Ecology
M
Mesopelagic
/ /ˌmɛsəʊpɪˈlædʒɪk/ /
Quick Definition
The ocean zone from 200–1000 m depth, the "twilight zone," characterised by dim light and abundant bioluminescent organisms.
📖 Full Definition
The mesopelagic zone (200–1000 m) lies below the photic zone and receives only traces of sunlight. Temperatures drop rapidly through this layer (part of the permanent thermocline). Many fish here possess photophores (light-producing organs) for counterillumination, bioluminescent lures, or species recognition. They include lanternfishes (Myctophidae), hatchetfishes, and bristlemouths (Cyclothone). Many mesopelagic fish perform diel vertical migrations — ascending to epipelagic waters at night to feed and descending at dawn.
💡 Examples & Context
The bristlemouth fish Cyclothone is probably the most numerous vertebrate genus on Earth, living in mesopelagic waters worldwide.