Why Don't Deep-Sea Animals Get Crushed by the Water Pressure?
– How Having No Air Lets Them Cope
Deep-sea documentaries are full of strange-looking fish and octopuses with soft, delicate bodies. The deep sea is said to be a place with such enormous water pressure that even a submarine built of sturdy metal can be wrecked if it goes wrong. So how do these fragile-looking animals live there without being squashed?
Have you ever dived a little deep in a pool or the sea? Even just 1 to 2 m below the surface, you may have felt a sharp, squeezing ache deep in your ears.
This is because the deeper you go in water, the greater the pressure pressing in from all sides (water pressure), so the explanation goes. Now think of the deep sea, thousands of metres down, a depth far beyond anything we can dive to. How much pressure is there? And why aren't the animals living there crushed?
In fact, the reason deep-sea animals aren't crushed is said to be a surprising secret that has little to do with being "tough".
For roughly every 10 m of extra depth, the water pressure rises by 1 atmosphere, so the rule of thumb goes.
Their bodies are filled not with air but with material much like water. So the pressure outside and the pressure inside are balanced, and they are thought to avoid being crushed.
Let's look, step by step, at how "having no air" makes all the difference.
Why does water pressure rise as you go deeper?
Water pressure is said to be the pressure created by the weight of the water above that spot. The deeper you dive, the more water sits on top of you, so the pressure keeps growing. Air has weight too, and the "atmospheric pressure" we feel on land is likewise the pressure from the weight of the air above us.
For seawater, a well-known rule of thumb is that pressure rises by one atmosphere (about 1 atm) for every 10 m or so of depth. At 1000 m, that works out to about 100 times the pressure at the surface.
By making the pressure inside and outside equal, they never create a "crushing force" in the first place.
Why deep-sea animals aren't crushed: a body with no air
An object is usually crushed by water pressure when the pressure inside it differs from the pressure outside. Sink an empty plastic bottle into the deep sea and the air inside is squeezed almost without resistance, so the bottle is flattened. This happens because the gap between the air pressure inside the bottle and the water pressure outside is very large.
By contrast, the bodies of most deep-sea animals are said to have almost no large pockets of air, and to be filled throughout with material much like water that barely shrinks. Water itself is very hard to compress. So however strong the pressure from outside, the pressure inside rises by the same amount, and almost no difference between inside and outside arises, it is thought. With no difference, there is no crushing force at all.
Compared with why submarines must be strong
A submarine, on the other hand, keeps its interior filled with air at about 1 atmosphere so the crew can breathe. As it dives, the outside water pressure climbs, while the pressure of the air inside stays almost constant, so the difference between inside and outside becomes very large. To withstand that difference, a submarine's hull needs a thick, sturdy metal shell. Humans have to bring in an "air room", which deep-sea animals lack, simply to breathe, and that is why such a strong structure is required.
Some fish have a gas-filled sac in the body called a swim bladder, but many fish living in the deep sea are said to have replaced the contents of the swim bladder with fat instead of gas. Fat is barely compressible, so it has the advantage of resisting collapse when the water pressure changes.
Things you can check yourself
- Near the pool surface, check how your ears feel (make sure there is no discomfort).
- Taking care to stay safe, dive about 1 to 2 m and pay attention to the feeling deep in your ears.
- Notice that even a very small change in depth brings a sense of pressure deep in your ears.
- Stop diving and come back up, and notice that the pressure eases quickly.
Inside the ear is a small air-filled space (the middle ear), so a difference arises between the water pressure and the air pressure inside the ear, and you feel it as pressure, so the explanation goes. You can feel for yourself how much the sensation of water pressure depends on whether there is an air space in the body.
Summary
The deep sea is a world of enormous pressure, where the water pressure rises by about 1 atmosphere every 10 m. Even so, deep-sea animals are not crushed, because they have no large air pockets in their bodies, so almost no difference arises between inside and outside, it is thought. Conversely, a submarine carrying an air room, or a plastic bottle containing air, needs strength to withstand that pressure difference.
Deep-sea animals look soft not because they are weak, but because no crushing force is acting on them in the first place.
Deep-sea life that copes not only with high pressure but also makes its own nourishment without sunlight is explained in the article on hydrothermal vents. In contrast to the pressure that builds quickly as you dive, air pressure falls steadily as you climb a mountain. We work through its effects in the article on altitude sickness. And what happens to humans who dive deep carrying air is covered in Whales can dive for an hour, so why must humans surface so slowly?
For those who want to know more – terms, numbers and links to textbooksFrom middle-school science to topics still under research, each item is labelled with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics"
- High school+High-school "Physics", or advanced or sidebar material in textbooks
- UniversityUniversity-level specialist content (biochemistry), not taught in high school
- ResearchTopics researchers are investigating right now, not yet taught as settled even at university
Middle schoolTerms: words about deep-sea pressure
- Water pressure: Pressure produced by the weight of water. It grows with depth.
- Atmospheric pressure: Pressure produced by the weight of air. About 1 atmosphere at ground level.
- Swim bladder: A gas-filled sac inside many fish, used to adjust how high or low they float.
High schoolChecking with a formula: finding water pressure from depth
Using the rule of thumb that "pressure rises by about 1 atmosphere per 10 m", let's calculate the water pressure at several depths.
| In symbols | P ＝ P0 ＋ ρ × g × h |
| In words | Water pressure ＝ atmospheric pressure ＋ water density × gravitational acceleration × depth |
| Where it comes from | What presses on a surface at a given depth is the weight of the column of water directly above it. This is the hydrostatic pressure formula, derived from the balance between weight and pressure. |
This formula gives the "1 atmosphere per 10 m" rule of thumb used in the main text. Let's put in a water density of 1000, gravitational acceleration of 9.8 and a depth of 10.
| Density × gravitational acceleration | 1000 × 9.8 ＝ 9800 |
| Multiply by a depth of 10 m (pascals) | 9800 × 10 ＝ 98000 |
| Compare with atmospheric pressure (101325 pascals) | 98000 ÷ 101325 ≒ 0.97 |
The weight of 10 m of water came to almost exactly 1 atmosphere. So it is fine to remember it as "1 atmosphere per 10 m".
Pressure (atm) ＝ atmospheric pressure (1) ＋ depth (m) ÷ 10
| Atmospheric pressure | The pressure at the surface. Said to be about 1 atmosphere |
| Depth ÷ 10 | Based on the rule of thumb that pressure rises by about 1 atmosphere per 10 m of depth |
| Increase due to depth (atm) | 1000 ÷ 10 ＝ 100 |
| Total water pressure (atm) | 100 ＋ 1 ＝ 101 |
| Result | About 101 atm (roughly 100 times the surface) |
| Increase due to depth (atm) | 6000 ÷ 10 ＝ 600 |
| Total water pressure (atm) | 600 ＋ 1 ＝ 601 |
| Result | About 601 atm (roughly 600 times the surface) |
Many deep-sea animals live under roughly 600 times the pressure at the surface. As long as they have no large air pockets in their bodies, even this much pressure creates almost no difference between inside and outside, it is thought.
* "1 atmosphere per 10 m" is a widely used approximation.
High school+With no "pressure difference", the forces balance
Whether an object is crushed is said to depend not on the size of the pressure itself, but on the difference between inside and outside. A deep-sea animal's body is made of material much like water, which compresses only very slightly. So as the outside pressure rises, the pressure inside rises by almost the same amount, and the difference stays extremely small, it is thought.
UniversityHow proteins stay stable under high pressure
In biochemistry, it has been reported that a substance called "TMAO (trimethylamine N-oxide)" builds up in the bodies of deep-sea animals, more so in those living deeper. Substances like TMAO are thought to prevent high pressure from distorting the three-dimensional shape of proteins, and so to keep the functions life needs.
📖 Derivations and further reading: Hydrostatic pressure (Japanese Wikipedia) ／ Trimethylamine N-oxide (Japanese Wikipedia)
ResearchWhat is still unclear
- How exactly substances like TMAO stabilise proteins at the molecular level is still being studied in biochemistry.
- The deep sea is thought to hold many organisms not yet observed or classified, and much about the overall picture of deep-sea ecosystems remains unknown.
- Research is also under way on applying the pressure-resistant protein mechanisms of deep-sea animals to industrial enzymes that work stably under high pressure.
The extreme environment of the deep sea offers rich themes where physics and biochemistry meet, and research continues today.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: force and pressure | Basic terms for water and air pressure |
| High school | Basic Physics: pressure | Calculating water pressure from depth |
| High school+ | Physics: pressure difference and balance of forces (advanced) | Why nothing is crushed without a pressure difference |
| University | Biochemistry | Protein stabilisation by TMAO |
| Research | Biochemistry and deep-sea ecology (under research) | Molecular mechanism of stabilisation; unknown deep-sea ecosystems |
- Explanations of water pressure and atmospheric pressure in physics textbooks.
- Explanations of deep-sea water pressure and biological adaptation in oceanography materials.
- Research reviews in biochemistry on TMAO accumulation and protein stabilisation in deep-sea animals.
- Explanations of deep-sea fish swim bladders (replacement with fat) in ichthyology materials.
- Explanations of submarine pressure-resistant structures in marine engineering.
* Water pressure values come from a rule-of-thumb approximation. Actual values are said to vary slightly with conditions such as water temperature and salinity.
* This article is a general-audience science explainer. When diving in water, don't push yourself, and pay close attention to your health and the state of your ears.