What Lives at the Bottom of the Mariana Trench?
The short answer
Swarms of amphipods, small scavengers that look like shrimp. The bottom of the Mariana Trench lies almost 11 km (about 7 miles) down, below the deepest fish. Sea cucumbers and giant single cells live there too. Our trash got there as well: in 1998 a robot camera spotted a plastic bag 10,898 m (35,755 ft) down.
Down there, whatever falls gets eaten. That includes plastic.
Shells that need armor
Shells contain calcium carbonate, and below about 4,000 to 5,000 m seawater dissolves it. Even so, the amphipod Hirondellea gigas, 2 to 5 cm long, has been caught at 10,897 m in the Challenger Deep. A gel of aluminum hydroxide covers its body and keeps its shell from dissolving. Scientists think the aluminum most likely comes from the mud on the seafloor rather than from the water.
Its gut makes an enzyme that turned dried wood into sugar in a lab test, so it could digest wood that sinks to the bottom. Nobody has filmed one eating a branch.
Amphipods also find food fast. At baited cameras, the first ones arrive 5 to 21 minutes after touchdown.
The fish that wasn't
On January 23, 1960, Piccard, one of the first two people to reach the bottom, reported "some type of flatfish, resembling a sole, about 1 foot long." Scientists who studied the report think it was probably a holothurian, a sea cucumber. No fish has ever been found below about 8,400 m. Sea cucumbers, though, have been filmed at the very bottom: 65 of them, at 10,876 to 10,908 m in the Challenger Deep.
The giant single cells are called xenophyophores. Each one is a single cell, often more than 10 cm (4 inches) across. Scientists found them as deep as 10,641 m in the Sirena Deep, in the Mariana Trench.
Plastic in the deepest place
The bag was seen by the robot Kaiko on May 20, 1998, on the trench floor, not in the Challenger Deep itself. Plastic also turns up inside the animals. One study checked amphipods caught between 7,000 and 10,890 m. Every one sampled from the Mariana Trench had at least one tiny particle in its gut. Not every particle was plastic, but the plastics found included nylon and polyethylene.
In 2014, scientists caught a new amphipod species in the trench at 6,010 to 6,949 m. One of the animals had a plastic fiber in its gut. Scientists named the species Eurythenes plasticus. Swallowing plastic is not the same as digesting it, and there is no evidence that these animals can.
Key numbers
| What | Number | Source |
|---|---|---|
| Depth of the plastic bag seen by a robot camera in the Mariana Trench (1998) | 10,898 m (35,755 ft) | Chiba et al. 2018, Marine Policy |
| Deepest known point, the Challenger Deep | about 10,935 m, give or take 6 m | Greenaway et al. 2021, NOAA repository |
| No fish has been found below | about 8,400 m | Yancey et al. 2014, PNAS |
| Depth where the aluminum coated amphipod Hirondellea gigas was caught | 10,897 m | Kobayashi et al. 2019, PLOS ONE |
| Length of Hirondellea gigas | 2 to 5 cm | Kobayashi et al. 2012, PLOS ONE |
| Depth below which seawater dissolves calcium carbonate | about 4,000 to 5,000 m | Kobayashi et al. 2019, PLOS ONE |
| Time for the first amphipods to reach a baited camera | 5 to 21 minutes | Jamieson et al. 2009, Proc. R. Soc. B |
| Size of the giant single cells (xenophyophores) | often more than 10 cm (4 inches) | Scripps Institution of Oceanography, 2011 |
| Deepest giant single cells seen, Sirena Deep | 10,641 m | Scripps Institution of Oceanography, 2011 |
| Sea cucumbers filmed in the Challenger Deep | 65, at 10,876 to 10,908 m | Gallo et al. 2015, Deep Sea Research I |
| Depth range of the amphipods checked for particles in their guts | 7,000 to 10,890 m | Jamieson et al. 2019, Royal Society Open Science |
| Where Eurythenes plasticus was caught (2014) | 6,010 to 6,949 m | Weston et al. 2020, Zootaxa |
What the film simplifies
- The aluminum coat is a gel of aluminum hydroxide, not metal foil. The film draws it as shiny chrome so the snailfish can ask if they come wrapped.
- "As big as your fist" rounds the source, which says the cells are often more than 10 cm (4 inches) across.
- The film shows amphipods tugging a thread out of the bag. Eurythenes plasticus was caught at 6,010 to 6,949 m, about 4 km above the bag, and its card carries a 6,949 M stamp. Nothing links its fiber to this bag.
- Piccard described a flatfish that looked like a sole. The film's sketch looks like an ordinary fish, and "probably a sea cucumber" is the best guess of the scientists who studied his report.
- "If it falls, somebody eats it" means swallowed. Nobody has shown that these animals digest plastic.
- Pressure as a giant waiter, the glowing 8,400 m line and the jealous snailfish are cartoon. The 8,400 m line marks the depth below which no fish has been found. It is not a wall.
At a glance
Runtime 1 minute 3 seconds · Topics: Deep sea life, Ocean trenches, Plastic pollution, Adaptation · Captions on YouTube · Full script below
Test yourself
Why do amphipods at the bottom of the Mariana Trench need a coat of aluminum?
Answer
Below about 4,000 to 5,000 m seawater dissolves calcium carbonate, which shells contain. A gel of aluminum hydroxide covers the amphipod and keeps its shell from dissolving.
The bag lay at 10,898 m, and Eurythenes plasticus was caught no deeper than 6,949 m. How far apart were they?
Answer
3,949 m, about 4 km. So nothing links the amphipod's plastic fiber to that bag.
In 1960 Piccard said he saw a flatfish at the bottom. Why do scientists think it was probably a sea cucumber?
Answer
No fish has ever been found below about 8,400 m, far above the bottom. Sea cucumbers have been filmed at 10,876 to 10,908 m in the Challenger Deep.
The script
Pressure: Yours?
Host: A robot spotted this bag at the bottom of the Mariana Trench, almost eleven kilometers down.
Snailfish: Let me see! What else is down there?
Pressure: No fish past this line!
Snailfish: Then YOU tell me!
Host: Scientists drop cameras with bait.
Pressure: Dinner's served!
Host: In minutes, the guests arrive: amphipods, like tiny shrimp!
Snailfish: Ooh! Snacks!
Host: Down here, the water dissolves shells. So these coat theirs in aluminum.
Snailfish: They come wrapped?
Host: Some cells down here are as big as your fist. One cell.
Snailfish: A BLOB gets to live down there?!
Host: In 1960, one of the first two people down here said he saw a fish.
Snailfish: A FISH? Down THERE?!
Host: It was probably a sea cucumber.
Pressure: Close enough!
Snailfish: That is NOT a fish!
Host: Sea cucumbers eat the mud, for the crumbs inside.
Pressure: Down here, if it falls, somebody eats it.
Host: Even plastic. One new amphipod had it in its belly.
Pressure: Hello, my name is...
Pressure: Plasticus.
Snailfish: So whose bag is it?
Sources
- A plastic bag was seen 10,898 m down in the Mariana Trench in 1998.Chiba et al. 2018, Marine Policy
- Hirondellea gigas from 10,897 m is covered in an aluminum gel that protects its shell; calcium carbonate dissolves below about 4,000 to 5,000 m.Kobayashi et al. 2019, PLOS ONE
- Hirondellea gigas is 2 to 5 cm long and has a gut enzyme that makes glucose from dried wood.Kobayashi et al. 2012, PLOS ONE
- The first amphipods reach baited cameras 5 to 21 minutes after touchdown.Jamieson et al. 2009, Proc. R. Soc. B
- Giant single cells, often more than 10 cm across, were found as deep as 10,641 m in the Sirena Deep.Scripps Institution of Oceanography, 2011
- Piccard's 1960 flatfish was probably a sea cucumber.Jamieson and Yancey 2012, Biological Bulletin
- 65 sea cucumbers were filmed at 10,876 to 10,908 m in the Challenger Deep.Gallo et al. 2015, Deep Sea Research I
- Every amphipod sampled from the Mariana Trench had at least one particle in its gut.Jamieson et al. 2019, Royal Society Open Science
- Eurythenes plasticus, caught at 6,010 to 6,949 m in the Mariana Trench, had a plastic fiber in its gut.Weston et al. 2020, Zootaxa
- The Challenger Deep is about 10,935 m deep.Greenaway et al. 2021, NOAA repository
- No fish has been found below about 8,400 m.Yancey et al. 2014, PNAS