Mesopelagial

The Mesopelagial, also known as the mesopelagic zone, is the middle layer of the open ocean. It extends roughly from 200 meters to about 1,000 meters below the sea surface, lying beneath the sunlit epipelagic zone and above the bathypelagic zone. Dim light penetrates only weakly here, so photosynthesis does not occur and temperatures are cooler with higher pressure.

The zone hosts a diverse assemblage of organisms adapted to low light, including zooplankton such as copepods and krill, small pelagic fishes like lanternfishes and bristlemouths, cephalopods, and gelatinous species. Many mesopelagic organisms are bioluminescent and possess large eyes or other adaptations for sensing in dim conditions. A key ecological feature is diel vertical migration: many species move upward to the upper layers at night to feed and descend during the day, effectively transferring energy and organic matter between depth zones and contributing to the ocean’s carbon cycle.

Environmental conditions in the mesopelagic vary by region and are influenced by factors such as temperature, oxygen levels, and productivity. In some areas, oxygen minimum zones intersect with the mesopelagic, shaping community composition and distribution.

Human relevance includes interest in the potential commercial harvest of mesopelagic fishes and the ecological implications of midwater fishing. Research typically focuses on physiology, sensory biology, bioluminescence, and the role of mesopelagic communities in ecosystem functioning and carbon transport.