Only 200 years ago it was generally assumed that the deep sea was completely void of life. Deep seas were comparable to deserts. It was believed that no animal life existed at depths greater than 500 meters under the surface.
 
But, In the 19th century, Norwegian scientist Michael Sars was able to produce a list of 427 marine animal species caught in the fjords and coastal waters off Norway’s shores at depths of 820 meters. The notion of the deep sea being similar to a desert now seemed difficult to uphold much longer.

The Challenger Expedition took place during the years 1872-76. It was one of the most significant events in the history of marine science. The British Navy supplied the ship and crew. The expedition’s international staff of scientists explored depths, temperatures, currents, and the biology of all the world’s oceans, except the Arctic ocean. 4,717 new species were described during this historic cruise.
 
The Challenger expedition was followed by The Michael Sars expedition. This was a four month cruise in the Atlantic Ocean which took place during the spring and summer of 1910, exploring the North Atlantic south to Africa. The steam ship, which was the most modern ocean research vessel of its time, was named Michael Sars. It was 38 meters long and weighed 226 tons.

The expedition was headed by Norwegian scientist Johan Hjort and British oceanographer John Murray, and it resulted in the description of more than 100 marine species previously unknown to mankind.
 
Following these expeditions, scientists Hjort and Murray both got previously unknown Cephalopods named after them:
The Cirrothauma murrayi lives in the dark at depths ranging from 1,500 to 4,500 m. It has very small eyes with no lenses. 
The Mastigoteuthis hjorthi has thousands of tiny suction cups on its long tentacles. Underwater video recordings have recently shown it hovering at a slanted angle over the sea-bed while feeding, using its tentacles to literally ”vacuum clean” the small creatures it feeds on.

Deep sea fish generally have some common physical features: They have large mouths, large teeth, large eyes, large stomachs and a dark color. Not only are many deep sea fish dark in color, but many have a special dark lining in their stomach that may serve to conceal light emitted from the light organs of devoured prey – protecting them from attracting other larger predators while they digest!

Many deep sea fish are bioluminescent. Some are able to produce their own light by mean of chemical reactions, while others have a partnership with bioluminescent bacteria that manufacture light for them. Some fish are able to control their bioluminescent activity by actively blocking or shutting off light production…
Scientists are not certain why fish use bioluminescence, and bioluminescence probably serves more than one purpose for most species. But the arrangement of bioluminescent organs provides some clues. Some are located on the underside of the fish, providing counter shading when the fish rise in the water column to feed. Others are arranged in distinct patterns and may thus be used for some kind of signaling, such as mating, or as a lure for prey.
Most deep-sea bioluminescence is blue or blue-green since light of these wavelengths penetrates furthest through water, but some fish do produce different colors of light, including shades of red and yellow.
For example: A fish called the Stoplight Loosejaw releases a flashing red light. This enables it to see its prey without being seen itself!
By the way, The Stoplight Loosejaw has another fascinating feature: It has an enormous lower jaw that can be extended far in front of the fish. The long, inward pointing teeth on the jaw act as hooks with which to grab the prey while the jaw retracts quickly, drawing the prey into the mouth.
Now, creatures in the deep sea are often few and far between. Predators must therefore be very efficient or capable of lasting for long periods without any food. Therefore, a number of deep water species, such as the Black Swallower, have stomachs which are often larger than the fish itself!
Wanna hear a love story? Some Deep Sea Anglerfish species form a weird kind of permanent relationship. Responding to the problem of individuals being spread very widely, females are very much larger than the males. The males have specially adapted biting teeth so that when they finally find a female they bite into her body forming what becomes a permanent attachment.