Environment
Only about 2% of known marine species inhabit the pelagic environment. This means that they live in the water column as opposed to the benthic organisms that live in or on the sea floor. Deep-sea organisms generally inhabit bathypelagic (1000m-4000m deep) and abyssopelagic (4000m-6000m deep) zones. However, characteristics of deep-sea organisms, such as bioluminescence can be seen in the mesopelagic (200m-1000m deep) zone as well. The mesopelagic zone is the disphotic zone, meaning light there is minimal but still measurable. The oxygen minimum layer exists somewhere between a depth of 700m and 1000m deep depending on the place in the ocean. This area is also where nutrients are most abundant. The bathypelagic and abyssopelagic zones are aphotic, meaning that no light penetrates this area of the ocean. These zones make up about 75% of the inhabitable ocean space.
The zone that deep-sea fish do not inhabit is the epipelagic zone (0m-200m), which is the area where light penetrates the water and photosynthesis occurs. This is also known as the euphotic, or simply photic, zone. Because the photic zone typically extends only a few hundred meters below the water, about 90% of the ocean volume is invisible to humans. The deep-sea is also an extremely hostile environment, with temperatures that "rarely exceed 3°C and fall as low as -1.8°C" (with the exception of hydrothermal vent ecosystems that can exceed 350°C), low oxygen levels, and pressures between 20 and 1,000 atmospheres (between 2 and 100 megapascals)..
Hydrostatic pressure increases by 1 atmosphere for every 10m in depth. Deep-sea organisms have the same pressure within their bodies that is being exerted on them from the outside, so they aren’t crushed by the extreme pressure. Their high internal pressure, however, results in the reduced fluidity of their membranes because molecules are squeezed together. Fluidity in cell membranes increases efficiency of biological functions, most importantly the production of proteins, so organisms have adapted to this circumstance by increasing the proportion of unsaturated fatty acids in the lipids of the cell membranes. In addition to differences in internal pressure, these organisms have developed a different balance between their metabolic reactions from those organisms that live in the epipelagic zone. David Wharton, author of Life at the Limits: Organisms in Extreme Environments, notes “Biochemical reactions are accompanied by changes in volume. If a reaction results in an increase in volume, it will be inhibited by pressure, whereas, if it is associated with a decrease in volume, it will be enhanced,”. This means that their metabolic processes must ultimately decrease the volume of the organism to some degree.
Most fish that have evolved in this harsh environment are not capable of surviving in laboratory conditions, and attempts to keep them in captivity have led to their deaths. Deep-sea organisms contain gas-filled spaces (vacuoles). Gas is compressed under high pressure and expands under low pressure. Because of this, these organisms have been known to blow up if they come to the surface. Other complications arise from nitrogen narcosis and decompression sickness, which also occur in humans. Nitrogen narcosis occurs because the absorption of gases in the blood, especially nitrogen, increase at greater depths. The result is similar to drunkenness. Decompression sickness occurs when excess gases cannot be removed from the blood stream fast enough when an organism rises in the water column. The decreased pressure makes the gases expand and small bubbles of nitrogen form in the blood stream as well as tissues. The result of this can be bone damage, extreme pain, physical debilitation, and even death. This can be seen in the case of the Frilled Shark that was found in shallow waters near Japan. They usually live at a depth of 1,500m, so when this specimen was transferred to a marine park it died within a few hours. For this reason little is known about them, as there are limitations to the amount of fruitful research that can be carried out on a dead specimen and deep-sea exploratory equipment is very expensive. As such, many species are known only to scientists and have therefore retained their scientific name.
Read more about this topic: Deep Sea Fish
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