mesopelagic

The mesopelagic, or the twilight zone, is the ocean layer spanning roughly 200 to 1000 meters below the sea surface. It lies below the sunlit epipelagic zone and above the deeper bathypelagic zone. Light levels are very low and insufficient for photosynthesis, though faint illumination persists, primarily blue. Organisms here rely on dim light for communication, camouflage, and prey detection.

Environmental conditions vary with depth and latitude; temperatures are cooler, pressures higher, and oxygen levels can be variable, with several regions coinciding with oxygen minimum zones.

Biota are diverse, including fishes, cephalopods, crustaceans, and gelatinous organisms. A defining feature is bioluminescence; many species possess photophores used for counterillumination camouflage, signaling, or attracting prey. Lanternfishes (Myctophidae) and related midwater taxa are often highly abundant in biomass.

Behaviors such as diel vertical migration, where many organisms ascend to the epipelagic zone at night to feed and return to depth by day, help funnel surface-derived organic matter to deeper waters. This movement plays a significant role in the ocean’s carbon cycle, contributing to the biological carbon pump through respiration, excretion, and sinking particles.

Ecological and economic relevance: the mesopelagic is a key link in marine food webs and is being studied for potential fisheries, though harvests may have ecological consequences. Research methods include acoustic surveys, midwater trawls, and submersible observations.