Here’s a mystery: below 8,400 meters there are no fish. There are other creatures: sea cucumbers, anemones, tiny worms, but no one has ever seen a fish. At 8,370 meters? There are fish. But not below 8,400 meters. At its deepest the ocean reaches roughly 11,000 meters, so there is plenty of space. And right below 8,400 meters it’s equally cold, equally dark, equally middle-of-no-where as it is right above 8,400 meters. But there is some magic line at 8,400 meters, below which fish apparently cannot go. No one understands why this line exists. Or if perhaps one day we’ll find a fish that can, in fact, cross it. But for now, scientists do have some ideas.
First, let’s talk about animals that can go below 8,400 meters. Lets talk about the deepest living animals on earth. The hadal amphipod is one of them:
This specimen was collected in the Sirena Deep . At 10,500 meters, the Sirena Deep is one of the deepest places on Earth, resting a little under 100 miles south of Guam. This creature is one of the Sirena Deeps’ few residents. The yellow patches on either side of its head may be simple eyes . The purpose of the red globe-like structures, along with much of the animal’s biology, remains a mystery. They are abundant below 8,400 meters.
Why do hadal amphipods thrive where fish do not? One person who has thought a lot about this is Dr. Paul Yancey, a deep-sea expert at Whitman College. The only real difference between 8,370 meters–the greatest depth at which fish have been found–and 8,400 meters, is pressure. Add an extra 30 meters, or about 100 feet, of seawater and you’ve added three extra atmosphere’s worth of pressure. Anyone who has tried to dive to the bottom of a pool and felt a squeeze in their ears can tell you that even a small change in pressure can make a big difference. The good news for fish is that water is not compressible like the air inside our ears. I had always assumed that, as long as an animal is made mostly of water, without any trapped gas to expand and compress, depth shouldn’t be an issue. But I was wrong, because pressure squeezes other things.
DNA. Proteins. Membranes. The very building materials of living animals are impacted by pressure. Proteins are particularly prone to problems. Proteins, which perform most of the work in our cells, including the pulling power of muscle, are crushed at high pressures. The muscles of shallow-water fish, for example, do not develop correctly under increased pressure . And so deep sea animals must adapt.
Dr. Yancey has two hypotheses for how deep-sea fish cope with the massive pressure squeeze . One possible adaptation is to counteract. You know that fishy smell from fish? That smell is from a special molecule known as trimethylamine oxide (TMAO). TMAO and regular old table salt have something in common. Have you ever spilled salt on a countertop in a humid climate, and come back to find little drops of moisture around the salt grains? Salt attracts water, and water-attracting molecules are called “osmolytes.” TMAO is also an osmolyte and it helps fish at shallow depths hold onto the water in their bodies, counteracting that other pesky osmolyte–salt–that they swim through all the time.
Dr. Yancey discovered that TMAO also helps cells function normally under pressure, by protecting proteins from crushing water molecules all around them. TMAO serves as a kind of protein stabilizer. Without TMAO, water molecules under pressure force their way into tiny protein folds, breaking apart protein structures and disrupting protein function. With TMAO, water molecules are not able to force themselves into these small cracks, and proteins keep working, even at great depths. Dr. Yancey suspects TMAO helps fish survive in the deep sea. But why the depth limit then? And what does this have to do with the mysterious 8,400 meter line?
“As fish go deeper, they need more TMAO,” Dr. Yancey explains . More pressure may require more TMAO to keep cells working. But you can have too much of a good thing. Remember how TMAO is an osmolyte, a water-attracting molecule? It’s possible, Dr. Yancey hypothesizes, that at 8,400 meters fish “would need so much TMAO to counteract the pressure of the water that water would start flowing uncontrollably into their bodies” . The same molecule that may protect against water pressure may ultimately cause water poisoning. In other words, the strange properties of TMAO may be responsible for the mysterious line in the deep sea, below which fish cannot go.
This hypothesis, however, is very hard to test. And Dr. Yancey concedes that “in another trench, maybe there is a fish that disproves my proposed depth limit.” If fish are there, they certainly are hard to find. And that’s good news for other denizens of the deep, like the hadal amphipod pictured above. These amphipods, and other animals below 8,400 meters, may have different adaptations to deal with such crushing pressures. This is especially fortunate for hadal amphipods, which are a favorite snack of fish above 8,400 meters. For now, the amphipods’ mysterious ability to survive in the deepest deep sea has afforded them the luxury of a fish-free freezing black abyss all their own.
(The video above is of a very deep fish, taken at 8,143 meters, and is a contender for the deepest fish, along with a tongue-twister of a fish: Bassogigas profundissimus, collected with a net from possibly as deep as 8,370 meters in the Puerto Rico Trench .)
 Photo: A hadal amphipod from 10,500 m in the Sirena Deep http://www.schmidtocean.org/file/show/3498
 Life Under Pressure – 100 Elephants on Your Head. Schmidt Ocean Institute expedition update. By Dr. Paul Yancey. http://schmidtocean.org/story/show/3236
 PH Yancey, ME Gerringer, JC Drazen, AA Rowden & A Jamieson (2014) Marine fish may be biochemically constrained from inhabiting the deepest ocean depths. PNAS. 4461–4465, doi: 10.1073/pnas.1322003111. http://www.pnas.org/content/111/12/4461.full
 The Deepest Living Animals. Schmidt Ocean Institute expedition update. By Dr. Paul Yancey. http://www.schmidtocean.org/story/show/3494/
 Staiger JC. 1972. Bassogigas profundissimus (Pisces; Brotulidae) from the Puerto Rico Trench. Bulletin of Marine Science22: 26–33. (PDF)