To celebrate Endangered Species Day, meet the scaly-foot snail, the most metal animal in the world

Nearly three kilometers below the waves of the Indian Ocean, a hydrothermal spring spews sulfur into the inky black waters. On its lip, a small snail not only basks in the poison, but transforms it into a partly iron shell. This gastropod is the scaly-footed snail (Chrysomallon squamiferum), perhaps the most metallic animal in the world.

Despite its unconditional nature, the future of the species is precarious. In 2019, the snail became the first species living in hydrothermal vents to be listed as endangered by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species – thanks to the threat of deep-sea mining companies who want to harvest the minerals in the vents where the snails live.

For a long time, scientists assumed that the intense pressure and toxic compounds gushing from the Earth’s crust through hydrothermal vents would make these structures and the water around them incompatible with life. But in 2001, researchers discovered that these snails lived happily among a myriad of other creatures that thrive in these unforgiving environments. The snails’ iron-rich shell is an essential part of their survival strategy, but not as protective armor, says Chong Chen, a senior scientist at the Japan Agency for Marine and Earth Science and Technology and a leading expert on the scaly-legged snail, who led efforts to map its genome. The shell is more similar to the human liver in that it helps remove toxins from the snails’ bodies, he explains.

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The reason for this has to do with how the snail obtains its nutrients. This creature does not eat in a conventional manner. Instead, like other species that live in hydrothermal vents, the snail is home to bacteria that feed on hydrogen sulfide, oxygen, and carbon dioxide and convert these chemicals into sugar. In exchange for housing the bacteria in its intestine, the snail uses this sugar as an energy source. Digestion of bacteria, however, produces toxic sulfur as a byproduct. To protect itself, the snail excretes sulfur, which mixes with iron in the water in the vents. The end result is a shell that is literally, albeit partially, made of metal, along with the tough scales that give the snail its name.

“The iron armor is not for defense, as people have thought for many years, but rather for symbiosis. The snail is completely happy without the iron armor, which is a byproduct formed by the hot vent environment,” Chen explains.

The snail, which measures less than two inches long, has already inspired some technological innovations. The US military, for example, has studied the snail’s scales for inspiration in designing new armor, and its chemical composition has sparked new ideas for how to make the pyrite nanoparticles found in solar panels.

Not only is the snail the only animal known to incorporate iron sulfide into its shell, but it has also evolved to exist in only eight sulfur-rich hydrothermal vents in the world. The space on Earth that can be inhabited by the snail is so small that it is about half the size of Disney World, says Jon Copley, a marine biologist at the University of Southampton in England.

The same environment in which the snail developed its signature is also what made its future uncertain. Underwater hydrothermal vents form at the edges of Earth’s tectonic plates, where seawater can flow through the crust. This water is heated by the magma below and rises upwards, bringing with it valuable minerals such as copper, zinc and gold, as well as the iron and sulfur found in the snail’s shell.

These minerals have attracted the attention of mining companies. Copper is particularly in demand due to its use in artificial intelligence data centers and green energy production. Although no deep-sea mining is yet underway in these areas, at least two of the vents where the snails live have been considered for possible operations, according to the IUCN.

“There is growing concern that if mining is permitted, habitat could be severely reduced or destroyed,” the organization wrote in its red list. The best way to protect the vents – and the snails – is to “simply not exploit active hydrothermal vents, period,” Chen says.

Instead, mining companies could target inactive vents, he says. “There are, for example, many inactive massive hydrothermal sulfide deposits in the Indian Ocean. These inactive vents no longer support the scaly-legged snail, and so exploitation of these sites would not impact the snails,” says Chen. Yet these vents are not a panacea, he adds. “We currently know very little about the uniqueness of inactive vents themselves in terms of biodiversity,” says Chen. “Ongoing research has discovered at least some groups of animals that appear to be specific to inactive vents, so mining there could impact these animals.”

Besides the snail’s contributions to modern armor and materials science, Copley says, there are philosophical reasons to preserve the future of a creature that lives in an environment as alien and inaccessible to humans as any ecosystem on our planet. Few people will see a scaly-footed gastropod, but that doesn’t change its value.

Chen is more pragmatic. The snail itself may not affect humans, but its native ecosystem, although distant, turns out to play a more important role than previously thought in the overall health of the ocean. The vents pump carbon and other nutrients into the water, and these nutrients maintain the beauty that humans admire and the food they eat.

“We are now beginning to understand that hydrothermal vents play a key role in regulating the supply of these elements to the ocean and therefore contribute significantly to the global biogeochemical cycles on which we all depend,” says Chen. “The world is a connected planet, more than we might believe. We live today with the consequences of the impacts of deforestation on the climate, of which we were not aware at the beginning. The exploitation of hot spots like hydrothermal vents could have a similar impact.”