In the pelagic class, epipelagic, mesopelagic, bathypelagic and abyssopelagic life forms live.

Epipelagic - the most inhabited the Ocean, extending from the surface of the Ocean up to depths on the order 100-200 m. The process of photosynthesis promotes the growth of organic substances. Therefore, biological processes observed in epipelagic zone are responsible for determining the life forms living in the other pelagic zones, where organic consumption occurs.

Mesopelagic - the "twilight" zone, located at depths from 100-200 up to 700-1,000 m. The seasonal and diurnal vertical migrations in the epipelagic zone are interconnected with searches for food are characteristics of the majority of the mesopelagic inhabitants. Vertical migration can be 600 m and more.

Bathypelagic - the zone of eternal darkness located at depths from 1,000 to 3,000 m. The quantity of living organisms is sharply reduced here, and is directly related to the decreasing volume of food descending from the upper zones. The specific varieties of fauna are also reduced.

Below the 3,000 depth and extending to the deepest parts of the ocean is the abyssopelagic zone. Inhabitants of the abyssopelagic zone are often colourless. The fishes and crabs most often have no eyes but some have special flashing organs used to assist them to find food.

For distribution of benthic organisms (living on the Ocean bottom) a number of zones have been allocated. The upper zone - littoral and sub-littoral includes places from the tide line to depths of about 200 m; bathyal includes the range from 200 to 3,000 m; abyssal takes in the zone from 3,000 m and ultra-abyssal takes in the zone from 6,000 m to the floors of the deepest trenches in the Ocean.

The littoral and sub-littoral are characterised by the greatest species varieties and abundance of food. Only in this zone does bottom vegetation develop..
 

Information provided by HDNO:  http://www.oceansatlas.com/unatlas/about/ContactInfoForHDNO.html