1 00:00:01,800 --> 00:00:06,920 Speaker 1: Welcome to brain Stuff, a production of I Heart Radio. Hey, 2 00:00:06,960 --> 00:00:11,440 Speaker 1: brain Stuff, Lauren vogel Bomb here online. It's become a 3 00:00:11,520 --> 00:00:16,440 Speaker 1: frequently asked question. How do eels, those long, writhing fish 4 00:00:16,560 --> 00:00:18,960 Speaker 1: you might have seen an aquarium or on a menu, 5 00:00:19,520 --> 00:00:25,000 Speaker 1: go about making eel babies? How to eels reproduce for real? 6 00:00:25,320 --> 00:00:28,560 Speaker 1: People have studied these creatures for literally thousands of years, 7 00:00:28,600 --> 00:00:31,320 Speaker 1: but even after all this time, some aspects of their 8 00:00:31,320 --> 00:00:35,560 Speaker 1: breeding lives are still shrouded in mystery. And to get 9 00:00:35,600 --> 00:00:38,839 Speaker 1: these skinny on eel reproduction for the articles, episode is 10 00:00:38,840 --> 00:00:42,440 Speaker 1: based on how Stuff Works. Spoke with an expert, Caroline Derriff, 11 00:00:42,560 --> 00:00:45,760 Speaker 1: an ecologist at the Norwegian Institute of Marine Research who 12 00:00:45,760 --> 00:00:51,040 Speaker 1: studies the habits of these incredible fish. Okay, first off, 13 00:00:51,320 --> 00:00:55,480 Speaker 1: calling something an eel doesn't make it one necessarily, no 14 00:00:55,600 --> 00:00:59,320 Speaker 1: doubt you've heard of the South American electric eel. Despite 15 00:00:59,440 --> 00:01:02,960 Speaker 1: common name, that dramatic fish isn't really an eel. It's 16 00:01:03,000 --> 00:01:08,720 Speaker 1: a Bizarro carp relative that's classified as a knife fish. 17 00:01:08,880 --> 00:01:13,080 Speaker 1: The order Anguilliforms contains all of the world's true eels. 18 00:01:13,520 --> 00:01:16,720 Speaker 1: There are more than eight hundred species from Mora's you know, 19 00:01:16,880 --> 00:01:20,839 Speaker 1: Ursula's sidekicks from the little Mermaid to the aptly named spaghetti. 20 00:01:20,920 --> 00:01:26,000 Speaker 1: Eels usually slender and elongated a true eels do not 21 00:01:26,160 --> 00:01:29,679 Speaker 1: have pelvic fins, which are twin appendages often found on 22 00:01:29,720 --> 00:01:34,119 Speaker 1: the underbellies of other fish. Eels can be scaleless, Many 23 00:01:34,160 --> 00:01:38,560 Speaker 1: species feel slippery to the touch, and salt water is 24 00:01:38,600 --> 00:01:42,440 Speaker 1: the preferred habitat for the majority of eels. However, there 25 00:01:42,480 --> 00:01:46,600 Speaker 1: are nineteen species of so called freshwater eels or anglid, 26 00:01:47,080 --> 00:01:50,120 Speaker 1: which spend some stages of their lives in fresh water. 27 00:01:50,960 --> 00:01:53,880 Speaker 1: They spawn in the ocean, that is, the adults produce 28 00:01:53,960 --> 00:01:56,960 Speaker 1: eggs and sperm, and then the resulting babies hatched there, 29 00:01:57,880 --> 00:02:01,120 Speaker 1: but they grow in freshwater before turning to the sea. 30 00:02:03,120 --> 00:02:05,680 Speaker 1: Saying that these fish have a complex life cycle is 31 00:02:05,760 --> 00:02:09,360 Speaker 1: a little bit of an understatement. If it survives to 32 00:02:09,440 --> 00:02:12,600 Speaker 1: reach sexual maturity, a freshwater eel will have gone through 33 00:02:12,800 --> 00:02:17,279 Speaker 1: five distinct stages. With each new phase, the animal experiences 34 00:02:17,360 --> 00:02:22,239 Speaker 1: both a physical transformation and shift in lifestyle. Derec explained. 35 00:02:22,639 --> 00:02:26,720 Speaker 1: The first stage is called the leptocephalus larva. They're called 36 00:02:26,800 --> 00:02:33,120 Speaker 1: leptocephalus because lepta means leaf and cephalis means head. True 37 00:02:33,120 --> 00:02:36,400 Speaker 1: to the name, the newborn larva have leaf shaped bodies 38 00:02:36,440 --> 00:02:40,639 Speaker 1: that appear broad and flattened in profile in biological jargon. 39 00:02:40,760 --> 00:02:46,560 Speaker 1: Their bodies are laterally compressed, being nearly transparent. They're also 40 00:02:46,800 --> 00:02:50,600 Speaker 1: very well camouflaged. Imagine trying to recover a lost contact 41 00:02:50,720 --> 00:02:55,200 Speaker 1: lens from the bottom of a swimming pool. Leptocephalus larva 42 00:02:55,320 --> 00:02:59,799 Speaker 1: are ocean going animals. Eventually, though instinct pushes them defined 43 00:03:00,080 --> 00:03:03,000 Speaker 1: change in scenery, which is where the change into the 44 00:03:03,000 --> 00:03:07,600 Speaker 1: second phase of the life cycle occurs. Derriff said, they 45 00:03:07,639 --> 00:03:11,760 Speaker 1: migrate for great distances. They drift through the gulf stream, 46 00:03:11,840 --> 00:03:14,760 Speaker 1: and then when they reach the continental shelf, they metamorphos 47 00:03:14,840 --> 00:03:19,560 Speaker 1: into glass eels. A glass eels are still more or 48 00:03:19,639 --> 00:03:24,160 Speaker 1: less transparent, but they're longer and skinnier by comparison, and 49 00:03:24,320 --> 00:03:27,520 Speaker 1: they're attracted to fresh water, so they head inland by 50 00:03:27,560 --> 00:03:31,440 Speaker 1: traveling up rivers, and that brings us to life stage 51 00:03:31,520 --> 00:03:35,560 Speaker 1: number three, a yellow eels. Unlike the transparent larva and 52 00:03:35,600 --> 00:03:39,280 Speaker 1: glass eels, these guys have body pigment with a yellowish 53 00:03:39,320 --> 00:03:42,600 Speaker 1: overall complexion. But it's not the last color change that 54 00:03:42,600 --> 00:03:45,600 Speaker 1: the fish will go through. A Derff said, oh, when 55 00:03:45,640 --> 00:03:49,160 Speaker 1: they're ready, they become silver eels, which is like puberty. 56 00:03:49,400 --> 00:03:51,680 Speaker 1: We often call them silver because they have a silver 57 00:03:51,760 --> 00:03:55,320 Speaker 1: belly and a black dorsal area. It's an adaptation to predation. 58 00:03:57,280 --> 00:04:00,720 Speaker 1: Many fish have the space of color pattern. When from below, 59 00:04:00,960 --> 00:04:03,840 Speaker 1: their silver bellies blend in with the brighter light coming 60 00:04:03,880 --> 00:04:07,240 Speaker 1: from the surface, and when viewed from above, their darker 61 00:04:07,240 --> 00:04:12,400 Speaker 1: backs blend in with the dimmer water below. But this 62 00:04:12,520 --> 00:04:16,599 Speaker 1: change in eels doesn't happen overnight. Transitioning from a yellow 63 00:04:16,600 --> 00:04:19,520 Speaker 1: eel to a silver eel can take twenty or thirty years. 64 00:04:20,279 --> 00:04:23,480 Speaker 1: Once the process finally ends, they return to their roots 65 00:04:23,560 --> 00:04:28,000 Speaker 1: and head seaward. Only then can the eels attain sexual maturity, 66 00:04:28,360 --> 00:04:32,400 Speaker 1: the fifth and final stage in their life cycle. However, 67 00:04:32,920 --> 00:04:35,880 Speaker 1: we don't know much about the reproductive stage of angle 68 00:04:35,920 --> 00:04:39,080 Speaker 1: at eels because no one has ever caught a sexually 69 00:04:39,160 --> 00:04:43,480 Speaker 1: mature eel alive in the wild. Likewise, no one has 70 00:04:43,480 --> 00:04:47,600 Speaker 1: observed these eels spawning in their natural habitat. The scientists 71 00:04:47,600 --> 00:04:50,400 Speaker 1: have yet to catch wild angleids in the act, if 72 00:04:50,440 --> 00:04:55,640 Speaker 1: you will, whatever happens out there. Experts think freshwater eels 73 00:04:55,760 --> 00:04:59,719 Speaker 1: die shortly after mating. The laboratory researchers have managed to 74 00:05:00,200 --> 00:05:03,400 Speaker 1: sexual maturity in silver eels by injecting them with hormones. 75 00:05:03,880 --> 00:05:08,200 Speaker 1: But after the transition their health declines, and Dereff said 76 00:05:08,560 --> 00:05:12,320 Speaker 1: the bones become decalcified like a woman during menopause. It's 77 00:05:12,360 --> 00:05:16,960 Speaker 1: super interesting, actually, and then their digestive tract their gut regresses. 78 00:05:18,600 --> 00:05:21,560 Speaker 1: Maybe that's just as well a breeding age. Freshwater eels 79 00:05:21,640 --> 00:05:24,440 Speaker 1: get together in places where their usual food options like 80 00:05:24,520 --> 00:05:28,000 Speaker 1: insects and small fish are probably rare or non existent. 81 00:05:30,000 --> 00:05:32,560 Speaker 1: Out in the Atlantic Ocean, there's a region called the 82 00:05:32,600 --> 00:05:38,120 Speaker 1: Sargasso Sea. Unlike the Mediterranean, the Red and most other seas, 83 00:05:38,120 --> 00:05:41,600 Speaker 1: this one is not bordered by any land masses. Instead, 84 00:05:41,839 --> 00:05:46,560 Speaker 1: its borders are formed by strong ocean currents. Both American 85 00:05:46,720 --> 00:05:50,480 Speaker 1: and European species of freshwater eels come here to reproduce. 86 00:05:51,120 --> 00:05:54,200 Speaker 1: A research suggests that they might use magnetic fields as 87 00:05:54,240 --> 00:05:58,280 Speaker 1: a navigation tool. The European eel has the longest path 88 00:05:58,360 --> 00:06:02,560 Speaker 1: to travel. Some individ jewels transverse around five thousand miles 89 00:06:02,640 --> 00:06:07,520 Speaker 1: or eight thousand kilometers to get there from Norway, half 90 00:06:07,560 --> 00:06:10,159 Speaker 1: a world away. The freshwater eels that live in and 91 00:06:10,200 --> 00:06:13,000 Speaker 1: around the Pacific Ocean have spawning areas of their own. 92 00:06:13,839 --> 00:06:16,119 Speaker 1: The Japanese eel is thought to breed at a site 93 00:06:16,120 --> 00:06:20,120 Speaker 1: west of the Mariana Islands. Other species could be procreating 94 00:06:20,160 --> 00:06:26,320 Speaker 1: somewhere between New Caledonia and Fiji. Eels release their eggs 95 00:06:26,400 --> 00:06:30,560 Speaker 1: underwater to be fertilized by clouds of expelled sperm that 96 00:06:30,600 --> 00:06:34,040 Speaker 1: goes for both freshwater eels and the non freshwater species 97 00:06:34,160 --> 00:06:38,719 Speaker 1: such as the aforementioned moras and conger eels, speaking of 98 00:06:38,720 --> 00:06:41,880 Speaker 1: which a derec explained that we know even less about 99 00:06:41,880 --> 00:06:45,640 Speaker 1: conger eels than anglids when it comes to reproduction. She said, 100 00:06:46,000 --> 00:06:48,480 Speaker 1: but we think there is at least a spawning area 101 00:06:48,680 --> 00:06:53,160 Speaker 1: in the Mediterranean Baby. Hopefully future research will shed some 102 00:06:53,240 --> 00:07:01,479 Speaker 1: light on their private lives. Today's episode is based on 103 00:07:01,480 --> 00:07:04,120 Speaker 1: the article how do eels reproduce? On how stuff works 104 00:07:04,160 --> 00:07:07,279 Speaker 1: dot Com, written by Mark Mancini. Brain Stuff is production 105 00:07:07,320 --> 00:07:09,400 Speaker 1: of I Heart Radio in partnership with how stuff works 106 00:07:09,400 --> 00:07:12,120 Speaker 1: dot Com and is produced by Tyler Clang. Four more 107 00:07:12,160 --> 00:07:14,760 Speaker 1: podcasts from my heart Radio, visit the iHeart Radio app, 108 00:07:14,880 --> 00:07:17,720 Speaker 1: Apple Podcasts, or wherever you listen to your favorite shows.