Volcano snails are smallish (0.5-5 cm) snails that have been found in several hydrothermal vent fields almost three kilometres deep in the Indian Ocean. Hydrothermal vents, sometimes called "black smokers", spew water laden with toxic metal salts out of the sea floor at temperatures of up to 750 C. The only reason they don't boil is because the pressure from the enormous depth prevents it.
To delicate surface-dwelling creatures like us, hydrothermal vent fields look like a reasonable approximation of some Medieval fantasy of hell. To volcano snails, they look like home.
One has to wonder how they got there. Did they evolve from some other mollusc that accidentally fell into the ocean deep and then somehow survived to breed? Or are they remnants of a long-lost Earth, when black smokers were common in the warm, shallow seas, and whole ecosystems thrived which are now reduced to a handful of species trapped on these abyssal islands? The evolutionary history of these deep sea communities is still an active area of research, and their story is complicated because they have to live without sunlight.
Everything in the upper reaches of the oceans depends ultimately on the sun for sustenance: photosynthetic algae, especially diatoms, use this energy to make simple carbohydrates, and get eaten by everything else. Some of them die and fall to the deep ocean floor, which sustains a sparse ecosystem of abyssal life.
Hydrothermal vents present a different kind of opportunity for life, because their rich chemistry is a source of energy. Chemo-synthesis, not photo-synthesis, is what sustains them. The existence of life in these chemically-driven communities implies the possibility of complex ecosystems in other regions of eternal darkness, including the ice-covered oceans of Jupiter's moon Europa, and Enceladus around Saturn.
Chemosynthetic micro-organisms (mostly bacteria) have a problem, though: they need to float in the water column to get a continuous supply of new chemicals. But if they do that, they'll drift away from the vent field and die. If instead they stick to rocks, they'll be exposed and get eaten.
This creates "selective pressure", which isn't literally anything pushing or pressing individuals to behave a particular way, but is a tendency to reward certain behaviours or capacities after the fact with a higher rate of reproduction for the individuals who exhibit them.
In this case, an individual snail that can accommodate chemosynthetic bacteria in its tissues is more likely to reproduce successfully than one that can't, because the snail can live off the bacteria. Individual bacteria that can grow within the snail's tissue get a stable home and plentiful water supply. Over generations of natural selection, this has resulted in the modern population of volcano snails, which has no source of food except the chemosynthetic bacteria growing within their enlarged oesophageal gland. The bacteria reduce hydrogen sulphide to extract energy, which results in the snails eliminating small pellets of sulphur from their gut, which for a snail would be a problem if they hadn't followed a literally twisted evolutionary path.
Evolutionary history can be seen through the mirror of modern taxonomy: different genera of the same class evolved from the same original species, and different classes of the same phylum did the same, but even further back in time. Some species travel a long way from their common root, others hardly at all: a lot of Americans came all the way from Europe, but most Europeans stayed home. This is why there are still people in Europe even though there are also people from Europe in America.
Snails belong to the class of gastropods, which includes things like slugs, which don't have shells, and whatever organism they both evolved from, it likely didn't have a shell either. So we need to think about the problems faced by a single-shelled organism in the course of its evolution, particularly: how does it defecate if its ass is covered by a shell?
The answer turns out to be: streptoneury, or "twisting", which is an evolutionary process by which internal organs re-arrange themselves over generations so that the anus and genital opening migrate up one side toward the front of the animal. The alternative to twisting would have been "folding", in which the organism bent double, but curiously this doesn't seem to ever happen. Evolution is an incremental, elaborative, process that allows organisms to migrate between stable forms by passing generation-by-generation through intermediate stages, but only some intermediate forms are viable. Presumably folded organisms suffered some disadvantages that their twisted cousins avoided.
Streptoneury is common in shelled gastropods, but the volcano snail's shell is something out of the ordinary: the inner layer is calcium carbonate, the same as most mollusc shells. Outside of that is the periostracum, which is a protective protein coating, also commonplace. The unique feature is the outer layer of iron sulphide. The sides of the foot are also scaled with iron sulphide "teeth", which likely help protect it from the crab-eat-snail world it lives in. The volcano snail is the only animal in the world that incorporates iron into its skeleton.
The scale-like "teeth" on the snail's foot are called sclerites, and give the snail its proper name: scaly-footed gastropod. It's the only living species with this feature, although similar scales are evident in many fossils from the Cambrian era. Before the Cambrian, lack of oxygen in the atmosphere and oceans restricted multi-cellular life to a few sesile, coral-like organisms, but about 540 million years ago some critical threshold was crossed and the "Cambrian explosion" saw a proliferation of novel, motile, forms over the following fifty million years, including the ancestors to all modern phyla, from molluscs to mammals.
So the volcano snail gives us a glimpse into the distant past, and the origins of complex, multi-cellular life on Earth, while at the same time suggesting what we might find in the dark oceans whose currents flow around the ice-moons within our solar system, and perhaps too beneath the surface on the ice-bound worlds that circle other stars.