WEBVTT Kind: captions Language: en 00:00:04.800 --> 00:00:10.020 During World War II, American sonar researchers encountered a mystery - 00:00:10.020 --> 00:00:13.220 an echo from what seemed like the ocean bottom, 00:00:13.220 --> 00:00:17.920 but at depths where no bottom should be. 00:00:20.200 --> 00:00:23.680 Even stranger - the false bottom moved. 00:00:23.680 --> 00:00:28.760 Deep in daytime, it crept closer to the surface as the sun went down. 00:00:28.760 --> 00:00:32.180 Was this “phantom bottom” enemy submarines? 00:00:32.180 --> 00:00:33.700 Water currents? 00:00:33.700 --> 00:00:36.880 Or maybe something more mysterious?... 00:00:39.680 --> 00:00:46.640 Trawling the depths with nets and dives by early submersibles revealed the answer: 00:00:46.640 --> 00:00:50.220 Fish. Trillions and trillions of them. 00:00:50.220 --> 00:00:56.020 The sonar mystery revealed life in a part of the ocean where none was thought to be: 00:00:56.020 --> 00:00:58.220 the mesopelagic zone. 00:01:01.540 --> 00:01:05.100 The open ocean - everything except the coast and bottom - 00:01:05.100 --> 00:01:07.640 is not the same at different depths. 00:01:07.640 --> 00:01:12.520 It’s made up of habitats defined by currents, pressure, and temperature, 00:01:12.520 --> 00:01:14.720 but most of all, by light. 00:01:14.720 --> 00:01:20.210 From the surface to around 200 meters is the epipelagic zone. 00:01:20.210 --> 00:01:25.890 Enough sunlight can penetrate here for photosynthetic organisms to thrive, and abundant life feeds 00:01:25.890 --> 00:01:28.650 on those organisms. 00:01:28.650 --> 00:01:34.090 The mesopelagic is below, from around 200 to 1000 meters. 00:01:34.090 --> 00:01:39.180 Though the ocean is often much deeper, that’s deep enough that almost no light penetrates, 00:01:39.180 --> 00:01:42.740 making it very hard to explore - until recently. 00:01:47.900 --> 00:01:54.120 New technology brought to bear by ships of exploration like NOAA’s Okeanos Explorer 00:01:54.140 --> 00:01:59.480 are starting to help expose the mysterious creatures of the mesopelagic. 00:01:59.480 --> 00:02:05.460 The water can seem empty…but remotely operated vehicles’ with advanced cameras 00:02:05.460 --> 00:02:09.960 reveal a wild riot of colors, shapes, and bioluminescence. 00:02:14.700 --> 00:02:16.240 Jellyfish… 00:02:16.240 --> 00:02:18.160 Siphonophores… 00:02:18.160 --> 00:02:20.080 Ctenophores… 00:02:20.080 --> 00:02:22.200 Hydromedusae… 00:02:22.200 --> 00:02:25.720 These jelly-like animals don’t survive net surveys, 00:02:25.720 --> 00:02:30.080 so their behaviors are often being seen for the first time. 00:02:30.080 --> 00:02:37.200 Krill and many types of squid are abundant… in some places, astonishingly so. 00:02:37.200 --> 00:02:42.000 There are also fish - like these saw-toothed eels that hover vertically in the water, 00:02:42.000 --> 00:02:45.780 waiting almost invisibly for prey to pass above. 00:02:45.780 --> 00:02:50.000 In fact, there are so many fish here that recent studies estimate that 00:02:50.000 --> 00:02:55.300 up to 10 billion tons of fish - or 90% of the fish in the world, by mass - 00:02:55.300 --> 00:02:57.300 are found in the mesopelagic. 00:02:58.580 --> 00:03:04.000 Just one family of fish called bristlemouths may number a quadrillion, 00:03:04.000 --> 00:03:05.940 which is a thousand trillion!!! 00:03:05.940 --> 00:03:09.280 This makes them the most numerous vertebrates in the world. 00:03:11.960 --> 00:03:16.100 Although many of the fish here are tiny, they occur in huge numbers, 00:03:16.100 --> 00:03:17.920 and they are always on the move. 00:03:21.380 --> 00:03:27.740 These animals - fish and invertebrates - make the up and down journey that researchers discovered, 00:03:27.740 --> 00:03:30.700 twice every day. 00:03:30.700 --> 00:03:35.860 What’s truly amazing is that we now know it’s part of the largest migration, by mass, 00:03:35.860 --> 00:03:37.760 in the world! 00:03:37.760 --> 00:03:40.820 This migration helps fill in some of the missing links 00:03:40.820 --> 00:03:43.960 in scientists’ understanding of the ocean food web. 00:03:47.820 --> 00:03:53.320 Mesopelagic animals rise in the evening because they’re feeding on plankton near the surface. 00:03:55.540 --> 00:04:00.440 As the sun rises they sink back into the dark to hide from predators. 00:04:02.700 --> 00:04:04.620 They don’t all succeed. 00:04:04.620 --> 00:04:09.700 One reason marine mammals like Elephant seals, King and Emperor penguins, 00:04:09.700 --> 00:04:16.700 and some large fish like Bigeye tuna and swordfish dive so deep is to feed on animals here. 00:04:16.700 --> 00:04:20.280 Understanding the connection between some of the fish we like to eat 00:04:20.280 --> 00:04:24.000 and mesopelagic creatures shows us that we need to be careful 00:04:24.000 --> 00:04:26.560 about impacting this habitat too much. 00:04:28.000 --> 00:04:30.840 And we wouldn’t be the only ones affected. 00:04:30.880 --> 00:04:35.040 When small mesopelagic creatures make waste, and when they die, 00:04:35.040 --> 00:04:38.419 they produce what is called “marine snow.” 00:04:38.420 --> 00:04:44.100 In the total darkness of the deep ocean, falling marine snow is the major food source 00:04:44.100 --> 00:04:50.220 for many bottom animals like corals and sponges that form their own amazing ecosystems. 00:04:51.260 --> 00:04:57.600 Creatures in the mesopelagic have, up until recently, been relatively safe from major human impacts, 00:04:57.600 --> 00:05:02.980 but increased pollution and overfishing could have huge consequences. 00:05:02.980 --> 00:05:08.580 Some fishing industries are now developing ways to harvest tons of small mesopelagic creatures, 00:05:08.580 --> 00:05:12.020 which could disrupt the food chain. 00:05:14.520 --> 00:05:18.960 But doing this important research now into the mysteries of the mesopelagic 00:05:18.960 --> 00:05:22.580 gives science a chance to get ahead of commercial exploitation. 00:05:23.080 --> 00:05:29.000 If we keep searching, good data will fuel smart policies to support sustainable use 00:05:29.009 --> 00:05:31.209 of this virtually unexplored frontier.