A newly discovered snail species is the smallest yet found on land. Angustopila psammion, discovered in cave sediment in northern Vietnam, has a shell just 0.48 millimetres high and a shell volume of only 0.036 cubic millimetres. This makes the species so small that you could fit about five individuals inside the average grain of sand.
Unsurprisingly, these snails are hard to spot. To find them, Barna Páll-Gergely, a land snail taxonomist at the Eötvös Loránd Research Network in Budapest, Hungary, and his colleagues gathered soil samples from caves and placed them in a bucket of water. They then removed the floating debris, dried it, sieved it and examined it under a microscope. “I cleaned the shells under the microscope with very precise brushes used by nail artists,” says Páll-Gergely.
The snails probably didn’t live in the caves though, says Páll-Gergely. “We assume that the sediment had fallen in through crevices in the rock, because it contains bleached, opaque shells of surface-dwelling terrestrial gastropods. The living snails presumably live deep in limestone crevices close to or on root systems.”
The researchers also discovered a not-quite-so-tiny snail in Laos, naming it Angustopila coprologos, which means “dung gatherer” in Ancient Greek. Standing a mighty 0.51 mm tall, it seems to arrange tiny granules of mud – possibly its own faeces – in a pattern of radial lines on the surface of its shell.
Why it does this is unknown. “If it is camouflage, what would prey on these tiny snails?” says Páll-Gergely. “If not camouflage, then what? It was surprising to see that in face of their extremely small size, these tiny snails have complex behavioural mechanisms that evolved as a response to certain environmental factors that we know nothing about.”
The snails’ miniature size does give them advantages. “It is probable that by being small, these snails can reach food particles no other species can consume and enter very narrow rock crevices,” says Páll-Gergely. They could also avoid predation by being smaller than the things their predators normally look for, he says.
Is A. psammion likely to be the smallest land snail possible? Finding anything smaller will be increasingly difficult, says Páll-Gergely.
There are smaller known snails in the sea – the record holder there is Ammonicera minortalis, with a diameter of between 0.34 mm and 0.46 mm, he says. That is probably close to the lower limit, which is determined by the number of neurons a newborn snail must have to be functional, and the shell of the adult snail being large enough to accommodate at least one egg.
Journal reference: Contributions to Zoology, DOI: 10.1163/18759866-bja10025
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