As the sun sets over the ocean, a massive wave of creatures rises up from the twilight zone to feed under cover of darkness at the surface. At daybreak, these animals return to the safety of the depths. This is the largest animal migration on Earth, one that happens every day, all around the world, and is critical for maintaining a habitable planet.
The Most Magnificent Migration
Hiding in Darkness
During the daytime, masses of animals hide out in the twilight zone, an ocean layer 200 to 1,000 meters (600 to 3,300 feet) below the surface. But at night, countless numbers of these animals migrate to shallower surface waters to feast.
Running the Gauntlet
Twilight zone animals take cues from the setting sun to begin their nightly migration from the twilight zone, known as the diel vertical migration. During the journey, predators like jellyfish ensnare unsuspecting travelers, and large fish like tuna, swordfish and some species of sharks dart through the migrating schools of fish and squid, snatching nutritious meals.
Once the sun has set, animals like krill dine on phytoplankton, which convert carbon dioxide and sunlight into energy through photosynthesis. And wherever the krill go, predators follow, which are in turn followed by other predators. As this nightly feeding frenzy happens, these animals perform one of the greatest services to life on planet Earth—they poop. As the nutrient-rich fecal matter sinks to deeper water, it feeds other animals and also transports carbon to the depths, where it may remain for hundreds and sometimes thousands of years.
A Carbon-rich Meal
When the sun begins to rise, twilight zone animals begin the long trek back home, pooping all along the way. These excreations—along with scraps of prey left lingering by sloppy predators—create a carbon-rich cloud of slowly sinking “marine snow.” The transfer of these particles into the deep ocean is known as the “biological carbon pump,” and it’s one important way carbon from the atmosphere is transported to the deep ocean, a process that helps regulate Earth’s climate.
Biological Carbon Pump
Once the sun has fully risen, the twilight zone’s creatures have returned home to their dark refuge. Some animals, however, never migrated in the first place, or only migrate seasonally. These are mostly larger animals, and they feed on smaller migrators and anything else that swims by.