In the absence of sunlight, which cannot penetrate so deep, the organic substance that feeds the life on the ocean floor is largely produced in the illuminated superficial layers. What is not consumed near the surface descends into the deep, supporting the abyssal trophic webs. However, animal communities that thrive in the proximity of black smokers have developed, in the course of evolution, a sort of “energetic autarchy”. Gases and metals dissolved in water feed a specialized microbial community that feed up the trophic network of this ecosystem. Indeed, the metazoa that live near ocean hydrothermal vents developed morphological and physiological adaptations that allow them not only to survive in these conditions, prohibitive for most animal species, but that also let them to exploit a trophic network based on chemoautotrophic bacteria. Unlike primary producers such as algae and plants, which use sunlight energy to transform inorganic carbon compounds into organic compounds, chemoautotrophic bacteria obtain the energy needed to fix organic carbon through the oxidation of compounds such as hydrogen sulfide and methane, in the absence of sunlight.

Among the particular animal organisms that populate these environments, one recently discovered in the Indian Ocean turned out to be even one of the most peculiar. It is the scaly-foot gastropod, a mollusk close relative of the common snails in our gardens, called Chrysomallon squamiferum Chen et al., 2015. At first glance it looks like a common gastropod, with a crawling soft body carrying a spiral shell on its back. Yet, on a closer examination, a first peculiarity emerges concerning the foot (i.e. the crawling organ that allows gastropods to move on the substrate). Unlike common snails and all the other terrestrial or aquatic gastropods known so far, the soft dorsal tissues of the foot of this mollusk are in fact covered with dermal mineralized scales made of iron sulfide. Therefore, like a modern cataphract, the soft parts of this animal appear covered with a sort of armor with metal plates. The peculiarities of this mollusk, however, do not stop at the external appearance.

Like many other animals living in this system, C. squamiferum seems to eat chemoautotrophic bacteria, but again it has interesting evolutionary innovations. Compared to other gastropods, it has a significantly enlarged circulatory system and heart, which supply the single gill and an esophageal gland. The latter is home to endosymbiotic chemoautotrophic bacteria (i.e., they live in symbiosis with mollusk, inside its body) and which provide the food of the mollusk. In practice, this mollusk seems to breed bacteria within its own body, offering them a suitable environment to feed, grow and reproduce Furthermore, it supplies them, through the hyper-developed circulatory system, with oxygen and hydrogen sulfide necessary for their survival. In return, mollusk feeds on these bacteria.

The armor that covers the soft parts of this animal could protect the mollusk from the boiling acidic hydrothermal fluids that come out of the volcanic vents, allowing it to live as close as possible to the source of the chemical compounds necessary for the sustenance of the chemoautotrophic bacteria. However, this armor could also be the result of metabolic processes aimed at eliminating the waste substances resulting from the biochemical processes carried out by the bacteria, depositing them precisely in the form of mineral scales on the surface of the foot.