The global ocean is a vast, layered environment. Beneath the sunlit surface waters lies the dimly lit realm known as the twilight zone, or the mesopelagic zone. This ecosystem covers roughly 60% of the Earth’s surface. It is a region of immense pressure, hosting abundant life that has evolved to survive its harsh conditions. The organisms here represent a substantial portion of the world’s total fish biomass, and their existence is interwoven with the health of the global ocean.
Defining the Twilight Zone
The mesopelagic zone extends from about 200 meters down to 1,000 meters below the surface. It sits beneath the sunlit surface layer (the epipelagic zone), which receives enough light for photosynthesis. In the twilight zone, less than one percent of sunlight penetrates, making it too dim to support the growth of phytoplankton.
The physical conditions change with depth. The upper boundary contains the thermocline, where water temperature rapidly decreases. Temperatures can drop from over 20°C near the top to around 4°C at the lower boundary. Hydrostatic pressure steadily increases, reaching up to 1,500 pounds per square inch. Oxygen levels also tend to decrease, often forming an oxygen minimum zone as sinking organic matter is consumed by microbes.
Survival Strategies and Adaptations
Life in the mesopelagic requires adaptations to low light, scarce food, and high pressure. A primary adaptation is bioluminescence, the production of light through a chemical reaction. Many organisms use this light for camouflage, employing counter-illumination to match the faint downwelling sunlight and conceal their silhouette from predators below.
Inhabitants often have highly specialized eyes to capture minimal light. Many species possess large, sometimes tubular, eyes directed upward to detect the faint silhouettes of prey against the dim surface glow. Other adaptations relate to high pressure and energy scarcity. Many mesopelagic fish have watery muscles and poorly developed skeletal structures. This helps them maintain neutral buoyancy without investing energy in a gas-filled swim bladder. These creatures also conserve energy by maintaining a slowed metabolism, since the food supply is limited to sinking organic matter.
The Fauna of the Mesopelagic
The fish biomass in the mesopelagic zone is estimated to be greater than that of all other ocean zones combined. The bristlemouth fish, a tiny inhabitant of this zone, is considered the most numerous vertebrate on Earth. Lanternfish (family Myctophidae) are also dominant, accounting for a large proportion of total fish numbers and exhibiting extensive bioluminescence.
Invertebrates, including copepods, krill, and various squid species, form large populations here. The defining behavioral pattern for much of this fauna is Diel Vertical Migration (DVM). Organisms ascend toward the surface waters at dusk to feed under the cover of darkness. They then descend hundreds of meters back to the deep water before dawn. This daily migration is considered the largest animal migration on the planet in terms of biomass involved.
Ecological Significance and Human Interaction
The mesopelagic zone regulates global climate through the biological carbon pump. Diel Vertical Migration is an active component of this pump, as animals consume carbon-rich plankton near the surface and transport that carbon to the deep sea when they descend. This process, along with the sinking of dead organisms and fecal pellets, helps sequester carbon away from the atmosphere for long periods.
This carbon-rich biomass has recently attracted commercial fishing interests. Deep-sea trawling targets mesopelagic fish for use in fishmeal and fish oil, posing a threat to this ecosystem. Disrupting these populations could interfere with the ocean’s ability to store carbon, and the climate impacts of such fishing may outweigh short-term economic benefits. The zone functions as a food source for many large marine predators, meaning human impact here could destabilize the food web across the entire ocean.