The twilight zone is defined by the amount of sunlight that penetrates from above. By day, there's just enough light for animals to see, but not enough to drive photosynthesis. Between approximately 200 to 1000 meters deep, depending on water clarity, it’s cold year-round and the pressures are extreme—as much as 1,500 pounds per square inch. The twilight zone is also one of the largest living spaces on the planet, making it a difficult place to study and understand.

WHAT is the Ocean’s Twilight Zone?

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WHAT IS IT?

WHERE IS IT?

Twilight Zone Basics

WHY IS IT IMPORTANT?

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CARBON TRANSPORT

HUMAN CONNECTION

WHY is the Ocean Twilight Zone Important?

LIFE

THE GREAT MIGRATION

TECHNOLOGY CHALLENGES

TOP-DOWN CONNECTIONS

Life in the Twilight Zone

The twilight zone may be the most biologically abundant habitat on Earth. It is estimated to contain more animals than all surface fisheries combined. This area of the ocean is home to an incredible number of species, from tiny zooplankton to larger gelatinous animals, all of which are uniquely adapted to their environment. Most of the organisms that live in the twilight zone are vulnerable to larger predators, but find safety in its darkness.

Moorings and Bouys

Gliders

Stingray

TZEx

MINIONs

Recent advances in robotics and sonar technology have enabled a new fleet of instruments for studying the twilight zone. Combined with traditional research tools like nets and ship-based sonar, these state-of-the-art sensors, samplers, and cameras are advancing our understanding of the twilight zone and its inhabitants like never before.

Technology Challenges

Top-Down Connections

Many well-known animals—like whale sharks, elephant seals, and tuna—rely on the twilight zone for food. Blue sharks and white sharks are known to ride miles-wide masses of swirling, nutrient-filled eddies of warm water to the twilight zone to feed.

The ocean plays a major role in absorbing the carbon dioxide that traps heat in our atmosphere. About a quarter of man-made CO2 emissions are dissolved in seawater and mixed throughout the ocean by horizontal and vertical currents. Under cover of darkness, twilight zone animals rise up to graze on grazing phytoplankton (which photosynthesize sunlight and CO2) at the surface and deposit carbon-rich poop in the depths by day. This “biological carbon pump,” locks tons of CO2 away from the atmosphere.

Carbon Transport

NIGHT: Some twilight zone animals come to the surface at night to feed.

The nightly journey of quadrillions of twilight zone animals to the ocean surface is the largest migration on Earth. It is so massive that in the 1940s, the U.S. Navy mistook it for the seafloor—until they discovered that it moved up at night and down at dawn. This migration attracts predators like swordfish, which follow their prey’s daily cycle. Twilight zone creatures play an essential role in the marine food web, serving as prey for a variety of fish, marine mammals and seabirds, and exchanging nutrients between the surface and deeper waters.

DAY: Twilight zone migrators return to the cover of darkness by day to avoid being seen by predators.

The Great Migration

The twilight zone plays an important role in carbon transport from the atmosphere to the deep sea, which helps keep our planet habitable. As the daytime habitat for many creatures, including the favorite prey of whales, tuna, swordfish, sharks, and other top predators, it also provides food for animals that are ecologically, commercially and nutritionally valuable to us.

Human Connections

The twilight zone stretches around the globe and from the Arctic to the Southern Ocean, just beyond the reach of sunlight. It exists mostly in the “high seas,” beyond the control of any one nation.

WHERE is the Ocean’s Twilight Zone?

Hadal Zone (Hadalpelagic)

~11000m

Sunlit Zone (Epipelagic)

Twilight Zone (Mesopelagic)

Abyssal Zone (Abyssalpelagic)

~1000m

6000m

0m

Midnight Zone (Bathypelagic)

4000m

~200m

While the twilight zone is roughly 800 meters in depth and comparatively smaller than the zones below, it covers nearly the entire planet and contains more fish biomass than any other zone.