Trillions of animals rise nightly from the ocean's twilight zone and sink more carbon than cars emit

Every night, one of the planet's biggest climate helpers rises from the deep ocean — and most people never see it happen.

A vast nighttime feeding run in the ocean's twilight zone sends trillions of tiny animals toward the surface; when they head back down into deeper water, they also move carbon in a natural cycle that may help keep the planet cooler.

What's happening?

Researchers have been aware of this once-a-day deep-ocean movement for a long time, yet its sheer size is still striking. Called diel vertical migration, it follows darkness across the world's oceans and involves an estimated 11 billion tons of biomass, the BBC reported.

The animals involved mostly live in the mesopelagic, or twilight, zone, 200 to 1,000 meters (656 to 3,280 feet) below the surface, where sunlight starts to fade. That layer contains an estimated 95% of all fish biomass, making it one of the ocean's most important — and least visible — regions.

Laura Hobbs, lecturer in Arctic Marine Science at the Scottish Association for Marine Science, said, "I like to think of it as a Mexican wave." At night, zooplankton swim upward to eat phytoplankton near the surface, then retreat downward during the day to avoid predators.

Each year, that trip may move as much as 6.6 billion tons of carbon into the deep ocean. According to the BBC, that is more than twice the emissions from cars worldwide.

Why does it matter?

The ocean stores carbon, and these tiny creatures are part of that system. When zooplankton feed near the surface and then descend, they move carbon deeper into the ocean, where it can remain stored for centuries or longer.

Jon Copley, professor of ocean exploration at the University of Southampton, explained why that shortcut matters so much. 

"Rather than [the organic matter] sinking and getting eaten on the way down, now it's inside whatever's swallowed it," Copley said.

The process helps regulate the climate while also supporting ocean food webs tied to communities and fisheries. Many species in the twilight zone serve as prey for larger fish such as tuna and swordfish, animals that support jobs, food supplies, and coastal economies.

There is still much that scientists do not know. Ocean warming, shifting food chains, and sea-ice loss could alter this migration, potentially affecting both marine life and the amount of carbon the ocean can store.

What's being done?

Some protections already exist in parts of the deep ocean, including marine protected areas in waters near Hawaiʻi, along England's southwest coast, and around Nova Scotia and the Azores. But those protections are generally designed for the seafloor, not the full water column where this nightly migration takes place.

That gap has prompted calls for stronger safeguards. According to the BBC, the International Union for Conservation of Nature has backed a pause on expanded mesopelagic fishing until researchers better understand this part of the ocean. Its Motion 035 would help protect the entire water column, not just the seabed.

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