The zone of twilight, formally called the mesopelagic zone, is the layer of ocean that stretches from 200 meters (660 feet) to 1,000 meters (3,300 feet) below the surface. It sits just beneath the sunlit waters where most marine life is visible, in a dim band where sunlight fades to near-total darkness. Despite its name suggesting emptiness, the twilight zone teems with life and plays a surprisingly large role in regulating Earth’s climate.
Where It Sits and What It’s Like
The ocean is divided into layers based on how much sunlight penetrates. The top layer, down to about 200 meters, gets enough light for photosynthesis and supports the familiar marine ecosystems most people picture: coral reefs, schools of fish, sea turtles. Below that threshold, you enter the twilight zone. Light still exists here, but only as a faint glow that weakens with every meter of depth. By the time sunlight reaches 200 meters, it has already dropped to roughly 1% of its surface intensity. Below that point, there isn’t enough energy for plants or algae to grow, so the entire food web operates differently.
Temperatures drop sharply compared to surface waters, and pressure increases steadily. At the bottom of the twilight zone, around 1,000 meters, the water pressure is about 100 times what you feel at the surface. Oxygen levels also decline through this zone, reaching their lowest concentrations around 700 to 800 meters deep, a band scientists call the oxygen minimum zone. Some areas here contain as little as 0.5 milliliters of dissolved oxygen per liter of water, which is barely enough for most animals to survive.
The Creatures That Live Here
Far from being a barren wasteland, the twilight zone may hold the largest concentration of fish on the planet. Lanternfish and bristlemouths are the most common species, both small enough to fit in your hand but staggeringly numerous. Early estimates put total fish biomass in this zone at about 1 billion metric tons, but more recent surveys using advanced sonar suggest the real number could be anywhere from 2.4 to 25 billion metric tons. Even the conservative end of that range is enormous.
Fish are just part of the picture. The twilight zone is home to jellyfish, squid (including juveniles of the giant squid and the even larger colossal squid), krill, shrimp, arrow worms, and a variety of tiny crustaceans called copepods. Siphonophores, which are colonial organisms that can stretch meters long, drift through these depths alongside comb jellies and small snail-like creatures called pteropods. Many of these animals feed on organic matter that sinks from the surface, essentially recycling the waste products of the brighter ocean above.
The Largest Migration on Earth
Every single day, an enormous wave of animals travels between the twilight zone and the surface in what scientists consider the biggest migration on the planet. At dusk, fish, squid, krill, and zooplankton rise hundreds of meters to feed on the plankton-rich surface waters under the cover of darkness. Before dawn, they descend again into the twilight zone’s dimness to avoid visual predators like seabirds, tuna, and dolphins that hunt by sight.
This behavior, called diel vertical migration, is so consistent and involves so many organisms that it was first detected accidentally. During World War II, Navy sonar operators noticed a mysterious “false bottom” on their screens that rose at night and sank during the day. It turned out to be a dense layer of fish and other creatures, many of them equipped with swim bladders that reflected sound waves just like the ocean floor would. Trawling the depths eventually confirmed the answer: billions of small fish making their nightly commute.
Bioluminescence: Living Light in the Dark
About 80% of animals living between 200 and 1,000 meters depth are bioluminescent, meaning they produce their own light through chemical reactions in their bodies. This makes the twilight zone one of the most light-filled environments on Earth, just not from the sun. Krill, shrimp, lanternfish, and many other species carry specialized light-producing organs across their bodies.
The exact purposes of all this living light are still not fully understood, but bioluminescence likely serves several survival functions. Some animals use it to attract prey the way an angler fish uses its lure. Others flash light to startle or confuse predators. Some species produce bioluminescent ink or mucus they release as a decoy while escaping. And certain fish use specific light patterns on their undersides to match the faint glow from above, effectively becoming invisible to predators looking up from below.
Why the Twilight Zone Matters for Climate
The twilight zone acts as a massive carbon sink. Animals that feed at the surface and then descend carry carbon from the upper ocean into the deep. Their waste, their respiration, and eventually their deaths all deposit carbon at depths where it can remain locked away for centuries. The total amount of carbon transported to the deep ocean through this process is estimated at 2 to 6 billion metric tons per year, according to Woods Hole Oceanographic Institution. That’s a significant chunk of the global carbon cycle, comparable in scale to the carbon absorbed by all the world’s forests.
This “biological pump” means the twilight zone isn’t just an interesting habitat. It’s an active player in how much carbon dioxide stays in the atmosphere versus getting stored in the deep sea. Any disruption to the animal communities living here, whether from deep-sea fishing, ocean warming, or declining oxygen levels, could have consequences for the planet’s ability to regulate its own climate.
How Scientists Study It
The twilight zone has historically been one of the hardest parts of the ocean to study. It’s too deep for scuba divers, too dark for satellite observation, and too vast to survey easily. Early research relied on trawl nets dragged through the water column, which gave scientists a snapshot of what lived there but missed fragile animals that were destroyed by the nets.
Modern exploration uses remotely operated vehicles equipped with high-definition cameras, which reveal a world that looks nothing like what nets suggested. NOAA’s research vessel Okeanos Explorer has deployed ROVs that capture footage of bioluminescent displays, bizarre body forms, and animal behaviors never before documented. Advanced sonar systems now map the daily vertical migration in real time across entire ocean basins. Autonomous underwater vehicles and environmental DNA sampling, which detects species from the genetic material they shed into the water, are filling in the picture of what actually lives in this zone and how abundant it is.
Despite these advances, the twilight zone remains one of the least explored habitats on the planet. Scientists estimate that the majority of species living here have yet to be formally described, making it one of the last great frontiers of biological discovery on Earth.