Discoveries of extreme life here on Earth often provoke speculation about what might lurk in other worlds. And so it was, when I reported on January 21 that fish were found living in an isolated corner of the ocean beneath 740 meters of ice in Antarctica: People asked what this might mean for finding life on distant worlds such as Europa, a moon of Jupiter that very likely harbors an ocean of liquid water beneath a crust of ice.

Astrobiologists wax poetic about the possibility that we might someday find carpets of microbial slime clinging to the aquatic underside of Europa’s ice, but might they be setting their sights too low? Might there be something more exciting gliding through Europa’s waters, like the spidery-legged, bioluminescent xeno-arachnids envisioned in a sci-fi sketch recently published in Nature? “The question will always be energy,” says Britney Schmidt, a planetary scientist at Georgia Institute of Technology who studies Europa as a possible habitat of life. “Fish require a lot of energy—a lot more than microbes.” (Scientific American is part of Nature Publishing Group.)

Fish require a multilevel ecosystem—the bioenergetic equivalent of a financial pyramid scheme. At the bottom of an aquatic ecosystem are single-celled microbes that use energy from either sunlight or chemical sources to pluck molecules of carbon dioxide out of the water in order to grow. Microscopic creatures called protists eat the microbes. And in the newly discovered Antarctic ecosystem crustaceans may eat the protists, and fish, at the top of the pyramid, may eat the crustaceans.

This transfer of carbon—or energy—up the food web is inefficient, says John Priscu, a microbial ecologist from Montana State University who was part of the team that discovered the Antarctic fish this month. “You lose about 90 percent of the energy as you go [each step] up a food web,” he says. So for every kilogram of fish living in that ecosystem under the ice, you might need up to 1,000 kilograms of microbes at the base of the food web to support it. Even if the newly discovered fish turn out to feed on microbes directly, that still means at least 10 kilograms of microbes per kilogram of fish.

It is this constraint—the extravagant energetic needs of animals, and the limited supply of energy—that will probably determine whether a food web with complex life could exist on Europa.

Most ecosystems on Earth are powered from the base up by sunlight, which drives carbon fixation through photosynthesis. But any life in Europa’s ocean, under 10 or 20 kilometers of ice, would have to use another source of energy. Studies of lakes sealed beneath hundreds of meters of ice in Antarctica have provided a compelling picture of how this might work.

The same team that discovered the ocean fish in Antarctica this year also drilled into one of these subglacial lakes in January 2013: Lake Whillans, which sits under 800 meters of ice, roughly 100 kilometers inland from where the fish were discovered—both expeditions were supported by the National Science Foundation (NSF). They found that Lake Whillans was thriving, with roughly 130,000 microbial cells per milliliter of water. But far more interesting was just how non-hostile this supposedly “extreme” subglacial environment turned out to be. At –0.5 degree Celsius, it was slightly warmer than the oceans surrounding Antarctica. And despite having sat under ice for thousands of years, it still contained levels of oxygen that some marine animals, such as brittle stars and worms, can actually survive on.

Ironically, the same shell of ice that cuts Lake Whillans off from the outside world also provides it with a steady supply of oxygen. Ambient geothermal heat emanating up from the seafloor melts the underside of the ice sheet at a rate of several penny thicknesses per year. This liberates ancient air bubbles that were trapped in the ice as it formed from falling snow thousands of years earlier.

Genetic studies suggest that the microbes in Lake Whillans use this oxygen to “burn” ammonium and iron minerals seeping up from the sediments below—providing energy to fix carbon—effectively replacing photosynthesis. “It’s like a battery,” says Priscu, whose laboratory retrieved and analyzed some of the samples from the lake in 2013. “The ice has oxidants and the sediments have reductants, and life evolves to fill the gap—the free energy gap.”

Kevin Hand, an astrobiologist at NASA’s Jet Propulsion Laboratory, believes that this same scheme also exists on Europa. Using spectral readings from telescopes at the Keck Observatory in Hawaii, Hand has found high levels of oxidative chemicals such as sulfate, oxygen, sulfur dioxide and hydrogen peroxide on Europa’s surface, which are produced as ionizing radiation from Jupiter scours it, splitting apart water molecules and sulfur compounds in the uppermost layers of its ice. Living organisms could use these oxidizing chemicals to burn fuels such as iron or methane seeping up from the rocky bottom of Europa’s ocean.

What makes this possible is that Europa seems to be geologically active, allowing these supplies of fuel and oxidants to be transported, mixed and constantly renewed. Schimdt has found evidence that warm ocean currents and convective forces beneath Europa’s frozen shell can cause large blocks of ice to overturn and melt, bringing vast pockets of water, sometimes holding as much liquid as all of the Great Lakes combined, to within several kilometers of the moon’s icy surface.

Scientists analyzing the cracks and ridges on Europa’s surface find that its icy skin is also slowly recycled through a process similar to continental subduction on Earth, with one icy plate slipping and buckling under the edge of another. The plate sinks and eventually melts back into the ocean below, carrying with it the oxidative chemicals that were formed on the surface.

The key to predicting what kind of life Europa’s ocean could support will be figuring out how quickly this occurs and how many thousands or millions of tons of oxidative chemicals are formed on the surface and injected into the ocean each year. The range of uncertainty is huge, with estimates of Europa’s energy supply varying by orders of magnitude. Refining these calculations will go a long way toward determining what kind of life might exist there—microbial slime or feathery-lobed xeno-arachnids.

Schmidt is part of a team of scientists in the early stages of developing a NASA mission, called Europa Clipper, which will investigate these questions by placing a reconnaissance spacecraft in orbit around Jupiter. Clipper will periodically swoop down to as low as 25 to 100 kilometers above Europa’s surface. It will use ice-penetrating radar to measure the thickness of the moon’s ice shell, map its internal rifts and faults (clues to the tempo of its geologic activity) and locate pockets of water near the surface. An onboard magnetometer will measure the depth and saltiness of the ocean and a spectrometer will measure chemicals in Europa’s uppermost layers of ice. Clipper would take a few years to build and launch, assuming it gets funded.

In the meantime other interesting questions linger about the recent discoveries in Antarctica. One such question is why fish were found along the ice-covered beach line of Antarctica, where its glaciers begin to float on the ocean, but only microbes were found 100 kilometers upstream in Lake Whillans. Both environments contain oxygen. Both hover around the same temperature. And both ecosystems will likely turn out to be powered by the same source of energy: microbes oxidizing ammonium, iron, sulfur and possibly methane in order to fix carbon.

Lake Whillans and its sister lakes in that region of Antarctica represent unforgiving environments for multiple reasons. They are shallow and transient—shifting their locations from one decade to the next, Schimdt says, “more like mud puddles.” That instability could make it hard for animals to hang on over tens of thousands of years. If some chance event exterminates them—for example the glaciers freezing fast onto their beds—then the likelihood of new animals making it in from the outside to recolonize the lakes afterward is extremely low.

I traveled with the expedition that drilled and sampled Lake Whillans in 2013, and during that time I frequently asked people what they expected to find in the lake. Perhaps it’s no surprise that everyone expected to find only microbes. But now the discovery of animals in an isolated, ice-covered nook of the ocean seems to have jarred people’s thoughts into a new space.

Priscu is quick to point out that the Lake Whillans microbes were actually fixing carbon about as rapidly as what has been seen in some ice-covered parts of the ocean that are known to harbor fish. “I would be surprised if there are fish [in the lake], but there’s enough energy for them” there, he says. “If we get another grant from NSF, it would be interesting to put down a fish trap and let it sit.”