Scientists Discover Animals Beneath the Ocean Floor, Offering Clues to Life Beyond Earth

A team of researchers has discovered macroscopic life beneath the seafloor of the deep ocean, one of Earth’s most extreme environments.

The discovery both complicates scientists’ understanding of the living communities around hydrothermal vents in the deep sea, and offers a tantalizing glimpse at what life may look like beyond Earth. Though researchers knew life existed in and around the hydrothermal vents, whether living things made use of the sediments beneath the miles of seawater itself was not clear.

In its recent work—published today in Nature Communications—the team used the remotely operated vehicle SuBastian on the research vessel Falkor (too) to identify animals typically associated with the ocean’s deep-sea hydrothermal vents in the subseafloor—meaning literally under the bottom of the sea. The team did their investigation about 8,250 feet (2,515 meters) beneath the sea on a stretch of ocean floor they nicknamed the Fava Flow Suburbs in the eastern Pacific Ocean.

“Our discovery shows us that animal life in the ocean is not restricted to what we see on the seafloor surface, but that animal life extends into the shallow ocean’s crust,” said study co-author Sabine Gollner, a researcher at the Royal Netherlands Institute for Sea Research, in an email to Gizmodo. “The extent of the subseafloor cavities—horizontally and how deep they reach—is not known yet,” Gollner added. “The subseafloor habitat could be also very important for the succession at hydrothermal vents, as the seafloor and subseafloor habitats are connected.”

The team found that animals like tubeworms wriggle through cavities in the subseafloor to move from Point A to Point B. Specifically, the team identified the tubeworms R. pachyptila and O. alvinae and the mussels B. thermophilus, as well as polychaete worms and limpets, among the species living in undersea cavities.

To get a sense of the conditions under the seafloor, the team used SuBastian‘s tool kit to drill small holes into exposed chunks of igneous rock on the sea bottom. The sub also lifted up sections of lava shelves to see what dwelled beneath. Before lifting the shelf, the team sampled the vent fluid through the holes they drilled, measuring the temperature inside the cavities (which was roughly 64° Fahrenheit, or 18° Celsius, on average).

Beneath the 4-to-6-inch-thick (10 to 15 centimeters) lava shelf, the team identified 4-inch-high (10 cm) cavities where macroscopic tube worms and mussels dwelled. Some of those subsea residents were clearly adults; at least one of the tube worms was over 16 inches (41 cm) long, indicating that the cavities weren’t merely a nursery. As adults, the tubeworms lose their mouths and gut and rely on the bacteria Candidatus Endoriftia persephone for nutrition, which lives inside the worms. With livable temperatures inside the undersea cavities, and their nutrition source within the worms themselves, the macroscopic animals were able to survive in the extreme environment without issue.

There are a few bare necessities for life as we know it. Water, carbon, nitrogen, luck—there are a handful of factors that appear necessary for living things, and which guide the search for life beyond Earth, a field of science called astrobiology. By probing some of Earth’s most extreme environments, scientists hope to better understand the kinds of conditions that may host life everywhere from distant exoplanets to the subsurface oceans of Jupiter’s icy moons.

“The study of the subseafloor biosphere for animal life has just begun,” the team wrote in the paper. “These efforts will lead to a better understanding of hydrothermal vent biogeochemistry, ecology, and evolution and its impact on global biodiversity, and connectivity, potentially leading to better management of seafloor surface and crustal subseafloor hydrothermal vents.”

Last year, a team using the same vessel found new hydrothermal vents teeming with life on the ocean floor. The Schmidt Ocean Institute coordinated that effort, which highlighted the need to protect such swathes of the mineral-rich seafloor, which can be targeted by deep-sea mining operations. The same year, Falkor (too) was used to find an octopus nursery deep off the coast of Costa Rica.

“The uniqueness of active hydrothermal vents is well recognized, and protection against potential future anthropogenic impact such as deep-sea mining has been suggested or is in place,” the team added. “The discovery of animal habitats in the crustal subseafloor, the extent of which is currently unknown, increases the urgency of such protections.”

The vessel Falkor—Falkor (too)‘s predecessor—shed light on the almost-alien lifeforms that eke out existence on the seafloor. In 2021, a team aboard the vessel found microbes in the deep sea that are invisible to the human immune system, indicating that our bodies’ ability to recognize foreign microbes isn’t so all-encompassing. Beyond these lifeforms being alien to our conventional sensibilities of life, they are so foreign to us that our bodies cannot even recognize them as living things.

The recent discovery could lend itself to the search for alien life. If creatures can make happy homes of the sediments beneath the sea, it broadens the sort of horizons that life may have in deep space. Sure, we may still be alone in the galaxy, but the universe is a big place, filled with opportunity.

Take a study published in PNAS Nexus back in April, for example, which found an ecosystem teeming with life under Chile’s Atacama Desert. The Atacama is an arid region in northern Chile that is one of the least hospitable places on Earth. Yet the team leading that research found several classes of bacteria eking out existence underground. The bacteria are protected from the ultraviolet radiation overhead under the soil, but remain near enough to the surface that they could get the energy to undergo photosynthesis.

It’s not yet clear how deep beneath the seafloor life can persist, but it’s clear that several macroscopic species are making it work. The team wrote that “due to increase in temperature, it has been predicted that life should be restricted to several meters below the seafloor.” Future studies could clarify exactly how far below the conditions become unsuitable for life.

The findings could hold water (no pun intended) as scientists peer deeper under the surface of Jupiter’s icy moons Europa and Ganymede, which are thought to harbor subsurface oceans. Even if alien life isn’t floating around the water column, the recent findings could indicate it’s worth digging–or diving–deeper to find it.