Thanks to the Monterey Bay Aquarium and their research and technology partner MBARI for partnering with us on this episode of SciShow.

They are super excited for folks to learn about the deep sea. [♪ INTRO] Below the surface of the ocean, before light disappears completely, there is a region where there isn’t enough light to support photosynthesis, but still enough light to see. This space, between the surface and the deep ocean, is known as the twilight zone, or midwater.

And this zone is home to the largest migration of animals on the planet. Which is impressive all on its own. But what these animals carry with them may have real consequences for all of us.

Because without even trying, these well-traveled organisms are fighting the climate crisis with every step, paddle, or stroke of a tail. The twilight zone officially begins 200 meters below the ocean surface, and it can be found across the entire open ocean, all over the world. Although it’s too dark here for photosynthesis, the twilight zone is not empty of life.

It is home to countless organisms: fish, jellies, worms, and squid. These animals avoid the surface during the day because predators can spot them more easily. Like, if you’re a distinct shadow against the bright shiny surface of the sea, it’s like hey, soup’s on.

But these organisms still depend on food that is produced at the surface. So during the night, they dash out to get it. It’s estimated that if you took all of the animals in the twilight zone and weighed them, their total biomass would be more than 100 times that of the animals that live near the surface.

Which means that if we ignore organisms in the twilight zone, our understanding of ocean life becomes very surface-level. Many bioluminescent organisms also make their homes here, where they can put their light signals to good use. And krill, the small crustaceans that feed whales and other marine life, also venture into the twilight zone.

But it’s less about where they live, and more about where they go. Because these marine commuters aren’t just participating in the rat race. They are affecting the whole planet.

Every day, countless animals travel from the twilight zone to the surface and back, to feed on the phytoplankton and other organisms that live in the surface layer. They swim up from the depths at night and feast on energy-rich organisms that grew on the surface. Because these animals are adapted to living in the dark, they can use the cover of darkness to hide from surface predators, who need light to hunt.

But when the sun comes up, they swim back down into the twilight zone, making a clean getaway before they become something else’s meal. This is known as diel vertical migration, and animals of all shapes and sizes join in. It happens every night, all over the ocean.

In terms of biomass, diel vertical migration is the largest movement of animals on Earth. In the morning, the animals that participate in it head back to the depths with food in their bellies. Food that is made of carbon.

The photosynthesizers at the ocean’s surface use carbon dioxide to build their bodies. When they, or the animals that eat them, die and decompose, that carbon is released back into the atmosphere. Unless, of course, that process happens somewhere that isn’t in contact with the atmosphere.

When that happens, that carbon can’t make it back into the air and it is sequestered, or basically removed from circulation. Because like any other organisms, twilight zone animals poop. And sometimes die.

And when they do, some of that carbon manages to make it back to the surface, but not all of it. Research shows that a surprising amount of that carbon falls down to the deep seafloor, where it stays. This is one of the ways that carbon dioxide naturally leaves the atmosphere, and it’s incredibly important when considering how future climate change might play out.

We use sophisticated computer models to forecast how carbon will cycle throughout the globe, and there’s a lot that needs to be accounted for. Diel vertical migration is an enormous contributor to carbon sequestration that has not been included in those models. In a 2019 paper, when researchers added diel vertical migration to their carbon models, they found that carbon sequestration increased by 14%.

Other studies have put the number even higher. This is important, because about a third of all carbon dioxide emissions are absorbed by the ocean as a whole. So migrating organisms running back and forth from the ocean’s surface on a daily grind may actually be playing a big part.

Of course, we cannot rely on marine life to do our work for us, and not even the mass migration can sequester enough carbon to stop the climate crisis. What this is about is learning everything we can about our planet in order to best protect it. And also, we get to be witness to a truly incredible event while we’re at it.

Thanks for watching this episode of SciShow, which is a collab with Monterey Bay Aquarium and MBARI! We are absolutely krilled to be working with them, and we’ve been having a whale of a time helping them in their mission to share their research and teach people about cool ocean stuff. They worked together on an exhibition, “Into The

Deep: Exploring Our Undiscovered Ocean,” to give visitors to the Aquarium a rare look at some of the animals that thrive in the least-explored area of the planet, the deep sea! So go check out our fronds at the Aquarium and MBARI, and kelp your eyes peeled for more collabs. [♪ OUTRO]