What is the twilight zone?

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Life in the twilight zone

Microbes, plankton, jellies, and fish—everything living in the twilight zone has amazing adaptations for life such an extreme environment. Many animals can produce their own light—called bioluminescence—that they use to camouflage themselves, to scare off predators, or even to attract prey.

Most life forms in the twilight zone are tiny—a few inches or less—but some, such as gelatinous siphonophores, can form chains that extend as much as 130 feet, making them among the biggest animals on Earth. But even the smallest twilight zone inhabitants can be powerful through sheer number. A tiny but fierce-looking fish called a bristlemouth is the most abundant vertebrate on Earth. And the combined biomass of all twilight zone fish may be more than in the rest of the ocean combined.

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The largest migration on Earth

Perhaps most remarkable, many twilight zone organisms participate in the largest migration on the planet. It happens around the globe, every day, sweeping across the world’s oceans in a massive, living wave. Every night, fish, squid, plankton, and other mid-ocean dwellers begin their journey up to surface waters to feed. By daybreak, they will be gone again, headed back to the relative safety of deeper, darker waters. Large, surface-dwelling predators such as sharks, swordfish, and whales routinely make the reverse trip, diving into the twilight zone to feed on the abundant life there.

Climate and the Twilight Zone

By migrating to and from the surface, eating and being eaten, dying—and even by pooping!—organisms in the twilight zone transport huge amounts of carbon from surface waters into the deep ocean. That process, known as the biological pump, plays an important role in regulating Earth’s climate.

Here’s how it works: In sunlit surface waters, tiny plantlike organisms called phytoplankton use energy from the sun to transform carbon dioxide into the food that allows them to grow. Those phytoplankton are eaten by tiny animals called zooplankton, which in turn are eaten by jellies, fish, and other animals—some of which carry that carbon into the twilight zone as part of their daily migration.

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Another key way that carbon gets from the surface ocean to deeper waters is via particles known as marine snow. The “snow” consists of clumps of dead plankton, shells, fecal pellets, bacteria, and other particles, which provide food for mesopelagic invertebrates and fish. It’s a kind of one-way shuttle bus for carbon.

About 90 percent of the carbon that makes it into the mesopelagic is eaten and stays there, but a small portion (about 200 million tons of carbon per year) sinks down deeper, where it can remain locked away for hundreds or even thousands of years—keeping heat-trapping carbon dioxide out of the atmosphere.

Urgency and Opportunity

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The twilight zone’s biological abundance makes it an attractive target for commercial fishing operations, and a potential source of life-sustaining protein to support growing human populations and the demand for food.

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A target for the future

Industrial fishing operations are already harvesting organisms that migrate between surface waters and the twilight zone. Every year, factory ships are vacuuming up increasing quantities of small crustaceans such as copepods and krill near the surface and are poised to begin reaching directly into the twilight zone. Most of what they catch is ground into fishmeal for aquaculture or processed into pet foods or “nutraceutical” oils. Many of these operations take place in international waters, outside the reach of national fishing regulations.

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Lessons from the past

Human activity has done irreparable harm to what once seemed to be near-limitless resources. Just a few decades ago, the Grand Banks cod fishery was one of the world’s most productive, with people coming from around the globe to harvest what was thought to be an unending supply of fish. As the cod population began to decline, frantic attempts at regulation failed and the fishery collapsed. Today, it is not clear if the cod will ever fully recover, even after a moratorium on fishing imposed more than a quarter-century ago.

What if we could do things differently this time?

Much of the twilight zone lies beyond national boundaries, in the “high seas” where relatively few laws or regulations apply. Knowing that the twilight zone is under imminent threat gives great urgency to efforts to study it and to understand how it is connected to other parts of the ocean and the planet. But this can also be seen as a great opportunity—to build a body of knowledge to support science-based policies that ensure ecosystem health and sustainable, equitable use of twilight zone resources.

Join our effort

The decisions we collectively make in the next decade will affect how the ocean looks for centuries to come.