The current biodiversity is astonishing, but it is essential to recognize that many organisms have evolved over a long time to survive on Earth and adapt to its environment, with each species developing different abilities.
To consume more food, they have developed very sharp claws, and their running speed has increased. Many prey species have learned survival instincts, such as changing color or camouflaging. Among various animal species, the most effective at self-protection are crustaceans, which have evolved hard shells in nature, such as crabs, turtles, and others.
Statistics show that there are over 20,000 types of crustaceans on Earth today. These crustaceans typically have no natural enemies, as when predators approach, they can retract into their shells, making it impossible for predators to consume them. One particular organism among them has the hardest shell in the world, made of iron.
Deep beneath the Indian Ocean, near the searing hydrothermal vents where hot water jets out in tall black columns of smoke, lives a peculiar gastropod known scientifically as Chrysomallon squamiferum. This species is a type of hydrothermal snail belonging to a family of deep-sea mollusks.
It is believed that even bullets cannot penetrate the shell of this snail, which operates at depths of over 2,000 meters underwater, generally residing at hydrothermal vent sites. These areas experience high water pressure, temperature, and acidity, while the oxygen content is very low. Their incredibly hard shells allow them to withstand significant pressure.
The harsh living environment has driven the scaly-foot gastropod to evolve a uniquely protective armor. Specifically, the shell of the scaly-foot snail consists of three layers, each believed to serve a specific protective function.
Hydrothermal snail living at the deep-sea floor.
The outer layer is about 30 micrometers thick and made of iron sulfide. This layer may shatter upon impact, but it also serves as a mechanism for the snail to absorb energy while deterring any would-be attackers.
The middle layer consists of organic keratin, similar to the thin protein coating found on other types of shells. This is also the thickest layer of the three (approximately 150 micrometers) and acts as an organic “pain-reducing” cushion for the snail.
The innermost layer is made of aragonite—a form of calcium carbonate commonly found in the shells of various mollusks and corals.
The sides of the scaly-foot snail’s foot are extraordinarily unusual, covered by hundreds of pieces of mineralized iron debris, including greigite and pyrite sulfides. These scales are not found in any other snail species on Earth. Their purpose may be for protection or a result of the deposition of toxic sulfide waste from symbiotic organisms.
Hydrothermal snail.
The harsh environment forces the snail to adapt with a three-layer shell.
In terms of nutrition, the scaly-foot snail is a symbiotic organism that relies on bacteria throughout its life. They obtain all their nutrients from the chemosynthetic processes of their symbiotic bacteria.
Regarding reproduction, this species is a simultaneous hermaphrodite (possessing both male and female reproductive organs that mature at the same time). Although there is no evidence yet, it is hypothesized that their larvae have a planktonic stage before settling on the seafloor.
Thus, each layer of the snail’s thick shell plays an essential role in its defensive effectiveness. Scientists state that this is the only known organism on Earth capable of using iron in its shell for protective purposes. The U.S. military is currently researching to develop a new effective type of armor.
According to the IUCN Red List, the scaly-foot snail is classified as a Vulnerable species. The threat to its survival primarily comes from the impacts of deep-sea mining activities.
