Pressure is the main restricting factor in our pursuit of knowledge about these deep sea organisms. It is only in the last 150 years that we have had the means to study these organisms at all due to the development of submersibles like Nereus, which is capable of diving to 11,000m (Whitcomb et al. 2010). For the organisms that live there, adaptations to cope with pressure are fundamental for survival in the deep sea. However, it has been difficult to collect samples, as when they are brought to the surface they “explode” due to the pressure change or some organisms like Siphonophores are unable to be caught by nets as they are too delicate and simply break apart.
It is only if the organisms move vertically in the water column that pressure becomes a problem, as their internal and external pressures are the same – it is only if the external pressure changes that some organisms experience problems. By having water filled bodies most organisms are able to overcome the extreme pressure, as water is almost not compressible (Byatt et al. 2001). However, it is mainly the gas filled swimbladders of fish which present problems. At the surface, diving to 10m below the surface the pressure doubles, causing the volume of air spaces in your body to halve. At depth however, moving from 1000m to 1001m causes only a 1% pressure difference, causing airspaces to only reduce by a further 1%. Remarkably some fish have gas filled swimbladders at depth, like the species Coryphaenoides armatus, who is able to secrete gas into its swimbladder through the use of specialised gas glands (Merret and Haedrich 1997). Some bathypelagic fish lack swim bladders and have developed thin muscle and feeble skeletons, which reduces their weight, making them weigh not much more than the surrounding water (Denton and Marshall 1958). This has been found tobe the case with two bathypelagic fish lacking swimbladders, the Elongated bristlemouth fish, Gonostoma elongatum, and the Bluntsnout smooth-head, Xenodermichthys copei (Figure 1) (Denton and Marshall 1958).