1. SHAAARK! Did you just get a mental image of a gaping mouth and pointy teeth? Think again. The roughly 500 known species of shark vary in size, shape, environment and diet.
2. Living shark species range from a few that could fit in your hand, such as the dwarf lanternshark, to a few you could fit inside, including the whale shark, which grows up to 40 feet long.
3. Angelsharks are nearly flat, like the rays and skates to which sharks are closely related, while sawsharks have a toothy snout that can be almost as long as their cylindrical bodies.
4. Sharks ply the waters of every ocean, from shallow, brackish estuaries to depths of nearly 10,000 feet.
5. Deep-sea dwelling Mitsukurina owstoni, the goblin shark, is the oldest living species among lamniform sharks, which go back about 125 million years and today include great whites, threshers and makos.
6. The first sharks evolved 400 million to 455 million years ago, but sharks’ flexible cartilage skeletons are rarely preserved, so the earliest species left little behind in the fossil record.
7. Fossilized denticles, tiny tooth-shaped scales that once covered their skin, are the oldest evidence we have for sharks — though researchers disagree on whether denticles alone are enough classify a species as a shark.
8. A few things make a shark truly sharky: All sharks have jawbones and multiple gill openings, and, unlike the vast majority of other fish species, that skeleton of cartilage rather than bone.
9. And while bony fish have an air-filled swim bladder to control buoyancy, sharks don’t. They use their large, oily livers as a kind of internal flotation device.
10. Many shark species — like most fish — are coldblooded, but some are warmblooded, including the great white shark.
11. Having a core body temperature that’s warmer than the water gives these animals all kinds of speed: They grow faster, swim faster and hunt more efficiently. The trade-off is that they need to eat up to 10 times more than a similarly sized coldblooded cousin.
12. You might assume a shark get-together turns into a feeding frenzy when food is around. But apparently it’s more of a dinner party. Researchers who observed great white sharks scavenge a whale carcass off the coast of South Africa found that multiple animals fed beside each other at the same time, displaying relaxed behavior such as a belly-up posture and a lack of ocular rotation.
13. Ocular rotation is, well, let’s let Jaws’ obsessive shark hunter Quint explain it: “The thing about a shark, he’s got lifeless eyes, black eyes, like a doll’s eye … he bites ya, and those black eyes roll over white.”
14. Quint got it half right. Only some species of shark, including the great white, use ocular rotation to protect their eyes. Other species guard their vision with a third eyelid called a nictitating membrane.
15. Jaws portrayed sharks as villains, and some etymologists believe the word shark may derive from earlier German and Dutch words for shifty characters. We can still see the connection in today’s loan sharks and card sharks.
16. Other researchers believe the word comes from Xoc (pronounced “shoke”) in Yucatec, a Maya language. According to this theory, English sailors visiting Caribbean waters in the 16th century picked up the local word for the “great fish.”
17. And talk about great: At more than 50 feet long, Carcharocles megalodon was the largest shark that ever lived before it went extinct about 2.6 million years ago.
18. Yet even C. megalodon was little once — well, relatively speaking. In 2010, paleontologists announced they’d found a 10 million-year-old megalodon nursery on the coast of Panama with newborns measuring more than 6 feet long.
19. While we’re talking big fish tales, you may have heard sharks don’t get cancer. That’s a load of rotten mackerel. Sharks do get cancer — and we’ve known that since at least 1908, when a malignant tumor was found in a blue shark.
20. Humans perceive sharks as a threat, but the opposite is true. Up to 100 million sharks are killed each year by finning: Fishermen cut off a shark’s dorsal fin to sell as a delicacy and dump the wounded animal back into the ocean to die. The practice imperils not only sharks, but entire food chains, which are disrupted as the animals’ numbers dwindle.