In what is arguably the most famous scene from Monty Python and the Holy Grail, King Arthur faces the Black Knight, relieving him of his limbs even though he's wearing some nice chain mail. But there’s a Black Knight in the depths of the Indian Ocean whose armor doesn’t yield to blows quite so easily.
I present to you the scaly-foot snail, which has evolved a shell made of iron sulfide. I’ll repeat that: It builds a shell out of iron. On top of this, the squishy part that protrudes out of the shell, known as the foot, is covered with iron plates, making the scaly-foot snail more metal than Ozzy Osbourne wrapped in tin foil. And it's all thanks to bacteria, which seem to be building the armor. No other animal on Earth can utilize iron this way. The thing is magnetic, for Pete's sake.
This is no ordinary snail, but then again, it lives in no ordinary environment. It's hanging around hydrothermal vents, where seawater percolates into the crust and is heated by underlying magma, reaching 750 degrees F or more, pouring out and bringing toxins with it. This is a very, very rough neighborhood.
According to biologist Shana Goffredi of Occidental College, among the animals down there are the mortal enemies of the scaly-foot snail: crabs and other snails. “It’s very strange because a lot of snails must have the same kinds of predators,” she said. “So I don't think there's anything special about the predatory challenges on them, but still it looks like they have really beefed up their shells for some reason.”
These shells aren’t totally rigid like plate armor. They’re more like the Black Knight’s chain mail—pliable, yet strong (well, maybe the Black Knight's armor wasn't all that strong, what with all the dismemberment). This is because there are three layers to the shell: the top layer is that iron-plated material and the bottom is a calcified material, with a thick, squishy organic layer in between. The iron provides the strength, while the squishy bit allows the shell to absorb the shock of, say, an ill-mannered crab clamping down on it.
The scales on the foot serve a rather more righteous purpose. Some predatory snails hunt by firing harpoons into the flesh of fish and other snails and injecting a venom. It’s thought that the iron plating of the scaly-foot snail deflects that missile, like a knight’s armor deflecting a lance.
Weirdly, there are two varieties of the scaly-foot. The other isn’t black, but instead whiter. It lives in the same environment as the Black Knight, yet it lacks the iron. Why?
The answer comes down to bacteria that the black variety has and the white lacks. “They have beneficial bacteria on and in them,” Goffredi says, “and we think that the beneficial bacteria on the outside of the animal are actually helping facilitate the production of these iron sulfides.” Sulfides pouring out of vents are really toxic, “but as soon as it gets mineralized and incorporated into a solid form, then it’s not toxic anymore,” said Goffredi. “I think that the microbes play a role in changing the nature of that compound by making it less poisonous.” So not only is the snail clad in badass armor, it’s armor is made out of poisons. Good god that's metal as hell.
That’s the working hypothesis, at least. The bacteria hasn’t been cultured in the lab, so we can’t know for sure. “There's another group that thinks that the snail is making the iron sulfides itself, but that is completely unprecedented,” Goffredi said. “We doubt it, but nobody can know for sure unless we could manipulate either of the players involved. We're left with speculating.”
Then there's the bacteria inside the scaly-foot snail. It's probably serving its host in an even more important way: chemosynthesis. That’s a five-dollar word meaning the snail isn’t eating food, but instead relying on bacteria for sustenance. Its digestive system is practically nonexistent, but it does have a gland—which is 1,000 times bigger than in other snails—where the bacteria live and produce food.
Like the kind of bacteria that builds the snail’s shell, this type takes the chemicals in the neighborhood and synthesizes them into grub for the snail, probably a kind of sugar (this bacteria also hasn’t yet been cultured in the lab, so we’re still a bit in the dark here, too). In exchange for its services, the microbe gets a nice little home to live in.
So the scaly-foot snail has stumbled upon a brilliant adaptation to life in the deep, where food is scarce—to put it mildly. “Any carbon that 'rains' down that deep has already been used by everybody else it has passed by, and they get essentially the dregs,” said Goffredi. “So, instead they've teamed up with bacteria that can harness that energy coming from the center of the Earth.”
You might ask why the snail doesn't move somewhere more hospitable. As it turns out, if you can manage to make a living here, the living is good. Something like a tropical reef is positively swarming with species, and that means lots of competition for resources. Down in the depths, and in particular around hydrothermal vents (see them in action above), there are niches begging to be taken. Evolve to survive the high pressures and temperatures and toxic waters, and you’ve got it made.
Wearing iron armor doesn’t hurt either. Just ask Robert Downey Jr.—I heard he dressed up like a robot for Halloween once.
