<P> Hydrostatic pressure increases by 1 atmosphere for every 10m in depth . 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 . 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 . </P> <P> 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 . </P> <P> The fish of the deep - sea are among the strangest and most elusive creatures on Earth . In this deep dark unknown lie many unusual creatures that have yet to be studied . Since many of these fish live in regions where there is no natural illumination, they cannot rely solely on their eyesight for locating prey and mates and avoiding predators; deep - sea fish have evolved appropriately to the extreme sub-photic region in which they live . Many of these organisms are blind and rely on their other senses, such as sensitivities to changes in local pressure and smell, to catch their food and avoid being caught . Those that aren't blind have large and sensitive eyes that can use bioluminescent light . These eyes can be as much as 100 times more sensitive to light than human eyes . Also, to avoid predation, many species are dark to blend in with their environment . </P> <P> Many deep - sea fish are bioluminescent, with extremely large eyes adapted to the dark . Bioluminescent organisms are capable of producing light biologically through the agitation of molecules of luciferin, which then produce light . This process must be done in the presence of oxygen . These organisms are common in the mesopelagic region and below (200m and below). More than 50% of deep - sea fish as well as some species of shrimp and squid are capable of bioluminescence . About 80% of these organisms have photophores--light producing glandular cells that contain luminous bacteria bordered by dark colorings . Some of these photophores contain lenses, much like those in the eyes of humans, which can intensify or lessen the emanation of light . The ability to produce light only requires 1% of the organism's energy and has many purposes: It is used to search for food and attract prey, like the anglerfish; claim territory through patrol; communicate and find a mate; and distract or temporarily blind predators to escape . Also, in the mesopelagic where some light still penetrates, some organisms camouflage themselves from predators below them by illuminating their bellies to match the color and intensity of light from above so that no shadow is cast . This tactic is known as counter illumination . </P>

Why do deep sea fish have large eyes
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