Antarctic krill are the lifeblood of the South Pole.
These shrimp-like crustaceans serve as a crucial food source for penguins, seals, and whales. "Most of them feed on Antarctic krill," says Kim Bernard, a biological oceanographer at Oregon State University and a National Geographic explorer. "And if they don't, they feed on something else that feeds directly on Antarctic krill. So they are absolutely fundamental to the functioning and health of that ecosystem."
Krill also recycle nutrients that allow tiny algae called phytoplankton to grow. In addition, krill are fished in large numbers to produce aquaculture feed and omega-3 fatty acids as dietary supplements.
But experts say this rich habitat in the Southern Ocean at the bottom of the world is shifting rapidly due to climate change. "It's one of the most rapidly warming areas on the planet," says Bernard. "And that's cascading through the food web and affecting the Antarctic krill population."
Now, Bernard and her colleagues report in the journal Communications Biology the discovery of a surprising habitat for the krill — hydrothermal vents, which are basically hot springs on the ocean floor. Characterizing the importance of this habitat, especially for reproductive females, may help inform how to safeguard these crucial crustaceans.
Venting feelings
Antarctic krill tend to be located in the upper layer of the ocean. "They can be found forming swarms and layers at the surface," says Bernard. "You can photograph them from airplanes."
They can also be found at the sea floor at different depths, but she says these deeper observations have been sparse and anecdotal.
Then, in late 2024, Bernard was in Bransfield Strait off the Antarctic Peninsula while aboard the National Geographic and Rolex Perpetual Planet Southern Ocean Expedition in collaboration with the Schmidt Ocean Institute.
One of her colleagues, UCLA benthic ecologist Andrew Thurber, was using a remotely operated vehicle in a trough some 3,600 feet below the surface to study and video record a hydrothermal vent. Picture "extremely hot water coming from deep within the Earth's crust," says Bernard, "and it's bringing with it metals and sulfides and all sorts of other chemicals that form almost like a tower."
It's a seemingly inhospitable place. Bernard wasn't studying the vent but she was eager to observe the sampling nevertheless.
"While having a little tea break upstairs in the galley," she recalls, "I was watching the big screens showing in real time what was happening."
And that's when she saw a flicker of movement in the corner of the screen. It was, unmistakably, a krill.
"It's like underwater ballet," she says, describing the krill's backflips. "It just looks like pirouetting. I pretty much threw the tea in the air and ran down to the control room, screaming, 'There's krill, there's krill!'"
Krill had never been spotted at a hydrothermal vent before. The other research team had planned to sample the geology and microbiology of the vent, but they gave Bernard a precious 15 minutes to try to catch a few specimens.
"They have what they call a suction sampler, which is essentially a vacuum that slurps whatever you want to slurp up," she explains. The team managed to nab four individual krill, all reproductive females, "very heavily laden with eggs. Should be releasing them any moment."
"And to see something like that at a hydrothermal vent," Bernard continues, "is completely opposite to what we would have normally expected."
Researchers had thought that females probably released their eggs near the surface over deep water, but it had never been observed before. So Bernard found herself with a single basic question: "Why on earth are they down there?," she says. "I really wanted to just try and understand, what are they getting from that site?"
It takes a lot of energy for little krill to get down that deep. In addition, they're exposed to potentially toxic heavy metals like cadmium, barium and lead.
So whatever is bringing the krill to the hydrothermal vent, Bernard figured, it must be worth it.
"It's really critical that we put our efforts into trying to understand the role of the deep sea environment for reproductive krill," she says.
Four little krill, one big question
To get a better sense of what was going on, Bernard brought the handful of krill to the surface and compared their stomach contents to others she collected elsewhere on the seafloor in shallower, non-vent waters. The four females had clearly been feeding on microbes living down there. (And such a microbial food source would be available year-round rather than seasonal phytoplankton that rely on sunlight and photosynthesis.)
"The rest of the krill tissue," she says, "I dried up and literally carried in my backpack on the way home. I didn't want to put it anywhere out of my sight because it was so important."
That tissue later revealed that the krill had accumulated higher levels of metals released by the vent, like manganese. "That's an important nutrient or mineral for embryo development in crustaceans," says Bernard. "And it's very scarce in the Southern Ocean. So potentially that's a reason for them to go down there."
The warmer waters near the vent may also speed up embryo development.
All or some combination of these reasons may motivate the krill to journey downwards to the hydrothermal vent repeatedly and to then stay there for some time.
"The study shows that krill can make use of hydrothermal vent habitats, but it doesn't provide enough evidence to suggest that they are dependent on these habitats," says Simeon Hill, a marine ecologist at the British Antarctic Survey who didn't participate in the research.
Still, he's reassured that krill can flexibly use different parts of the ocean to live and feed as it demonstrates their resilience.
And he says that even though hydrothermal vents are reasonably well protected from destructive human activities, additional conservation measures may be needed.
"It makes me think it would be sensible to develop a marine protected area process that allows new vulnerable habitats to be added as they're discovered," says Hill.
That kind of protection may be especially necessary if those habitats are critical to the krill's reproductive cycle.
Two months after the original research cruise, the same vessel visited another Antarctic hydrothermal vent some 1,000 miles away. That vent also abounded with egg-laden female krill.
