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Historically, the hadal zone was not recognized as distinct from the abyssal zone, although the deepest sections were sometimes called "ultra-abyssal".During the early 1950s, the Danish Galathea II and Soviet Vityaz expeditions separately discovered a distinct shift in the life at depths of 6,000–7,000 m (20,000–23,000 ft) not recognized by the broad definition of the abyssal zone.
The lifecycle of deep-sea fish can be exclusively deep-water, although some species are born in shallower water and sink upon maturation. Regardless of the depth where eggs and larvae reside, they are typically pelagic. This planktonic — drifting — lifestyle requires neutral buoyancy.
At depths of 4,000–6,000 m (13,000–20,000 ft), [2] this zone remains in perpetual darkness. [ 3 ] [ 4 ] It covers 83% of the total area of the ocean and 60% of Earth's surface. [ 5 ] The abyssal zone has temperatures around 2–3 °C (36–37 °F) through the large majority of its mass. [ 3 ]
The fertilized eggs are buoyant and planktonic; the larvae and juveniles drift with the currents—likely at much shallower depths than the adults—and upon metamorphosis into adult form, they descend to deeper waters. Dolichopteryx species are noted for their paedomorphic features, the result of neoteny (the retention of larval characteristics).
The ocean depths still remain a largely unexplored part of the Earth, and form a relatively undiscovered domain. Scientific deep-sea exploration can be said to have begun when French scientist Pierre-Simon Laplace investigated the average depth of the Atlantic Ocean by observing tidal motions registered on Brazilian and African coasts circa the ...
The largest species, the 90-cm (3-ft) long abyssal halosaur (Halosauropsis macrochir) is also one of the most deep-living fish, recorded at depths of 3,300 m (11,000 ft). Halosaurs have developed certain adaptations to life at these extreme depths, where no light penetrates.
Stomiidae is a family of deep-sea ray-finned fish, including the barbeled dragonfishes.They are quite small, usually around 15 cm, up to 26 cm. These fish are apex predators and have enormous jaws filled with fang-like teeth. [1]
Grenadiers or rattails are generally large, brown to black gadiform marine fish of the subfamily Macrourinae, [1] the largest subfamily of the family Macrouridae.Found at great depths from the Arctic to Antarctic, members of this subfamily are amongst the most abundant of the deep-sea fish.