In the deep ocean, the waters extend far below the epipelagic zone, and support very different types of pelagic fishes adapted to living in these deeper zones. In deep water, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. Its origin lies in activities within the productive photic zone. Marine snow includes dead or dying plankton, protists (diatoms), fecal matter, sand, soot and other inorganic dust. The "snowflakes" grow over time and may reach several centimetres in diameter, travelling for weeks before reaching the ocean floor. However, most organic components of marine snow are consumed by microbes, zooplankton and other filter-feeding animals within the first 1,000 metres of their journey, that is, within the epipelagic zone. In this way marine snow may be considered the foundation of deep-sea mesopelagic and benthic ecosystems: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source.

Some deep-sea pelagic groups, such as the lantern fish, ridgehead, marine hatchet fish, and lightfish families are sometimes termed pseudoceanic because, rather than having an even distribution in open water, they occur in significantly higher abundances around structural oases, notably seamounts and over continental slopes. The phenomenon is explained by the likewise abundance of prey species which are also attracted to the structures.

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.

Layers of the pelagic zone

In the deep ocean, the waters extend far below the epipelagic zone, and support very different types of pelagic fishes adapted to living in these deeper zones. In deep water, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. Its origin lies in activities within the productive photic zone Marine snow includes dead or dying plankton, protists (diatoms), fecal matter, sand, soot and other inorganic dust. The "snowflakes" grow over time and may reach several centimetres in diameter, travelling for weeks before reaching the ocean floor. However, most organic components of marine snow are consumed by microbes, zooplankton and other filter-feeding animals within the first 1,000 metres of their journey, that is, within the epipelagic zone. In this way marine snow may be considered the foundation of deep-sea mesopelagic and ecosystems: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source.

Some deep-sea pelagic groups, such as the lantern fish, ridgehead, marine hatchet fish, and lightfish families are sometimes termed pseudoceanic because, rather than having an even distribution in open water, they occur in significantly higher abundances around structural oases, notably seamounts and over continental slopes. The phenomenon is explained by the likewise abundance of prey species which are also attracted to the structures.

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.

Humans seldom encounter frilled sharks alive, so they pose little danger (though scientists have accidentally cut themselves examining their teeth). 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 a frilled shark found in shallow waters near Japan. Frilled sharks usually live at a depth of 1,500 metres, and 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 limits to the amount of useful research that can be carried out on dead specimens and deep-sea exploratory equipment is very expensive. As such, many species are known only to scientists and only by scientific names.