Water pressure increases with depth because the weight of the water above compresses the water below, every 10 meters underwater increases by around 1 atmospheres (atms), so in the deep sea there are pressures ranging from 180 atms onwards, this is a huge amount of pressure to be exerted on something. You may have seen examples before of plastic cups that are taken down with deep sea exploration vessels that get compressed to a fraction of their original size due to the sheer force of pressure. Hydrothermal vents prove how strong the effects of pressure are, when water reaches 100oC boils to steam as we all should know, but hydrothermal vents release temperatures up to 475oC (Koschinsky 2008) and yet the water stays liquid. This because the pressure is so great that water particles simply don’t have enough energy boil under pressures of 300 atms, which is the pressure at depths were hydrothermal vents are found. And yet, surprisingly life still flourishes.
There are methods of adaptation that allow these pressures to be withstood. Most teleost fish at this depth have very soft cartilaginous bone, much like elasmobranchii skeletons. More elastic bone tissue can resist pressure much more easily without breaking, it is for this reason that boned fish and sharks are somewhat rare at these depths. They are still found but the majority of species found at depth are invertebrates that do not have the problems of heavy bone material, making animals such as echinoderms, molluscs and cnidarians much more common. Boned animals do have other adaptations to the depths however. Studies show that DNA molecules in deep sea species of fish have molecules that are manipulated and replaced to make the DNA coils tighter and more compact (Morita 2010). Although it is unclear why this occurs one theory is that it makes their DNA harder to compress under pressure reducing damage that could lead to numerous problems caused by damaged DNA such as various forms of cancer. Looking back at invertebrates, specifically molluscs that often have heavy shells, they too have adapted thicker more resistant shells such as Bathymodiolus spp.
Bacterial life has other methods of adaptation, many strains have been found to have cell walls with heavy amounts of lipids (Delong 1985) or unsaturated fats (Grossi 2010). By having these fats and lipids the cell walls become much more flexible helping resist compression in much the same fashion as teleost fish reduce bone tissue to cartilage. This is visually evident with more than bacterial species however as often when deep sea residing species are brought up to the surface the pressure on their bodies is released making them become more fluid, many samples simply fall to pieces when not under pressure. This is a drastic adaptation, by becoming better adapted to the depths they can longer live without pressure.