The mesopelagic zone, also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at the depth where only 1% of incident light reaches and ends where there is no light; the depths of this zone are between approximately 200 to 1,000 meters below the ocean surface.
Sediment trap sample, Thermaikos Gulf, Greece, 2000. Stereoscopic image of the collected material above 63-μm pore size net. Calcareous shells and skeletons of planktonic organisms can be identified.
Sonar data. The green layer in the water column is the deep scattering layer of diel vertically migrating mesopelagic zooplankton and fish.
Illustration by Charles Frederick Holder of various bioluminescent fish that live in the mesopelagic zone
Helmet jellyfish, Periphylla periphylla
The aphotic zone is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1 percent of sunlight penetrates. Above the aphotic zone is the photic zone, which consists of the euphotic zone and the disphotic zone. The euphotic zone is the layer of water in which there is enough light for net photosynthesis to occur. The disphotic zone, also known as the twilight zone, is the layer of water with enough light for predators to see but not enough for the rate of photosynthesis to be greater than the rate of respiration.
The migration of bioluminescent organisms from aphotic zone
