Why No Fish Lives Below 8,400 Metres

Chapters

1. 0:00 A fish at 5,990 metres
2. 1:05 Voyage to the Ridge 3, dive 09
3. 1:43 Three and a half hours down
4. 2:43 Landing at 5,998 metres, and a can
5. 4:01 The rocks that were mud
6. 6:17 16:05 UTC: a snailfish?
7. 7:05 What the frame shows
8. 7:43 A blind cusk eel, from the chat
9. 9:28 The eggs, and what the archive kept
10. 10:40 Two families at the door
11. 11:11 8,370 m in an open net
12. 11:46 Cold, oxygen, food, dark
13. 13:11 A sponge that eats meat
14. 14:28 The number that keeps rising
15. 15:26 Pressure, proteins and TMAO
16. 16:52 Five snailfish from 7,000 metres
17. 17:17 The arithmetic: about 8,200 metres
18. 18:05 Why starfish do not stop
19. 19:13 What the cameras found
20. 19:41 The floor of the Atlantic
21. 20:33 Oil in the winch, weed in the engine
22. 21:49 What is still open

Transcript

A fish at 5,990 metres 0:00

Five thousand nine hundred and ninety metres down, in the Puerto Rico Trench, a pale fish swims into the lights of a robot. The robot is built for six thousand metres, and on this day it is at its limit. The trench under it keeps going for more than two thousand metres. Nobody on board can follow. So here is the question this episode answers. How much deeper can a fish go? The answer is a number. It was not measured with a camera. It was measured inside fish muscle.

The deepest quarter of the ocean, from about eighty-four hundred metres to eleven thousand, has never produced a single fish. Not in a net, not on a camera. In twenty fourteen, a team measured one molecule in fish muscle and predicted where fish should stop. Every moving picture in this episode is NOAA footage, from one dive to six thousand metres and a handful of dives above it. Every number comes from the dive record, a paper, or our own reading of the robot's instruments.

Voyage to the Ridge 3, dive 09 1:05

The ship is NOAA Ship Okeanos Explorer, on an expedition called Voyage to the Ridge three. The date is the thirty-first of August, twenty twenty-two. The site is north of Puerto Rico, on a fault zone in the trench. Two vehicles go down together. Deep Discoverer is the robot, with the arms and the main camera. Seirios hangs above it on the cable, and films it from above. Over three legs that summer, Voyage to the Ridge made nineteen dives, from three hundred and forty-seven metres to six thousand and twelve. This is the deepest of them.

Three and a half hours down 1:43

They go in at twelve twenty-one UTC. The way down takes three hours and twenty-seven minutes of open, empty blue. Six kilometres of cable pay out above them. Near the end, the pilot is watching two depth readings that disagree, and a sonar that has not found the floor yet. Then the floor comes up into the lights.

Landing at 5,998 metres, and a can 2:43

Fifteen forty-eight. The robot settles at five thousand nine hundred and ninety-eight metres. The mud is so soft that every touch of a thruster raises a cloud. The first thing in view is not an animal. It is a can, lying on its side in the mud, hung with long orange threads of rust. The summary calls them rusticles, and the room already has an idea about who made them.

Scattered over the mud there are brown tangles, and they are sargassum. It is seaweed that floats at the surface of this part of the Atlantic, six kilometres straight up. It sinks, and it ends here. Keep that in mind, because it will matter twice. Once as food for the animals on this floor. And once more, at the end of the dive, for a different reason.

The rocks that were mud 4:01

The team has permission to collect today, because this spot is outside Puerto Rico's territorial waters. So they try. A small tube anemone sits on what looks like a rock. The arm closes on the rock, and the rock dissolves. It was never a rock. It was mud, packed tight enough to hold a shape, and it collapses in the claw. They try for the anemone alone. The arm cradles it, lifts it toward the sample box, and then.

For the next half hour the floor keeps playing the same trick. Angular blocks, with edges and layers and holes, lie all over the slope. They look exactly like rock. The pilots reach for at least four of them. Every one comes apart in the claw. The dive summary says it plainly. It was surprising that all of them were unconsolidated. A second anemone does make it into a box, at sixteen twelve. No rock comes home from this dive. Here is the room, looking at the inside of one.

16:05 UTC: a snailfish? 6:17

Sixteen oh five. Something moves at the edge of the lights. A fish, and a small sea urchin beside it. The pilot drives closer. The fish does not flee. It turns along the robot and shows its whole body, and the room reacts.

What the frame shows 7:05

Now look at what the camera gives you. NOAA's still camera took a full frame at sixteen oh seven. A pale, rounded head, and a small dark eye. Long, low fins running along the back and the belly, meeting in a thin tail. And one detail that is easy to miss. Under the throat hangs a thin thread. That is where a cusk eel carries its pelvic fins, reduced to filaments. Most snailfish carry a sucking disc there instead. In this frame there is no disc to be seen. The picture points one way. It does not settle a species.

A blind cusk eel, from the chat 7:43

On a dive like this, the room is not only the people on the ship. Specialists watch the live video from shore and type into a chat. One of them is Ken Sulak, a fish biologist. A few minutes after the fish, the science lead reads out his message.

Leucicorus is a genus of deep cusk eels, and it is rare in collections. A twenty twenty-two revision in Zootaxa counts about thirty specimens caught since eighteen ninety-nine, and about thirty-five more seen by robots in the Pacific, between thirty-eight hundred and fifty-seven hundred metres. This is one of them, filmed by the same robot in twenty sixteen, near the Mariana Trench, at forty-nine hundred and thirty-eight metres, and annotated as Leucicorus. Same pale body, same tapering tail. The revision also names a new species from the Atlantic, with eyes that have no lens. It is named after Mackenzie Gerringer, a biologist of hadal fishes. Remember that name.

The eggs, and what the archive kept 9:28

There is one more guess on this dive. Before the fish, the robot passed small white spheres on the mud. The dive summary calls them two small white things that might have been fish eggs, with a question mark. Then the chat offers what kind.

Now look at what the archive kept. NOAA files this dive in five-minute files, and each file carries keywords typed by the people watching. The file from sixteen hundred says Liparidae, the snailfish family. The file from sixteen ten says Leucicorus. The dive summary, written after, lists a cusk eel and no snailfish. Nothing was collected, not the fish and not the eggs. So the deepest snailfish in this catalogue is a guess, twice over.

Two families at the door 10:40

Why does it matter which one it was? Because these two families are the ones standing at the door. In the trenches of the Pacific, the deepest fish ever filmed are snailfish. The Mariana snailfish lives from sixty-two hundred to eighty-one hundred and seventy-eight metres. In twenty twenty-two, a baited camera in a trench off Japan filmed a juvenile snailfish at eighty-three hundred and thirty-six metres. And the record for the deepest fish ever caught belongs to a cusk eel.

8,370 m in an open net 11:11

Abyssobrotula galatheae. Caught in nineteen seventy by the research ship John Elliott Pillsbury, at eighty-three hundred and seventy metres, in this same trench. It is still quoted as the record. Hadal biologists doubt it. The trawl could not close, so it was open all the way down and all the way up. The species is common between three and six thousand metres, and it is known to swim far above the floor. Nobody has seen one that deep since. The record can be neither confirmed nor refuted.

Cold, oxygen, food, dark 11:46

So fish reach six thousand metres easily, and eight thousand with effort. Something stops them before eleven. The obvious suspects are cold, oxygen, food and darkness. This dive measured every one of them, and not one of them explains a floor.

Start with cold. The robot carries a Sea-Bird CTD, and NOAA publishes its raw file. We decoded it with the instrument's own calibration, and checked it against the dive summary. Both say two point oh three degrees at the bottom. At the surface the water is twenty-nine degrees. About four kilometres down it is two point three. From there to the floor, it drops by three tenths of a degree. Cold stops changing long before fish do.

Oxygen. Just after landing, the robot fired a water bottle at five thousand nine hundred and ninety-four metres. Seven point two nine millilitres of oxygen per litre. This water is well breathed. Food. The sargassum on this floor came from the surface. Amphipods, the small crustaceans a snailfish hunts, are more abundant in trenches, not less. And darkness is not new. It has been completely dark since about a thousand metres, and fish live through five thousand metres of it.

A sponge that eats meat 13:11

The food web down here is real enough to have predators nobody expects. The last sample of the dive, at sixteen forty-seven, is a small sponge, Asbestopluma. It belongs to a family of sponges that eat meat. The biology lead explains it while the arm goes in.

The number that keeps rising 14:28

One number never levels off. At the bottom, the same CTD reads six thousand one hundred and twenty-one decibars. That is about six hundred times the pressure at the surface, and it keeps rising, one atmosphere for every ten metres, all the way to the bottom of the deepest trench. On this dive, even the robot noticed.

Pressure, proteins and TMAO 15:26

Pressure does not crush a fish. A fish is mostly water, and water barely squeezes. What it does is quieter. A protein works by changing shape, and each change shifts the water packed around it. At high pressure, water is forced into the protein, and enzymes stop working the way they should. Deep fish carry a defence, a small molecule called TMAO. It changes the structure of the water around the protein, and it holds the shape.

And the deeper the fish, the more of it they carry. Here is the ladder, on the same expedition. About six hundred metres, off San Juan: a sea toad, the fish that holds its breath. A shallow sea fish carries forty to fifty millimoles of TMAO in each kilogram of muscle.

About fourteen hundred and fifty metres, on the Mona Block, west of Puerto Rico: a halosaur, long and slow over the mud. Every rung down, the same robot, the same lights, and more of the same molecule in the muscle.

Forty-nine hundred and thirty-eight metres, in the Pacific: the blind cusk eel again. At about this depth, Yancey and colleagues had measured TMAO in many species of fish, from several families. At forty-eight hundred and fifty metres, it reached two hundred and sixty-one millimoles per kilogram.

Five snailfish from 7,000 metres 16:52

The deepest point on the line was the hardest to get. In the Kermadec Trench, north of New Zealand, the team caught five intact snailfish at seven thousand metres. Notoliparis kermadecensis. Their muscle held three hundred and eighty-six millimoles of TMAO per kilogram. Two of the five authors of that paper were Jeffrey Drazen and Mackenzie Gerringer.

The arithmetic: about 8,200 metres 17:17

Now the arithmetic. TMAO is dissolved in the fish, so it adds to how salty the fish is inside. A shallow fish sits around three hundred and fifty milliosmoles. Seawater is about eleven hundred. The snailfish at seven thousand metres was at nine hundred and ninety-one. Draw the line through the points, and a fish reaches the saltiness of the sea itself at about eighty-two hundred metres.

Past that point, the only way to add protection is to become saltier than the sea. Then water flows into the fish, all the time, through the gills. In the paper's words, that would mean significant physiological reorganization. Not impossible in principle. But no fish has done it.

Why starfish do not stop 18:05

And that explains the animals on this floor that do not stop. This starfish, a brisingid, the urchins, the amphipods. Most invertebrates in the sea are already as salty inside as the water around them. So when they need protection, they do not add TMAO. They swap it in, in place of the molecules they used in shallow water. A twenty eighteen study of amphipods, from the shore down to the Mariana Trench, found exactly that. TMAO up, the shallow molecules down. An amphipod has been hauled up from the Challenger Deep, at ten thousand eight hundred and ninety-seven metres. The fish have a wall. The crustaceans have a trade. Even at six thousand metres, the room was still being surprised by the colours.

What the cameras found 19:13

The cameras agree. When the paper came out, baited cameras had found fish on every deployment down to seventy-seven hundred metres, and on none of the fourteen deeper. Since then the record has moved twice. Eighty-one hundred and seventy-eight metres in twenty seventeen. Eighty-three hundred and thirty-six in twenty twenty-two. Both snailfish, and both inside the band where the chemistry says fish should run out.

The floor of the Atlantic 19:41

Which brings us back to this trench. The deepest point of the whole Atlantic is here, at roughly eighty-four hundred metres. The floor for fish, about eighty-two hundred. The Pillsbury's cusk eel, eighty-three seventy. The robot, six thousand. The floor of the Atlantic sits almost exactly on the line where fish should end. The room said it out loud.

Oil in the winch, weed in the engine 20:33

Sixteen thirty-one. The room calls the bridge. The winch that holds six kilometres of cable is leaking oil, at full depth. The team keeps working for another quarter of an hour and takes the last sample. At sixteen forty-nine they pull fifty metres off the bottom so the engineers can get to it. And there is a second problem. The ship keeps drawing sargassum into its engine intakes. The same weed that feeds this floor is now choking the ship. Here is the call.

What is still open 21:49

One hour and seven minutes on the bottom. Here is what is still open, exactly. The fish at five thousand nine hundred and ninety metres was filmed, not caught, and its name rests on a live message and a family guess. The eggs on the mud were never picked up. The Pillsbury record cannot be settled from a net that never closed. And the floor at eighty-two hundred metres is a line drawn through fish from seven thousand. To test it, someone has to bring a fish up from below eight thousand and measure its muscle. No robot on this ship can go there.

What the record holds is one dive at the limit of a machine. A fish with two names in the archive. Water that is cold, clear and full of oxygen. And a single number that never stops rising. Next time, a place. On the Mid-Atlantic Ridge, between the Azores and Iceland, there is one known field of high-temperature black smokers, Moytirra, found in twenty eleven. In twenty twenty-two, this robot went to look. Next episode, the only hot vents between the Azores and Iceland.

Description and sources

EX2206 dive 09 spent 67 minutes on the floor of the Puerto Rico Trench at the full operating depth of ROV Deep Discoverer. At 16:06 UTC the room called a pale fish a snailfish; minutes later ichthyologist Ken Sulak wrote in from shore that it was a cusk eel, probably of the blind genus Leucicorus. NOAA's index kept both guesses. We decoded the robot's raw CTD file: the water is 2.03 °C at the bottom and changes by only 0.3 °C over the last 2,100 decibars, and the dive's water bottle found 7.29 ml of oxygen per litre. The number that keeps rising is pressure, 6,121 dbar here. Yancey and colleagues (2014) showed that fish fight pressure with TMAO, which climbs from 40-50 mmol/kg in shallow fish to 386 in snailfish at 7,000 m, and that a fish reaches the saltiness of seawater at about 8,200 m. The deepest point of the Atlantic, in this trench, is about 8,400.

PRINT-READY, FROM THIS CHANNEL

Read This Dive, Vol. 01 — the method, and five dives read with it

https://therepository.gumroad.com/l/read-this-dive

PRIMARY SOURCES

• NOAA Ocean Exploration, EX-22-06 ROV dive 09 summary (Main Ridge, Puerto Rico Trench, 31 August 2022): https://repository.library.noaa.gov/view/noaa/49989/noaa_49989_DS10.pdf
• NOAA Ocean Exploration, EX2206 dive video, track, CTD and ancillary data: https://www.ncei.noaa.gov/waf/okeanos-rov-cruises/ex2206/
• Yancey, P.H., Gerringer, M.E., Drazen, J.C., Rowden, A.A. & Jamieson, A. (2014). Marine fish may be biochemically constrained from inhabiting the deepest ocean depths. PNAS 111(12): 4461-4465. https://doi.org/10.1073/pnas.1322003111
• Gerringer, M.E. (2019). On the success of the hadal snailfishes. Integrative Organismal Biology 1(1): obz004. https://doi.org/10.1093/iob/obz004
• Schwarzhans, W.W., Nielsen, J.G. & Mundy, B.C. (2022). Revision of the circumglobal deep-sea genus Leucicorus (Teleostei, Ophidiidae) with two new species. Zootaxa 5100(4).
• Linley, T.D. et al. (2016). Fishes of the hadal zone including new species, in situ observations and depth records of Liparidae. Deep-Sea Research I 114: 99-110.
• Natural History Museum (2023). Deepest ever fish filmed at a depth of 8,336 metres: https://www.nhm.ac.uk/discover/news/2023/april/deepest-ever-fish-filmed-depth-8336-metres.html
• Downing, A., Wallace, G. & Yancey, P. (2018). Organic osmolytes of amphipods from littoral to hadal zones. Deep-Sea Research I 138: 1-10.
• Stewart, H.A. & Jamieson, A.J. (2019). The five deeps. Earth-Science Reviews 197: 102896.

ARCHIVE PLATES — public-domain documents, with author, date, archive and licence.

• 2022-08-31 | NOAA Ocean Exploration | public-domain

Abyss Report reads the deep ocean through the record that resolved it -- instrument, date, position and the primary sources above.

About the material. Every moving frame is NOAA Ocean Exploration video, public domain, credited on screen with dive, depth and position: EX2206 dive 09 (Puerto Rico Trench, 31 August 2022) and, for the depth ladder, EX2206 dives 01, 02, 05 and 06 and EX1605 leg 1 dive 03 (Mariana region, 2016). Dive 09 depths come from the vehicle's 1 Hz track; EX1605L1 and EX2206 dive 05 positions use the dive tracks and summary, not the catalogue coordinates. Twelve cuts drop the narration and play the original control room audio; each window was re-transcribed in isolation twice and used only where both passes matched. The temperature and pressure profile is our own decode of the robot's raw SBE 911plus file, checked against the dive summary. The fin reading on the 16:07 frame is ours. Every diagram is drawn in code from the sources listed.

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