Water Worlds

Deep sea fish
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Humpback anglerfishDeep sea fish is a term for fish that live below the photic zone of the ocean. Examples include the lanternfish, flashlight fish, cookiecutter shark, bristlemouths, and anglerfish.

Environment
Because the photic level zone lies 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 pressures between 20 and 1,000 atmospheres (between 2 and 100 megapascals), temperatures between 3 and 10 degrees Celsius, and a lack of carbon dioxide. The fish that have evolved to adapt to these harsh environments are not capable of surviving in the environments with which humans are familiar, and any attempts to keep them in captivity have led to their deaths. 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 names.

Characteristics
California headlight fishThe fish of the deep sea are among the most elusive and unusual looking creatures on Earth. 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 deep sea fish are bioluminescent, with extremely large eyes adapted to the dark, and they can have long feelers to help them locate prey or attract mates in the pitch dark of the deep ocean. The deep sea angler fish in particular has a long fishing-rod-like adaptation protruding from its face, on the end of which is a bioluminescent piece of skin that wriggles like a worm to lure its prey. The lifecycle of deep sea fish can be either permanently deeply submerged or they are sometimes born in shallower water and sink on becoming adults.

Due to the poor level of photosynthetic light reaching deep sea environments, most fish need to rely on organic matter sinking from higher levels, or, in rare cases, hydrothermal vents. This makes the deep sea much poorer in productivity than shallower regions, with most deep sea fish noticeably smaller and have larger mouths and guts than those known to us. It has also been found that the deeper a fish lives, the more jelly-like its flesh and the more minimal its bone structure. This makes them slower and less agile than surface fish.

Endangered Species
A recent study by Canadian scientists , has found five species of deep sea fish – roundnose grenadier, onion-eye grenadier, blue hake, spiny eel and spinytail skate – to be on the verge of extinction due to the shift of commercial fishing from continental shelves to the slopes of the continental shelves, down to depths of 1600 metres. The slow reproduction of these fish – they reach sexual maturity at about the same age as human beings – is one of the main reasons that they cannot recover from the excessive fishing.