Each evening, as night moves across the sea, something extraordinary begins. From the depths of the twilight zone, countless creatures start to rise.
Squid
Tiny, shrimp-like crustaceans, lanternfish, squid, gelatinous drifters, transparent hunters, a multitude of fish and many more. Together, they ascend towards the surface in immense numbers, coming with the protection of darkness to feed in the richer waters above. Then, before dawn, they sink once more into the dimness of the depths. This happens every night in oceans around the world, yet most people have never heard of it.
The largest migration on Earth
The great herds of Africa, the inter-continental travels of birds, the migrations of whales—all are spectacular. But none compare in scale to the ocean’s hidden vertical migration. Every day and night, trillions upon trillions of marine organisms move hundreds of metres through the water column in what scientists call the “deep scattering layer migration” or diel vertical migration. In total biomass, it may be the largest migration on Earth. And it unfolds unseen.
The false ocean floor
For years, early sonar operators were puzzled by a strange phenomenon. Their instruments detected what seemed to be a solid layer rising towards the surface at night and sinking again by day—almost like a phantom seafloor moving through the ocean. Only later did scientists learn that the “false bottom” was alive. The sonar waves were reflecting from immense concentrations of fish, squid and drifting organisms moving together through the twilight waters. The ocean itself appeared to rise and fall with life.
Why rise at night?
The migration is driven by a simple danger: light. During the day, many small animals remain in deeper, darker water where predators cannot easily see them. But surface waters hold far more food, especially microscopic plankton nourished by sunlight. The cloak of darkness offers them safety. Feeling protected, vast communities rise upwards to feed before descending again at dawn. It is an ancient rhythm repeated every 24 hours all around the planet.
A living conveyor belt
This hidden migration does more than feed marine life. It may help regulate Earth’s climate. Animals feeding near the surface carry carbon-rich material back into deeper waters when they descend. Their waste, bodies and respiration transport carbon downward through the ocean in a vast biological process sometimes called the biological carbon pump.
Thus, tiny migrating creatures help move carbon away from the atmosphere and into the deep sea. The nightly rise and fall of life influences the chemistry of the planet itself.
The twilight travellers
Some creatures travel hundreds of metres every night—an immense journey for such small beings. They are perfectly adapted to the dim mid-water world. Lanternfish glow with rows of bioluminescent organs. Transparent shrimp vanish into the blue darkness. Squid flash signals invisible to most predators. And scientists continue discovering new participants in this migration, for the twilight zone is still poorly explored.
An invisible pulse of the planet
To human eyes, the sea surface may appear empty after sunset. But beneath it, the oceanic ecosystems are in motion. Columns of living creatures rise like slow underwater storms. Predators follow them upward. Then, with the return of dawn, the migration reverses and the deep ocean's inhabitants sink back into shadow.
The sea is never still. It breathes, circulates, glows and migrates in secret rhythms hidden from our eyes. For perhaps the greatest mystery is not that this migration exists, but that it remained unknown for so long. Every night on Earth, the largest movement of animal life on the planet unfolds beneath the waves. Silent, invisible and ancient beyond imagination.
Right now, somewhere on Earth, beneath a darkening ocean, the ascent has begun.
Ethologist Ila France Porcher, author of Yes, Fish Feel Pain, The True Nature of Sharks and six other books on wildlife behaviour, spent 15 years closely observing fish and shark behaviour in Tahiti, resulting in several scientific papers. Her writings are based on decades of first-hand observations of wildlife and focus on the individuality and intelligence of individuals, challenging traditional views of animal minds. Her work has been featured on Shark Week, in scientific discussions, conservation debates and international media for its unique blend of field observation, art and science.
