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  • You're at the beach one day, going for a pleasant swim under the hot sun.

  • It's a perfect day to spend floating lazily in the ocean - aside from that cut in your

  • hand you got from a nasty barnacle on an underwater rock.

  • But you don't pay much attention to it, you're too busy relaxing out in the water.

  • So relaxed, in fact, that you don't even notice the fin cresting out of the water towards

  • you.

  • Just because you haven't been paying attention to the cut on your hand doesn't mean nobody

  • has.

  • In fact, to some residents of the ocean, you smell downright delicious...

  • Sharks: They're some of the oldest vertebrates on the planet, having existed before even

  • dinosaurs.

  • From the tiny, parasitic cookie-cutter shark, to the massive, filter-feeding whale shark,

  • to the majestic but deadly Great White, the shark family has dominated the ocean for the

  • past 420 million years.

  • They've been feared, respected, and even worshipped by seafaring civilizations for thousands of

  • years, and in recent times have been unfairly targeted out of fear due to movies like Jaws

  • and Deep Blue Sea portraying sharks as homicidal monsters.

  • But it's easy to see where that reputation comes from - even though sharks only rarely

  • target humans, they possess a number of adaptations that make them the ocean's perfect killers.

  • Probably the most famous and feared of these adaptations is a shark's ability to smell

  • blood in the water from miles away, but that's just one part of the amazing biology that

  • makes the shark the greatest hunter in the ocean.

  • Before we answer our main question, let's take a quick crash course in Why Sharks Are

  • Incredible 101.

  • There are 500 species of shark, and all are members of the Selachimorpha family, a group

  • that also includes rays.

  • This family is unique among fish because unlike other vertebrates, who have skeletons composed

  • of bone, sharks and rays have skeletons composed of cartilage - the same material that forms

  • the structure of your ears and nose.

  • Cartilage is sturdy, but also flexible, and much lower density than bone, allowing sharks

  • to be buoyant in water without needing a swim bladder like other marine animals.

  • In addition to this lack of a swim bladder, sharks also have a very specific pattern of

  • fin placement.

  • Ask anyone to draw a shark and they'll know where to place the fins - a large dorsal fin

  • on the back, a fin on either side, and a vertical fin on the end of the tail.

  • It's no mistake that pretty much all shark species have this fin configuration - it's

  • the perfect arrangement to maximize speed and maneuverability.

  • Fish that have swim bladders are kind of like hot air balloons - they can only change depth

  • by squeezing out some of the gas stored in their swim bladders which then refills over

  • time as they take in oxygen from the water around them.

  • But because sharks don't need a swim bladder to stay afloat, they're more like a fighter

  • jet - able to dive up and down in the water using only the propulsion of their back fins.

  • That's why the arrangement of a shark's fins look a little like the wings on an airplane.

  • The two side fins are used to change direction while the upper dorsal fin acts as a stabilizer.

  • So, unlike other fish, sharks can cruise at high speeds, stop quickly, swim directly up

  • or down, and turn on a dime.

  • This makes it very easy for them to chase their prey.

  • Sharks are also unique from other fish in the texture of their skin.

  • Despite what you might think, sharks aren't smooth.

  • Instead of having scales like other fish, they're actually covered in small teeth called

  • denticles, which gives their skin a rough texture like sandpaper.

  • These denticles provide a layer of lightweight armor, but are arranged in a way that minimizes

  • drag while swimming, making sharks extremely hydrodynamic.

  • Like all fish, sharks breathe by filtering oxygen out of water as they swim, but their

  • gills are structured a little bit differently.

  • You might've heard it said that sharks will suffocate if they stop moving, and while that's

  • not true of all sharks, it is true for a number of shark species.

  • While some species, like the wobbegong, are ambush predators and have the capability of

  • laying still for long periods of time, most sharks lack a gill pump and breathe through

  • a process called ram ventilation.

  • As long as the shark is moving, water will flow into the gill slits, where it rushes

  • over the shark's gill filaments, allowing the oxygen in the water to diffuse into the

  • shark's bloodstream.

  • Now, back to our central question.

  • Another commonly repeated fact about sharks is that they can locate a single drop of blood

  • in an olympic swimming pool's worth of water.

  • That might sound impossible, but sharks really do have amazing senses at their disposal that

  • they use to hunt.

  • Sharks have incredibly keen hearing, able to detect sounds below the range of human

  • hearing.

  • Eyesight varies from species - the Great White and other species that hunt close to the surface

  • of the water have quite poor eyesight, which is the main reason behind most shark attacks.

  • The hammerhead, in contrast, has an extremely wide field of view owing to its bizarrely

  • shaped head.

  • Sharks also have a very highly developed sense of taste, with some species in fact being

  • very picky eaters.

  • Great whites in particular are known for their pickiness, even though they're the largest,

  • most powerful, and most deadly of the macropredatory sharks - macropredatory meaning sharks that

  • eat anything larger than plankton.

  • Great Whites normally feed on seals, otters, dolphins, and large fish like tuna and mackerel,

  • but because of their poor eyesight, they rely on smell and movement to hunt, and will usually

  • take an initialtest biteof whatever prey they find.

  • If the taste isn't to their liking, they'll spit it out and abandon the kill entirely.

  • This is what happens in most shark attacks - sharks will mistake humans for their regular

  • prey, then leave them after one bite once they realize what they've just chomped on.

  • So, if you're afraid of sharks, maybe you can take a little comfort in knowing that

  • Great Whites at least don't find people very appetizing.

  • But, the most impressive of a shark's senses is, as we said before, their sense of smell.

  • Sharks have two forward-facing nostrils, which passively take in water as the shark swims.

  • The water is filtered through folds of tissue that contain millions of sensory cells that

  • can pick up even the tiniest trace of blood.

  • That alone is impressive, but what really gives the shark's sense of smell a killer

  • edge is the fact that it's directional, much like our sense of hearing.

  • The position and shape of the nostrils means that each one detects smells separately, so

  • a shark can tell whether the scent of blood is being picked up by the left or right nostril.

  • Using that information, the shark can follow the trail and hone in on the exact location

  • of the wounded animal that the blood is coming from.

  • On top of their amazing senses of smell, sharks have another sense that is unique to them,

  • as well as their close cousins, rays and skates.

  • This sense is called electroreception, and it's exactly what it sounds like - the ability

  • to, more or less, smell electricity.

  • All animals conduct electricity - it's what allows our muscles to move us around.

  • The more we move, the more electricity our muscles release.

  • Air is a poor conductor of electricity, so on land the ambient charge created by living

  • creatures is simply dissipated into the air or down through our feet into the ground.

  • Water, however, especially salt water, is an excellent conductor, so when animals swim

  • through the water, the tiny charges that our bodies create influence an electrical field

  • that extends far away from us.

  • Sharks, using a cluster of specialized pores on the tips of their noses called the ampullae

  • of lorenzini, can detect those charges from the field.

  • Like their sense of smell, their sense of electroreception is directional, because the

  • ampullae of lorenzini is connected to the shark's lateral line.

  • The lateral line is an organ that most fish and amphibians possess - it's a kind of tube

  • that runs along their bellies to the end of their tails that helps them sense things like

  • water pressure and currents.

  • This helps them maintain a sense of direction in the featureless open ocean.

  • A shark's sense of electroreception , combined with their extremely keen sense of smell,

  • are probably the shark's two greatest weapons when it comes to locating prey.

  • When their noses pick up a trace of blood, they can then use their ampullae to sniff

  • out the electric currents being released by whatever distressed animal the blood is coming

  • from.

  • Then, using their directional sense of smell, they follow the scent, along with the electrical

  • currents, until they're able to pinpoint the exact location of their prey.

  • Because sharks have such keen senses and are such excellent trackers, one whiff of fish

  • guts can attract dozens of sharks at a time.

  • And because sharks are usually solitary predators, this can lead to a lot of competition over

  • prey.

  • When competition occurs, it leads to a feeding frenzy.

  • You've probably heard the term used before, and you've probably heard it used many times

  • to refer to Black Friday shoppers or kids at a party when the pizza finally arrives.

  • How it typically goes is, a few sharks will be attracted by the large electrical signals

  • being put out by multiple prey in one location, then once they arrive, they will fight over

  • the prey.

  • The commotion caused by the sharks fighting will add to the electrical signals, making

  • them stronger and attracting even more sharks.

  • Sometimes this can be relatively uneventful, but other times the sharks will start fighting

  • each other, repeating the process, and pretty soon it turns into an all-out battle royale.

  • Shark experts theorize that shark feeding frenzies are less about actually feeding and

  • more about defending territory.

  • They often engage in cannibalism as a show of dominance, with bigger sharks taking bites

  • out of the smaller ones to reinforce the pecking order.

  • This doesn't seem to bother the small sharks as much as you'd think, though, as sharks

  • in frenzy mode are so fixated on claiming their share that there have been reports of

  • frenzied sharks continuing to feed even while being mortally wounded by other sharks.

  • Chillingly, some of the biggest shark feeding frenzies ever recorded happened at the sites

  • of shipwrecks.

  • Because we're not used to swimming, our movements in water require more energy and produce a

  • higher amount of electrical current.

  • Combine that with our large body size relative to most fish, the stress of having to tread

  • water while stranded at sea, and the fact that some people in a shipwreck may end up

  • injured and bleeding, and you have the perfect storm to cause a truly epic shark frenzy.

  • And while most sharks don't like the taste of human flesh at all, a frenzied shark will

  • attack and eat anything, just so that the other sharks won't be able to.

  • You probably remember the scene in Jaws where Captain Quint describes the crew of the USS

  • Indianapolis being picked off by sharks after the ship was torpedoed.

  • Well, that was based on a true story, and it's not even the only one like it from naval

  • history.

  • Oceanic whitetip sharks, one of the most common species found in open water, were known to

  • make buffets out of many a sinking ship during the major naval battles of World War 1 and

  • World War 2.

  • The steamship Nova Scotia, sunk during the second world war, had around a thousand crew

  • members when it went down, and only 192 survived to be rescued.

  • By their accounts, many of the crew who survived but were injured in the initial disaster were

  • devoured by frenzied oceanic whitetips.

  • Not all sharks are so viciously territorial, however.

  • The sub-group of sharks known as dogfish are so named because of the fact that they have

  • been seen hunting cooperatively in dog-like packs.

  • Whether they're pack hunters or working alone, gentle filter-feeders or frenzied man-eaters,

  • whether they're ambush predators or roving assassins, sharks are some of the most unique

  • and fascinating animals on the planet.

  • Their incredible senses make them unparalleled when it comes to tracking their prey, and

  • because of this they've filled a vital role in the ocean's ecosystem for hundreds of millions

  • of years.

  • No other predator is quite as good at what they do as the shark, but unfortunately, many

  • species of shark are on the decline.

  • Overfishing plays a part in this decline, but another part is simply fear and misunderstanding.

  • Sharks can die getting tangled in anti-shark nets, and often if a shark attack occurs in

  • an area, this leads to all the local sharks being killed out of over-cautiousness.

  • So, even though it's perfectly natural to be scared of these apex predators, keep in

  • mind that they're just doing what they were made to do, and next time you go fishing or

  • swimming or jet-skiing, try to be mindful of the amazing creatures you're sharing the

  • ocean with.

  • Cause the blood being spilled there is more often theirs than ours.

  • Now check out “I was Attacked By a Sharkand “I Was Lost At Sea for 76 Days With

  • Sharks Circlingfor more fascinating shark facts!

  • Thanks again to The Great Courses Plus!

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B2 shark water smell fin prey swim

How Sharks Actually Smell Blood

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    Summer posted on 2021/04/22
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