We are used to imagining the life of the ocean as straining toward the light.
At the surface, microscopic algae absorb solar energy. Krill feed on them. Fish, penguins, seals and whales follow the krill. It seems that this entire vast system must be oriented upward — toward where photosynthesis begins.
But the robotic vehicle sinks deeper and deeper. Sunlight disappears. In the cold darkness, mineral towers arise, cracks in the Earth's crust, and streams of heated water carrying chemical substances from the planet's interior.
And among them moves krill.
Not random, lost individuals, but females ready to reproduce. They gather near hydrothermal vents, feed on local microorganisms, and connect two worlds that previously seemed almost independent: the illuminated surface of the ocean and its volcanic depths.
A Scientific Discovery at the Fire of the Earth
The study was led by the biological oceanographer Kim Bernard of Oregon State University. The international team included Angelo Bernardino, Andrew Thurber, Rachel Kaplan, Aaron Micallef, Sarah Seabrook and other specialists.
The observations were conducted as part of the National Geographic and Rolex expedition Into the Southern Ocean together with the Schmidt Ocean Institute. The scientists worked aboard the research vessel R/V Falkor (too), while the seafloor was explored by the remotely operated vehicle SuBastian.
In 2024–2025, the vehicle's cameras studied two active hydrothermal regions of the Southern Ocean — Hook Ridge and Quest Caldera. It was there, more than 900 meters below the surface, that researchers first saw Antarctic krill gathered near deep-sea vents.
Previously, this most important inhabitant of the Southern Ocean had been studied mainly in the upper 300 meters of water. Isolated observations at great depths already existed, but the presence of krill directly at hydrothermal vents has been documented for the first time.
They all turned out to be females
At the Hook Ridge site, the vehicle collected four individuals. The sample is small, but its composition proved astonishing: all four were mature females, ready to reproduce and recently mated.
During an observation lasting almost nine hours, the cameras captured other females near the mineral chimneys as well. The researchers did not see a single male during this period.
At the Quest Caldera site, observations continued for about 21 hours. Here, near the active vents, dense aggregations of reproductive female krill were already appearing.
For comparison, the scientists collected 35 individuals at an ordinary seabed site in the Antarctic Sound, far from hydrothermal activity. The control group contained young animals, males and females — the usual mixed composition of the population.
For now it is impossible to assert that deep-sea vents serve as a special "maternity ward" for krill. However, the unusual predominance of females ready to spawn at two hydrothermal regions is hard to consider an insignificant detail.
Food created without sunlight
The main surprise was hidden inside the animals themselves.
The upper layers of the ocean depend on photosynthesis: algae use sunlight, creating organic matter that becomes the foundation of the food web. But at deep-sea vents there is no solar energy.
Here the beginning of the food chain is created by chemosynthesis. Microorganisms draw energy from sulfur and other chemical compounds rising from the Earth's crust, and turn it into organic matter.
The scientists studied the krill's gut microbiome, the chemical characteristics of its tissues and its trace-element content. The results showed that the females had indeed been feeding on microorganisms associated with the hydrothermal environment. The chemical traces also pointed not to a chance brief visit, but to repeated contact with the food and mineral water of the vents.
This changes the familiar picture.
Krill is capable of drawing energy not only from a system that begins with sunlight. At great depth it enters another food web — one fed by the chemical energy of the Earth.
Why undertake such a journey?
A vertical migration of hundreds of meters requires energy. It must be especially costly for females, whose resources are already directed toward forming offspring.
This means the deep-sea environment may offer advantages capable of justifying such an energy-expensive journey.
The researchers are considering several possible explanations.
First, hydrothermal microorganisms may provide the females with additional nutrition during a period of high energy demand. Certain bacterial lipids participate in forming the cell membranes of krill ovaries and embryos.
Second, the water near the vents contains elevated concentrations of manganese, zinc and iron — elements important for reproduction and the early development of offspring. At the same time, high levels of toxic metals, including lead and cadmium, were also found in the tissues. Therefore, the mineral environment may carry both benefit and risk.
Third, locally warmer water could theoretically accelerate the development of embryos. But this assumption has not yet been tested experimentally.
The main answer is also unknown: whether the females release their eggs directly at depth or, after feeding, return closer to the surface to spawn.
The study does not definitively reveal why krill comes to the vents. It establishes something no less important: reproductive females do indeed use this deep-sea world and find food in it.
A small creature on which a vast world depends
Antarctic krill may seem an almost inconspicuous creature only a few centimeters long. But researchers estimate its total mass at roughly half a billion tons — possibly the largest biomass among all wild animals of a single species on Earth.
Krill is eaten by whales, seals, penguins, seabirds and fish. Even those predators that do not eat it directly depend on the animals linked to this food web.
Krill also takes part in the carbon cycle. By consuming phytoplankton and releasing rapidly sinking organic particles, it helps transport carbon from surface waters into the depths.
At the same time, its populations are under pressure from ocean warming, shrinking sea ice and industrial fishing. But protecting the species is difficult while we do not know where the most important stages of its life cycle take place.
If females ready to reproduce regularly use hydrothermal regions, these areas cannot be considered isolated geological curiosities. They may be part of the environment on which the reproduction of one of the central species of the Southern Ocean depends.
This is especially important when planning marine protected areas and discussing possible deep-sea mining.
The vertical dimension of life
This story seems to reveal a hidden axis of the ocean.
At the top, sunlight creates food for phytoplankton. Below, the chemical energy of the Earth's interior feeds the microorganisms of hydrothermal vents. And between them moves a tiny creature capable of entering both systems.
Krill carries substances and energy through the water column. It is eaten by the largest animals on the planet. Its presence connects the surface, the depths, the atmosphere and the rocky shell of the Earth into a single continuous process.
The ocean again shows: wholeness does not mean immobility. It is born of movement — of the constant exchange between top and bottom, light and darkness, water and fire.
And perhaps the most astonishing part of the discovery lies not in the fact that krill turned out to be so deep. But in the fact that life had long since joined these worlds — before humans were able to see the path passing between them.
