Pressure
A satellite image of the Earth's atmosphere, which presses down on the planet's surface and creates pressure. Underwater, the pressure is much greater.
The pressure at the bottom of the ocean is many times greater than on the surface, and it increases as you move into deeper waters. How do marine creatures cope?
The weight of air in the Earth’s atmosphere is 1kg per square centimetre at sea level. In the oceans, the pressure is far greater because of the water pressing down heavily.
Only 10m below the water surface, the pressure doubles. At 20m, it is 3 times greater than at the surface. And at 4km below the surface, half a tonne of weight presses on every square centimetre.
Many deep sea animals survive in these conditions because their bodies are mostly made of liquid, which can’t easily be compressed. However, some fish have gas-filled buoyancy bladders, and these are much more compressible.
To cope with the pressure, fish have to be able to add or remove air from the bladder as they move up and down in the water. Otherwise, it would expand as they moved upwards, into an area of lower pressure, and deflate at lower levels where pressure is greater.