字幕表 動画を再生する 英語字幕をプリント This episode of Real Science is brought to you by Skillshare. Home to thousands of classes that can teach you a new life skill. 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 using a flexible 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 existed. Strange species like the Stethacanthus, a shark with what looks like an anvil on its head, the 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 take a strange 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 blochii, whose 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 baffling. Being 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 prey, an 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 degrees, but a very narrow binocular portion in front. You can see this pattern throughout the animal kingdom - prey and predator species distinguishable by eye position. But 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 overlap, nearly 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 stingrays, octopuses, 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 streamlined, pointed 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 drag. It’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 means “head 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 lift, but 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, down, left, right. And the hammerhead follows suit. Supporting this hypothesis is the winghead shark, who 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 herrings - which 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 is a much 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 species. In 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 senses—sight, smell, taste, touch, hearing—useless. 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 encounter. So 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 niche. It’s a fun thing to think about - what else will appear on earth in the next 50, 100 million years? What 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 near and dear to my heart on a platform with such a wide audience is something I once could only have dreamed about. After finishing college I knew that science communication was my passion - I just needed to find a way to make that my career. Working with Real Engineering and then starting this channel made that dream a reality. 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