Mesopelagic zone

The ocean layer, generally 200–1,000 meters deep, where sunlight is too weak for photosynthesis but still supports dim-light vision.

Photic zone: The ocean region where sunlight supports photosynthesis. Its lower boundary marks the transition into the mesopelagic zone.
Diel vertical migration: The daily movement of aquatic organisms between deeper water and the surface. Many mesopelagic animals rise at night to feed and descend by day.
Acoustic scattering layer: A layer of marine organisms that strongly reflects sound and often changes depth over a daily cycle. Echo sounders detect dense mesopelagic animals as migrating scattering layers.
Mesopelagic biomass: The total mass of organisms inhabiting the ocean’s mesopelagic layer. Its scale remains difficult to estimate because species and acoustic measurements vary.
Bathypelagic zone: The ocean layer below roughly 1,000 meters, where sunlight is absent. It begins where the mesopelagic zone’s faint daylight disappears.
Counterillumination: Camouflage in which an animal emits light to match the brightness above it. Some mesopelagic animals use it to hide their silhouettes from predators below.
Mesopelagic fishery: Commercial harvesting of fish living in the ocean’s mesopelagic layer. Proposed fisheries depend on uncertain estimates of biomass and ecological effects.
Ocean deoxygenation: The decline of dissolved oxygen in ocean waters. Changes in oxygen can compress habitable mesopelagic space and alter animal distributions.
Ocean stratification: The layering of ocean water by density, temperature, salinity, or other properties. Water-column structure helps shape the mesopelagic zone’s physical environment.
Mesopelagic fish: Fishes that inhabit the ocean’s dimly lit midwater layer, typically around 200–1,000 meters deep. Their sensory and feeding adaptations show how animals live in this layer.

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