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  • 550 million years ago, the  ocean was a simple ecosystem,  

  • full of reefs made by bacteria, and a gooey  mat of microbes that covered the ocean floor.  

  • Creatures were simple, often  amorphous. None were yet predatory.

  • But within a few million years, this simple  ecosystem would disappear, replaced by an ocean  

  • full of diverse, mobile, and highly effective  animals. The world’s first predators emerged  

  • during the Cambrian explosion, 540 million years  ago, in the form of giant shrimp like creatures  

  • like, Anomalocaris, which trapped its prey in  its mouth lined with hooks, or the five-eyed  

  • Opabinia, which caught its victims usingflexible clawed arm attached to its head.  

  • Soon, the first fish emerged - the jawless  Agnatha, of which two groups still survive today:  

  • the lampreys and the hagfish. And by 450 million  years ago, the ocean was populated by the ancestor  

  • of what is now the most fearsome  predator of the sea - sharks.

  • The first modern sharks arrived in the  late Devonian 370 million years ago,  

  • taking the iconic shape we know today. They  were six feet long, with a streamlined body,  

  • 5-7 gill slits, and dorsal fins.

  • Soon, sharks dominated the oceans. The  Carboniferous Era was a period with some  

  • of the most unique sharks that ever existedStrange species like the Stethacanthus,  

  • a shark with what looks like an anvil on its headthe Eugeneodontida, a shark with a tooth whorl at  

  • the end of its bottom jaw, and the Falcatus,  a shark with a long, sharp horn on its head.

  • But these strange iterations of the  shark have long since gone extinct.  

  • The sharks that prevailed were largely  streamlined, with a pointed snout,  

  • large pectoral and dorsal fins, and a strong  crescent-shaped tail, like the great whites or the  

  • shortfin mako. Sharks like these have dominated  the seas for hundreds of millions of years.

  • That is, until evolution decided to takestrange turn. Around 20 million years ago  

  • evolution created the newest shark to enter  the water - arguably the strangest one of  

  • them all. With a mallet shaped head full of  sensory organs, and eyes set on either end,  

  • the hammerhead is one of the most recognizable  and downright bizarre looking animals on earth,  

  • their body plan a drastic departure  from the other sharks that roam the sea.

  • They are found in temperate and tropical  waters worldwide and can often be seen in  

  • massive numbers as they migrate to colder water.  6 meters long and weighing up to 450 kilograms,  

  • hammerheads are a formidable and  dominant force across the world’s oceans.

  • Why only recently did shark evolution take such  a surprising turn, making something so different  

  • from the rest? And what does their odd-shaped head  do that gives hammerheads their evolutionary edge?

  • The iconic hammer of the hammerhead  shark is called a cephalofoil,  

  • and the size of it varies from species to  species. It’s easy to assume this weird  

  • shape is a rubbery extension of flesh, but it  is actually a flattened and stretched out skull.

  • The smallest is the modest bonnethead, Sphyrna  tiburo, also known as the shovelhead. The largest  

  • is the winghead shark, Eusphyra blochiiwhose wing-like head is so big its width is  

  • nearly 50% of its total body length. All other  hammerheads fall between these two extremes.

  • And on the tip of all of the hammerhead’s  cephalofoil are their weird beady eyes. This  

  • configuration is a little bafflingBeing as spread out as they are,  

  • it would seem like each eye would see the  world independently, with no overlap in  

  • each eye’s vision - not something that  would be very helpful for a predator.

  • The visual field in all creatures is the expanse  of space visible to them without moving their  

  • eyes. In humans, our forward facing, horizontal  visual field is around 190-degrees. And our  

  • binocular vision, where the vision of each  eye overlaps, giving us depth perception,  

  • covers 120 horizontal degrees. In fact  most predators have large binocular fields,  

  • to help quickly scan the environment for preyan ability made possible by having eyes that face  

  • forward. By contrast, most prey animals  have eyes on the sides of the heads,  

  • to help them be aware of danger coming  from any direction. Pigeons, for example,  

  • have a visual field of around 310 degreesbut a very narrow binocular portion in front

  • You can see this pattern throughout  the animal kingdom - prey and predator  

  • species distinguishable by eye positionBut when you look at a hammerhead shark,  

  • it’s not immediately obvious what’s going  on there. They are obviously predators,  

  • but it’s eyes are far apart, and in a totally  unique configuration from all other vertebrates.

  • Does the weird shape of the  hammerhead hurt, or help their vision,  

  • and thus their predatory ability?

  • In 2009, researchers started to get to the  bottom of it. They compared the visual fields  

  • of three hammerhead shark species to two  sharks with a more typical head morphology,  

  • to see which type of shark body plan  offers a more enhanced binocular field.

  • All sharks in the study had a full 360 vertical  visual field, with similar vertical binocular  

  • overlaps. But when looking at the horizontal  visual field, the differences were profound.  

  • The total monocular visual fields ranged from 308 degrees to 340 degrees,  

  • with the hammerheads on the upper end. And  when comparing binocular field of view,  

  • the hammerheads were the clear winners. The lemon  shark had a mere 10 degrees of binocular overlap,  

  • the blacknose just 11. The modest  bonnethead had a bit more with 13,  

  • and the scalloped hammerhead had 32 degrees of  overlap. And the winghead shark, the one with the  

  • widest head, had 48 degrees of binocular overlapnearly four times that of the typical sharks.

  • It’s clear then that the binocular overlaps in  

  • hammerheads increases as the  width of the head increases.

  • This gives them an advantage when hunting for  prey, by giving them exceptional depth perception.  

  • Out of all the sharks, they have the clearest  view of the underwater world, and it shows.  

  • Hammerheads are some of the most effective  predators among the sharks - easily  

  • catching and devouring stingraysoctopuses, and even other sharks.

  • This alone may have been enough to influence  the evolution of the hammerhead cephalofoil.  

  • But the long, flat shape of the head does more  than give the shark better vision. It also  

  • gives the hammerhead unique hydrodynamics  found nowhere else in the animal kingdom.

  • When you think of agility and speed in the  ocean, you think of animals like mako sharks or  

  • bottlenose dolphins - animals with a streamlinedpointed nose, that cuts through the water.

  • But a hammerhead shark is basically the opposite  of that. It’s like an airplane with a wing  

  • attached to its front. Hammerheads have to use  much more energy than other, normal shark species  

  • just to swim because of the increased dragIt’s a lot of work to push that thing around.

  • So the obvious question is  - why would nature do this?  

  • What benefit does this give  to the hammerhead, if any?

  • Elasmobranchs, like sharks and  rays, don’t have a swim bladder,  

  • so they have to constantly swim  to avoid sinking to the bottom.  

  • So for a long time, it was thought that  the cephalofoil indeed acts like a wing,  

  • producing lift forces that help the hammerhead  stay vertically positioned in the water column.

  • This theory seems to make sense, when you  compare the cephalofoil to an airplane wing.  

  • The hammerhead hammer looks just like  one - the structure’s technical name,  

  • cephalofoil, even meanshead  wing.”

It’s easy to assume then  

  • that the flow of water over the cephalofoil  works just like the flow of air over a wing.

  • To test this theory, researchers laser  scanned the heads of 8 species of hammerhead.  

  • Each digitized head was then placed  in a virtual underwater environment,  

  • allowing them to measure  water pressure, drag and flow.  

  • They then did the same for a few shark  species with more typical pointy heads.

  • And surprisingly they found that the cephalofoil  does not create lift when the shark is swimming  

  • in a regular, forward motion. But when the  head is tilted up or down, strong forces  

  • quickly come into play. When the angle of attack  changes, the shark can rapidly ascend or descend.

  • The hammer is not for liftbut for maneuverability.

  • And this type of motion is essential for how the  hammerhead hunts. Unlike mako sharks that chase  

  • down prey in long pursuits, hammerheads swim just  above the sand, looking for bottom dwelling prey.  

  • Once detected, these prey animals, like  stingrays or squid, will erratically dart  

  • away to try to escape, zig zagging up, downleft, right. And the hammerhead follows suit.

  • Supporting this hypothesis is the winghead sharkwho has the biggest head of all the hammerheads  

  • compared to its body. It has the largest amount  of drag- but also shows the greatest change in  

  • lift as the attack angle changes. Of all the  hammerheads, it has the best maneuverability.

  • And when you look at its diet, you can see why  evolution would create something so extreme.  

  • Most hammerheads eat crabs or  stingrays - creatures that are quick,  

  • but not known for their sheer agility.

  • But the winghead diet consists of about 93%  

  • teleost fishes - like herringswhich are very fast, and very agile.

  • The cephalofoil gives the hammerheads  an agility unmatched in the world  

  • of sharks - allowing them to fill an  ecological niche that other sharks cannot.  

  • But on top of this agility, hammerheads possess a  6th sense, something which we have no equivalent  

  • to - their ability to detect minute and  invisible electric fields in the water.

  • The underwater world for us is distorted - our  vision blurred, our hearing muffled. We can  

  • immediately tell that the ocean is not where  we belong. But on top of our senses becoming  

  • out of whack, there is so much more going on  under the waves than we could ever perceive,  

  • a world of stimuli that we can’t pick  up on at all, a world of electricity.

  • Electroreception is a 6th sense  to many aquatic creatures.  

  • It’s an ability to detect the electrical  fields that permeate the water,  

  • giving navigational cues and information about the  location of prey. In fact, it is observed almost  

  • exclusively in aquatic animals since water ismuch better conductor of electricity than air.

  • Many members of the elasmobranch  fish family share this trait,  

  • but sharks' electroreception  abilities are the most finely tuned.

  • Sharks receive tiny electrical  signals from their environment  

  • via a series of pores peppered over their heads. These pores are distributed in discrete patterns,  

  • varying somewhat among elasmobranch speciesIn this picture of a great white shark,  

  • you can see the clusters of pores  around its eyes and nostrils.

  • These pores are filled with an electrically  conductive jelly, and lead to tiny bulbous cells,  

  • called ampullae of Lorenzini. And this is the  key to their amazing power. All animals generate  

  • electricity around them as their muscles  contract in movement and their heart beats,  

  • and this current radiates away from them in the  water. When these electrical currents travel  

  • towards the shark and through the jelly, they  stimulate cilia - hairlike projections on the  

  • ampullae, which then trigger the sensory neurons. This then triggers neurotransmitters in sharks'  

  • brains, which tells them they  are close to something alive.

  • This sense works even when the  conditions underwater render the five  

  • other common sensessight, smelltaste, touch, hearinguseless.  

  • It works in turbulent water, in total darkness  and even when prey are hidden beneath the sand.

  • And for the hammerheads, this  sense is even more extreme.

  • With a wider head, hammerheads have a greater  number of electrosensory pores. The pores are also  

  • located over a broader area, which increases the  surface area that the head can sample, and thus  

  • increases the probability of a prey encounterSo when the hammerhead swims above the sand, it  

  • waves its head like a metal detector looking for  treasure - its treasure being a buried stingray.

  • And the sensitivity of this metal detector head  is profound. Researchers found that newborn  

  • Bonnethead Sharks can detect electric fields  less than 1 nanovolt per square centimetre.

  • This is around the equivalent to the  intensity of a voltage gradient that  

  • would be created in the sea by connecting one end  of a 1.5volt AA battery to the Long Island Sound,  

  • and the other end in the waters  off of Florida.Theoretically,  

  • a shark swimming between these places could  tell when the battery was switched on or off

  • Such incredible electrical sensitivity is over  five million times greater than anything we  

  • could ever feel. Even our best technology  struggles to detect something that minute.  

  • It is likely the most powerful  electrical sensing in the animal kingdom.

  • We often think of the weirdest, cartoon-iest  animals being things of the past. Creatures that  

  • were giant, strange, or sinister. But hammerheads  show us that evolution is never finished.  

  • What seems illogical, or even detrimental, to  an animal’s survival can be the key to them  

  • fitting into a very specific environmental nicheIt’s a fun thing to think about - what else will  

  • appear on earth in the next 50, 100 million yearsWhat will sharks look like given that much time?  

  • Some reef shark species have  recently evolved the ability to walk,  

  • even above the water at low tide. Other species  have recently been discovered to glow in the dark,  

  • others can live in freshwater in addition to  salt water. What else does the future hold  

  • for sharks like the hammerheads? It’s hard to  speculate, but the possibilities are endless.

  • Sharks have dominated the seas since the  end-Cretaceous, and as a group, have survived  

  • all 5 mass extinctions so far, in large part due  to their ability to fill many varying ecological  

  • niches. But the hammerheads, as the newest species  of shark, have not yet faced such an event. That  

  • is, until now. Many believe we are currently  living through the 6th mass extinction due  

  • to human activity, and sharks, especially the  hammerheads, are among the most at risk. One  

  • study in Australia reported that 80% of scalloped  hammerheads have been lost. They are threatened  

  • by commercial fishing, mainly for the shark fin  trade - where the fins are cut off and the rest  

  • of the body discarded. Their numbers, like many  sharks, are dwindling, and if we, as a species,  

  • are not careful, we may never get to see what  the future holds for such incredible creatures.

  • Being able to talk about the  science and conservation subjects  

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  • Working with Real Engineering and then  starting this channel made that dream  

  • a reality. By growing my video making skills  and then learning how the youtube world works,  

  • I have been able to create videos that I deeply  care about while also building a business.

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The Insane Biology of: Hammerhead Sharks

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    joey joey に公開 2021 年 06 月 09 日
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