naturally more salty than inside their body so they must drink water and excrete the salt back out. Sharks maintain a high concentration of urea in their blood, high enough that water naturally enters their blood through their gills via osmosis. This gives them a little bit of bouyancy as well! Even though they don't drink water, they still have kidneys that clean their blood and they urinate out of a cloaca.
Must they keep swimming?
Some sharks have to keep swimming or they'll suffocate. These are called Obligate Ram Ventilators, because they are obliged to ram water into their mouths in order to breathe. The Great White Shark is an excellent example of this. It is a big, high energy shark that needs a lot of oxygen to keep moving and it needs more the faster it goes! The Whale Shark isn't so fast but it's so large that it's an obligate ram ventilator as well. The alternative is called Buccal Pumping, where the muscles in the mouth and throat are used to pull water in through the mouth or holes called spiracles. This lets the shark sit still and continue breathing.
No Swim Bladder
Sharks also managed to split from the rest of fish before the most basic of lungs was ever developed. In some fish, this would become a true lung like ours. Other fish turned it into an internal balloon to manage their buoyancy. Sharks got neither! Some sharks store a lot of fat in their livers to help them float while others just sink to the bottom when they aren't actively swimming.
