Editing Deep-sea fish (section)

==Environment==
[[File:Pelagiczone.svg|thumb|right|150px|Scale diagram of the 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.{{sfn|Moyle|Cech|2004|pp=585}} 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]], [[protist]]s ([[diatom]]s), 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 [[microbe]]s, [[zooplankton]] and other filter-feeding animals within the first {{convert|1,000|m|ft}} 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]] [[ecosystem]]s: as sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source. Since there is no light in the deep sea (aphotic), there is a lack of primary producers. Therefore, most organisms in the bathypelagic rely on the marine snow from regions higher in the vertical column.

Some deep-sea pelagic groups, such as the [[lanternfish]], [[ridgehead]], [[marine hatchetfish]], and [[Phosichthyidae|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 [[seamount]]s and over [[continental slope]]s. The phenomenon is explained by the likewise abundance of prey species which are also attracted to the structures.

Hydrostatic pressure increases by 1&nbsp;atm (0.1&nbsp;MPa) for every {{convert|10|m|ft|abbr=on}} in depth.{{sfn|Wharton|2002|pp=198}} Deep-sea organisms have the same pressure within their bodies as is exerted on them from the outside, so they are not 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.{{sfn|Wharton|2002|pp=199, 201–202}} 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".{{sfn|Wharton|2002|pp=199}} This means that their metabolic processes must ultimately decrease the volume of the organism to some degree.

[[File:Frill shark.jpg|thumb|right|Humans seldom encounter [[frilled shark]]s alive, so they pose little danger (though scientists have accidentally cut themselves examining their teeth).<ref name="compagno">{{cite book |author=Compagno, L. J. V. |year=1984 |title=Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date |publisher=Food and Agricultural Organization of the United Nations |isbn=92-5-101384-5 |pages=14–15}}</ref>]]

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.{{sfn|Wharton|2002|pp=199}}