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So we spent the last few days talking about sharks here on The Brain Scoop but we haven't really answered the question, "What is a shark?"

Well duh, Emily, they're major fearsome apex predators with razor-sharp teeth and sometimes their craniums more closely resemble hardware equipment than it does feasible skulls, and on occasion they swim around with their mouths gaping open as though they're stuck in a perpetual state of dumbfoundedness.

Right? What makes a shark, a shark?

We're going to keep it relatively simple in our explanation. So, for our fundamental learning purposes, there are three types of fish that are still swimming in our oceans today. 

There are the jawless fish, like the hag fish and the lampreys; bony fish are those with skeletons of bone; and the cartilaginous fish, those with skeletons made out of cartilage, the Chondrichthyes.

Sharks are one of two sub-classes within this cartilaginous order, the other being the chimeras, which includes the creatively named rat, rabbit, and elephant fishes. So, in the taxonomic sense, a shark is any member that falls under the sub-class Elasmobranchii, which has twelve orders of its own.

Let's get started.

Hexanchiformes are the most primitive order of shark, many of which are so ethereal looking that they perpetuate myths of sea serpents. You're probably most familiar with the frilled sharks in this order, which have a stunning number of needle-like teeth in rows and bodies that more closely resemble eels than they do sharks. 

There's Carcharhiniformes, the ground sharks, which is the largest order and contains some of the most recognizable species. It includes such creatures as blue sharks, tiger sharks, and hammerheads - like this one. An extreme example of the hammerhead variety is the winghead shark, whose head - with its incredible binocular vision - is about 50% as wide as the entire animal is long. 

Squaliformes is another huge order, which includes some strange animals like the sleeper sharks, which look more like manatees than anything. Some of these sleeper sharks live in deep, frigid, polar waters, like the Greenland shark. Its skin is extremely poisonous, due to having high concentrations of TMNO, trimethylamine N-oxide, which works as an anti-freezing agent by stabilizing enzymes and proteins in its flesh.

Next are the Heterodontiformes, the bullhead sharks. These sharks are relatively small, but they have some of the more interesting eggs of the animal kingdom. The egg of the uncommon crested bullhead shark possesses long tendrils that are attached to a conical spiral, which are used to anchor itself to seaweed or nearby sponges. Also, the Port Jackson shark has a face that looks like a pig. 

The Lamniformes are the mackerel sharks. The most famous in this order is the great white, but it also includes its weirdo deep sea cousin the goblin shark, which has an uncanny ability to hyper-extend its jaw during feeding. Basking sharks, the world's second largest fish, only smaller than whale sharks, are in this order too.

Then there's Orectolobiformes, the carpet sharks and wobbegongs. The wobbegongs are pretty spectacular in that they look like swimming shag carpets, with frilly beards and spectacled patterns to assist in camouflage. These nocturnal bottom dwelling sharks are heavily threatened by the commercial fishing industry. Their small size means they're frequently swept up in nets and get caught in lobster traps. 

Squatiniformes are more commonly known as angel sharks, and they kind of look like a cross between a shark and a ray. Mostly they look like what would happen if you ran a shark underneath a rolling pin. They are masters of disguise, spending their time flat against the ocean floor and covered in a fine layer of sediment, waiting for an unsuspecting fish or squid to swim by before they strike.

Pristiophoriformes are the sawsharks, not to be confused with Pristiformes, the sawfish, which are actually more closely related to rays, but they're all still sharks. These two orders differ in a number of ways. Sawsharks are smaller than sawfish, who stick to coastal waters and have gills on the underside of their bodies, as opposed to sawsharks that have gills on the side. And when I say that sawfish can be large, I'm talking about mythically gigantic here. This is the rostrum from a single individual, and it's chock-full of sensory pores that help it to detect electrical impulses when it's searching in the muddy ocean floors for food.

Lastly, let's talk about the other orders of the skates and rays.

Rajiformes are the skates. Easily recognizable from this order are the devil fish but also the oddly-shaped and aptly-named guitarfish. Or you can have something that's sort of in between like this guy, which is actually a guitarfish that was modified in order to kind of look like a devil baby, and then sold as a tourist item in Mexico in the '50s. It's not a devil baby.

Next is Myliobatiformes, the stingrays, which are named for the venomous barb at the end of their tails, a defense typically only deployed if they're stepped on or otherwise harassed. One mentionable member of this order that doesn't have a stinger is the giant oceanic manta ray, which has broken records for its immense size. The biggest of these ever recorded was 30 feet, or 9 meters across. Imagine swimming next to a three story building.

And, to wrap up this summary of the twelve orders of sharks, we've got Torpediniformes, the electric rays and numb fishes. Maybe their name gave it away, but one of the most unique characters of these animals is there electrogenic nature. They can store energy in modified muscle fibers and use it to send out signals to detect organisms in murky water, or use it to shock nearby predators and deter them from attacking. Very large electric rays can give out a shock of about 500 volts. It's not enough to kill a human, but it's enough to get a very strong message across. 

Well, now that you're familiar with all the twelve orders of sharks, I hope you'll take some time to investigate the hundreds of other species we didn't even talk about today. Or check out one of our other videos from five consecutive calendar days dedicated to predatory cartilaginous fishes. 

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It still has brains on it.