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The largest species of shark is also one of the most easygoing. Whale sharks have been known to give a ride to hitchhiking swimmers, and cruise through the water atop them. But marine life experts caution against popularizing this sport. "When people spend a lot of time and a lot of pressure on a fish, it takes away that slime covering and potentially has negative health impacts for the fish," Marine biologist Bruce Neill told ABC News.

You thought nine months seemed like a while, but the spiny dogfish species of shark can take two years to gestate before delivery—making it the longest gestation period of any vertebrate.

Just when you thought it was safe to go back into the lake… While sharks live in all of the world's oceans, a few species are also known to inhabit freshwater lakes and rivers. For example, bull sharks are found in tropical rivers and have evolved to swim between salt and fresh water. River sharks, true to their name, have been found in rivers in areas of South Asia, New Guinea, and Australia

All told, there are almost 500 species of shark, including angel, bullhead, carpet, and dogfish sharks, not to mention weasel, mackerel, crocodile, zebra, and even cat sharks. They range in size from a few inches to 40 feet long, living in a wide range of habitats and boasting a strange assortment of physical characteristics.

Partly due to the fact that they need to carry shark babies, females tend to be larger in most shark species. And for more fun ocean facts, don't miss the 33 Missing Treasures Experts Say Are Real.

Reaching lengths of 40 feet long, the whale shark is seriously huge and holds the title of largest fish in the sea. But in the unlikely situation that you encounter one of these in the water, don't be alarmed: their main meal is plankton, which they eat by "filter feeding," in which they scoop up a huge amount of ocean water and scoop out the tiny plants and animals—it's tough to catch a person in that. And for more the seven seas, check out the 17 Floating Hotels That Are Simply Magical

Another advantage of hammerheads' weird head is that they have incredible vision. A 2009 study found that the placement of their eyes gives them impressive binocular vision and the ability to see 360 degrees. "The eyes of hammerhead sharks are tilted slightly forward," as BBC Earth put it, "allowing the field of vision of each to significantly overlap.

Hammerhead sharks have that funny-looking head for a reason. It contains a whopping 3,000 ampullar pores for picking up on electrical fields in the ocean. As MNN reports, "The hammerhead's increased ampullae sensitivity helps it track down its favorite meal, stingrays, which are usually hidden under the sand.

In addition to their killer sense of smell, sharks also can detect prey by tapping in to the small electrical fields that other animals generate using tiny organs called the ampullae of Lorenzini. These small pores, located near their nostrils, around the head and beneath their snout, are something of a second sight. The pores connect to long, jelly-filled bulbs that connect to nerves below their skill. And for more ways to maximize your ocean knowledge, check out the 30 Reasons Why the Ocean Is Scarier Than Space

Sharks are so tough, their embryos are known to attack one another. The largest embryo in a shark litter is known to eat its fellow embryos, in an act known as intrauterine cannibalism. Researchers looked at this phenomenon in sand sharks, noting that, "While 12 littermates may start out the journey, all but one is devoured by the biggest in the pack. That strategy allows sand tiger sharks to have much larger babies at birth than other shark species, making the little ones relatively safe from other predators." And for more amazing trivia straight from the animal kingdom, don't miss these 40 Amazing Animal Facts.

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