1 00:00:08,113 --> 00:00:12,245 In 1800, the explorer Alexander von Humboldt 2 00:00:12,245 --> 00:00:16,234 witnessed a swarm of electric eels leap out of the water 3 00:00:16,234 --> 00:00:19,834 to defend themselves against oncoming horses. 4 00:00:19,834 --> 00:00:24,435 Most people thought the story so unusual that Humboldt made it up. 5 00:00:24,435 --> 00:00:28,796 But fish using electricity is more common than you might think; 6 00:00:28,796 --> 00:00:32,935 and yes, electric eels are a type of fish. 7 00:00:32,935 --> 00:00:35,125 Underwater, where light is scarce, 8 00:00:35,125 --> 00:00:37,774 electrical signals offer ways to communicate, 9 00:00:37,774 --> 00:00:38,861 navigate, 10 00:00:38,861 --> 00:00:43,692 and find—plus, in rare cases, stun—prey. 11 00:00:43,692 --> 00:00:49,135 Nearly 350 species of fish have specialized anatomical structures 12 00:00:49,135 --> 00:00:52,807 that generate and detect electrical signals. 13 00:00:52,807 --> 00:00:55,246 These fish are divided into two groups, 14 00:00:55,246 --> 00:00:58,608 depending on how much electricity they produce. 15 00:00:58,608 --> 00:01:03,587 Scientists call the first group the weakly electric fish. 16 00:01:03,587 --> 00:01:06,406 Structures near their tails called electric organs 17 00:01:06,406 --> 00:01:12,507 produce up to a volt of electricity, about two-thirds as much as a AA battery. 18 00:01:12,507 --> 00:01:14,597 How does this work? 19 00:01:14,597 --> 00:01:19,068 The fish's brain sends a signal through its nervous system to the electric organ, 20 00:01:19,068 --> 00:01:21,288 which is filled with stacks of hundreds 21 00:01:21,288 --> 00:01:27,258 or thousands of disc-shaped cells called electrocytes. 22 00:01:27,258 --> 00:01:31,438 Normally, electrocytes pump out sodium and potassium ions 23 00:01:31,438 --> 00:01:36,807 to maintain a positive charge outside and negative charge inside. 24 00:01:36,807 --> 00:01:39,920 But when the nerve signal arrives at the electrocyte, 25 00:01:39,920 --> 00:01:43,549 it prompts the ion gates to open. 26 00:01:43,549 --> 00:01:47,709 Positively charged ions flow back in. 27 00:01:47,709 --> 00:01:52,029 Now, one face of the electrocyte is negatively charged outside 28 00:01:52,029 --> 00:01:54,699 and positively charged inside. 29 00:01:54,699 --> 00:01:58,771 But the far side has the opposite charge pattern. 30 00:01:58,771 --> 00:02:01,420 These alternating charges can drive a current, 31 00:02:01,420 --> 00:02:06,491 turning the electrocyte into a biological battery. 32 00:02:06,491 --> 00:02:10,649 The key to these fish's powers is that nerve signals are coordinated 33 00:02:10,649 --> 00:02:15,031 to arrive at each cell at exactly the same time. 34 00:02:15,031 --> 00:02:21,080 That makes the stacks of electrocytes act like thousands of batteries in series. 35 00:02:21,080 --> 00:02:24,323 The tiny charges from each one add up to an electrical field 36 00:02:24,323 --> 00:02:27,061 that can travel several meters. 37 00:02:27,061 --> 00:02:30,140 Cells called electroreceptors buried in the skin 38 00:02:30,140 --> 00:02:33,121 allow the fish to constantly sense this field 39 00:02:33,121 --> 00:02:38,022 and the changes to it caused by the surroundings or other fish. 40 00:02:38,022 --> 00:02:40,492 The Peter’s elephantnose fish, for example, 41 00:02:40,492 --> 00:02:43,673 has an elongated chin called a schnauzenorgan 42 00:02:43,673 --> 00:02:46,103 that's riddled in electroreceptors. 43 00:02:46,103 --> 00:02:48,892 That allows it to intercept signals from other fish, 44 00:02:48,892 --> 00:02:50,792 judge distances, 45 00:02:50,792 --> 00:02:53,793 detect the shape and size of nearby objects, 46 00:02:53,793 --> 00:02:58,801 and even determine whether a buried insect is dead or alive. 47 00:02:58,801 --> 00:03:01,302 But the elephantnose and other weakly electric fish 48 00:03:01,302 --> 00:03:05,402 don't produce enough electricity to attack their prey. 49 00:03:05,402 --> 00:03:08,573 That ability belongs to the strongly electric fish, 50 00:03:08,573 --> 00:03:11,794 of which there are only a handful of species. 51 00:03:11,794 --> 00:03:16,392 The most powerful strongly electric fish is the electric knife fish, 52 00:03:16,392 --> 00:03:20,513 more commonly known as the electric eel. 53 00:03:20,513 --> 00:03:25,400 Three electric organs span almost its entire two-meter body. 54 00:03:25,400 --> 00:03:27,354 Like the weakly electric fish, 55 00:03:27,354 --> 00:03:30,960 the electric eel uses its signals to navigate and communicate, 56 00:03:30,960 --> 00:03:34,684 but it reserves its strongest electric discharges for hunting 57 00:03:34,684 --> 00:03:40,273 using a two-phased attack that susses out and then incapacitates its prey. 58 00:03:40,273 --> 00:03:43,995 First, it emits two or three strong pulses, 59 00:03:43,995 --> 00:03:46,444 as much as 600 volts. 60 00:03:46,444 --> 00:03:49,734 These stimulate the prey's muscles, sending it into spasms 61 00:03:49,734 --> 00:03:53,045 and generating waves that reveal its hiding place. 62 00:03:53,045 --> 00:03:55,925 Then, a volley of fast, high-voltage discharges 63 00:03:55,925 --> 00:03:59,294 causes even more intense muscle contractions. 64 00:03:59,294 --> 00:04:03,095 The electric eel can also curl up so that the electric fields 65 00:04:03,095 --> 00:04:07,249 generated at each end of the electric organ overlap. 66 00:04:07,249 --> 00:04:10,939 The electrical storm eventually exhausts and immobilizes the prey, 67 00:04:10,939 --> 00:04:14,890 and the electric eel can swallow its meal alive. 68 00:04:14,890 --> 00:04:18,550 The other two strongly electric fish are the electric catfish, 69 00:04:18,550 --> 00:04:20,935 which can unleash 350 volts 70 00:04:20,935 --> 00:04:23,906 with an electric organ that occupies most of its torso, 71 00:04:23,906 --> 00:04:29,466 and the electric ray, with kidney-shaped electric organs on either side of its head 72 00:04:29,466 --> 00:04:32,846 that produce as much as 220 volts. 73 00:04:32,846 --> 00:04:35,718 There is one mystery in the world of electric fish: 74 00:04:35,718 --> 00:04:39,436 why don't they electrocute themselves? 75 00:04:39,436 --> 00:04:42,148 It may be that the size of strongly electric fish 76 00:04:42,148 --> 00:04:44,797 allows them to withstand their own shocks, 77 00:04:44,797 --> 00:04:48,465 or that the current passes out of their bodies too quickly. 78 00:04:48,465 --> 00:04:53,047 Some scientists think that special proteins may shield the electric organs, 79 00:04:53,047 --> 00:04:57,957 but the truth is, this is one mystery science still hasn't illuminated.