Mesopelagic
The mesopelagic (Greek μέσον, middle) (also known as the middle pelagic or twilight zone) is that part of the pelagic zone that extends from a depth of 200 to 1000 metres (~660 to 3300 feet) below the ocean surface. It lies between the photic epipelagic above and the aphotic bathypelagic below, where there is no light at all.
Although the temperature varies less at any one height than the epipelagic, the mesopelagic is the location of the thermocline; and in warmer regions of the world, the temperature varies from over 20 °C (68 °F) at the top to around 4 °C (39 °F) at the boundary with the bathyal zone. Water generally moves slowly in the mesopelagic with a residency time of about a century though a variety of animals move vertically through the zone on a daily basis and various debris sink down in relatively short time frames.
The region was initially discovered by American researchers during World War II in 1942 during anti-submarine research with sonar. Sonar at the time could not penetrate below this depth due to the large amount of creatures obstructing sound waves.
Although some light penetrates the mesopelagic zone, it is insufficient for photosynthesis. The general types of life forms found are daytime visiting herbivores, detritivores feeding on dead organisms and fecal pellets, and carnivores feeding on the former types.Algae do not inside the mesopelagic layer, due to lack of light.
Examples of animals in the mesopelagic zone are: bristlemouth, swordfish, squid, wolf eels, cuttlefish and other semi-deepsea creatures. The bristlemouth is the Earth's most abundant vertebrate, numbering in the hundreds of trillions to quadrillions. The small amount of sunlight is sufficient for animals, such as the chain catshark, to be fluorescent.
...
Wikipedia