09/06/2026
BENEATH THE OCEAN’S SURFACE, BILLIONS OF ANIMALS BEGIN ONE OF THE LARGEST MIGRATIONS ON EARTH EVERY SINGLE NIGHT — AND A MARINE BIOLOGIST WHO STUDIES THEM SAYS SOME ARE STRANGER THAN ANYTHING SCIENCE FICTION COULD INVENT.
Far below the waves is a world most humans will never see directly.
There are transparent animals that seem to disappear in the water.
Squid capable of surviving enormous changes in temperature and oxygen.
Fish that produce red light in a place where almost no other animal can see red.
Crustaceans that resemble giant versions of backyard pill bugs.
Tiny animals that release glowing material when attacked.
And gelatinous creatures whose role in Earth’s climate may be far more important than their strange appearance suggests.
Marine biologist Dr. Lloyd Trueblood recently opened a Reddit AMA to answer questions about this hidden ecosystem and the animals he has spent his career studying.
Trueblood is a professor of biology at La Sierra University in Riverside, California, specializing in marine invertebrate physiology.
In simpler terms, as he likes to describe it, he studies the ocean’s “squishy animals” and investigates how they manage to survive in environments that would be hostile to most familiar forms of life.
His research has included octopuses, Humboldt squid and salps—transparent, barrel-shaped animals related more closely to vertebrates than their jelly-like appearance might suggest.
Much of his work asks deceptively simple questions.
How does an animal continue functioning when oxygen becomes scarce?
How does it cope when the surrounding water becomes warmer?
What happens as carbon dioxide increases and ocean chemistry changes?
And how can tiny drifting animals influence processes happening across the entire planet?
The answers take scientists into one of Earth's largest habitats: the ocean's midwater.
The open water between the sunlit surface and the seafloor contains an almost unimaginable amount of living space.
NOAA estimates that the water column accounts for at least 95 percent of the planet's habitable volume.
Yet compared with forests, grasslands and even the surface of Mars, much of it remains remarkably difficult to observe directly.
One of the greatest spectacles in this hidden world happens every evening.
As sunlight disappears, enormous numbers of fish, crustaceans, jellies and other animals begin swimming upward.
Some travel hundreds of meters.
They approach food-rich surface waters under cover of darkness, feed during the night and then retreat toward deeper water before daylight makes them easier for visual predators to detect.
Scientists call this diel vertical migration.
By number of animals and total biomass involved, NOAA describes it as the largest organized migration of animals on Earth.
Think about that for a moment.
The greatest migration on this planet is not wildebeest crossing the Serengeti.
It is not birds flying between continents.
Much of it happens silently in darkness beneath the ocean every night.
And most people never notice.
The migration may also affect Earth's climate.
Animals feed on carbon-containing material near the surface.
When they descend, they carry some of that carbon downward inside their bodies.
They breathe, excrete waste and produce f***l material at depth.
Some of that carbon can then remain away from the atmosphere for long periods.
Scientists describe this movement as part of the ocean's biological carbon pump, and researchers are still working to determine exactly how large the contribution from migrating animals really is.
Salps are particularly interesting.
They look almost impossibly delicate—transparent cylinders drifting through open water.
But they are efficient filter feeders.
They consume tiny particles and package waste into dense f***l pellets capable of sinking rapidly.
Trueblood's published research specifically examines salp physiology because these animals can play an important role in moving carbon through marine ecosystems.
Then there are the animals that simply seem alien.
Asked during the AMA to name some of the strangest creatures he had encountered, Trueblood struggled to choose only one.
Giant isopods were high on his list.
Imagine a pill bug or roly-poly enlarged until it is roughly the size of a football.
These deep-sea crustaceans belong to the same broad group as familiar terrestrial isopods, but their enormous size gives them an appearance that seems better suited to a science-fiction film.
Anglerfish are even stranger.
In several deep-sea anglerfish species, tiny males attach themselves to much larger females.
Eventually, their tissues can fuse.
The male becomes dependent on the female's bloodstream and functions primarily as a reproductive partner.
Evolution has effectively transformed one s*x into a permanently attached mate.
Dragonfish have another extraordinary adaptation.
Some deep-sea dragonfishes can generate red bioluminescence.
Because most deep-ocean animals are poorly sensitive to red wavelengths, this can function something like a private flashlight: the dragonfish may illuminate prey without producing light that many potential victims can detect.
Trueblood jokingly compared the ability to having night-vision goggles in the deep.
The ocean has stranger defenses still.
During the discussion, Trueblood highlighted copepods capable of releasing glowing material when threatened.
Bioluminescence is widespread in the deep ocean.
For an animal surrounded by darkness, producing light can attract a mate, confuse a predator, lure prey or—in some cases—potentially illuminate an attacker strongly enough to attract an even larger predator.
In the deep sea, turning on a light can sometimes become a weapon.
One of Trueblood's favorite research animals, the Humboldt squid, demonstrates how extreme life in the ocean can become.
These large, fast predators make major vertical movements through the water.
They can encounter dramatic differences in temperature and move into oxygen-minimum zones where oxygen concentrations are exceptionally low.
Trueblood's doctoral and later research has examined how squid and other marine animals cope physiologically with such environmental extremes. His published work also documents the remarkably complex visual displays of Humboldt squid, including numerous color, posture and movement patterns.
But one of the most memorable moments he described during the AMA involved no laboratory experiment at all.
While traveling aboard a research vessel from the Gulf of California toward San Diego, Trueblood and others stood near the bow during a moonless night.
The sky was dark enough for the Milky Way to be clearly visible.
Then they noticed green trails approaching the ship.
They came closer.
Dolphins were swimming beside the bow.
Bioluminescent microorganisms in the water were lighting up around their moving bodies, outlining the dolphins in glowing green points.
Trueblood described it as resembling a surreal Disney scene and called it one of the most amazing things he had ever witnessed.
Experiences like that help explain why scientists remain fascinated by the deep.
We often speak about exploration as though the great age of discovery ended when people finished mapping the continents.
It did not.
An enormous frontier still covers most of our planet.
We simply cannot see through it from the surface.
Deep-ocean animals live in darkness, under immense pressure, sometimes with very little food and little oxygen.
Evolution has responded by producing organisms with transparent bodies, expandable stomachs, extraordinary sensory systems, enormous eyes, reduced eyes, luminous organs and reproductive strategies that can seem almost impossible.
And scientists still encounter organisms whose biology remains poorly understood.
Trueblood acknowledged during his AMA that he has not personally pulled up an animal and known immediately that it represented a species new to science.
But he also pointed out how many organisms appear in research nets and how easy it is to imagine that undescribed species have passed through scientists' hands without being recognized at the time.
That may be the most exciting thing about deep-ocean biology.
The bizarre creatures we already know are probably not the end of the story.
They are the beginning.
Every expedition has the potential to reveal a new behavior, an unexpected adaptation or an animal science has barely encountered before.
And these creatures are not merely curiosities.
Their movements shape marine food webs.
Their waste transports carbon.
Their responses to warming, acidification and oxygen loss can reveal how the ocean may change in the future.
The strange animals living hundreds or thousands of feet below us are connected to the same planetary systems that make life possible at the surface.
The deep ocean can look like another world.
But it is not another world.
It is most of ours.
