Armed with things including a sieve and a bucket of water, scientists in limestone caves in Vietnam and Laos have discovered two remarkable snail species.

Who the sheet Contributions to Zoology opens the door and examines the latest additions to the world of microsnails, you may not be immediately impressed. Because at first sight the two new species presented by the international team of researchers – including some Dutch people – strongly resemble the snails we find in gardens and public gardens. But appearances are deceiving, as the scale number in the images reveals. Because these snails are tiny. One even goes down in the books as the smallest land snail that researchers have described to date.

The smallest

The smallest specimen – Angustopila psammion called – has a snail shell about 0.5 millimeters in size. “That house is slightly wider than it is high and it is white and brownish in colour,” says Menno Schilthuizen, one of the discoverers and an evolutionary biologist associated with the Naturalis Biodiversity Center initiate.

Angustopila psammion. Image: Contributions to Zoology 2022; 10.1163/18759866-bja10025.

only the house

Strikingly enough, Schilthuizen and colleagues can only guess about the appearance of the inhabitant of this tiny snail shell. “We have only found the houses,” the biologist tells “That is often the case with micro snails and has everything to do with the way we detect these snails. We scrape off some soil or – as in this case – some grit from a crevice in a limestone cave, sieve it and then throw the sieve into a bucket of water and see what floats. Because uninhabited snail shells are filled with an air bubble, they float and so do we A. psamion discovers.” After the discovery of this new species, the scientists also scoured the walls of the cave, hoping to find a living specimen. But without result. “We think they live underground.” The recovered snail shells may have been washed away – along with sediments – and eventually ended up in the fissure of the limestone cave in which the researchers found them.

Unanswered questions

The fact that the researchers only found the home of the microsnails clearly doesn’t stop them from describing and presenting the new species. “It is very common with land snails to do this on the basis of snail shells,” explains Schilthuizen. “In snail science, the entire taxonomy is actually based on the houses instead of on the critters themselves. It is sometimes a bit strange for outsiders, such a science that is completely based on a secretion product, but in snail science that is tradition. And it is also quite possible to describe new species on the basis of the house alone, because the houses often have many clear species characteristics.” On the other hand, it does mean that many questions remain unanswered. “We know little about what the animal looks like,” Schilthuizen admits. “We don’t know, for example, whether the animal also has color patterns or what the genitals look like.”

Small is beautiful

What the researchers do know is that the animal must be tiny to fit in its micro house. Such a small size undoubtedly has evolutionary advantages. For example, it enables the micro snails to crawl into spaces that are inaccessible to just or much larger snails. Because of their small size, they create a niche in which they can flourish. But miniaturization – the process of shrinking that eventually resulted in the tiny… A. psamion – knows limits. That is, a snail cannot keep getting smaller; it stops for a while. “If you become very small, you encounter physiological problems,” says Schilthuizen. In the case of snails, dehydration is probably one of them. “With snails, the evaporation of moisture takes place via the surface. And the smaller the snails become, the larger that surface becomes in relation to the content and the greater the chance of dehydration. That is also the reason that we know very small snails, but no micro-slugs; without a protective shell, tiny snails are doomed to dry out.” And so there are even more physiological limitations that prevent snails from getting smaller and smaller. “That way they also have to be able to produce an egg – that happens at the top of the house – and that egg must then also be able to come out. And because that egg already contains the beginning of a snail’s shell, that egg cannot become infinitely smaller either.” This automatically brings us to the fascinating question of whether there are still much smaller land snails than A. psamion to exist. “Of course, that can never be completely ruled out.” But what is striking is that many recently discovered microsnails differ in size from A. psamion cling to. It cautiously suggests that at least when it comes to land snails, we are gradually approaching the limits of miniaturization.

The land snails may be small; they are clearly fine. “This is probably not a rare species. We have A. psamion found in large numbers in our samples. And that suggests that – even though we don’t see them – they are very common.”

Another newcomer

Next A. psamion Schilthuizen and colleagues present another new microsnail in their research article: Angustopila coprologos. “This one looks about the same, but is slightly larger. In addition, A. coprologos all sharp ridges on his house that are decorated with grains.” These granules are attached to their shell by the snail in a regular pattern. “And it seems to be feces. Nobody knows why A. coprologos that does.” More snails are known to cover their houses with waste. “And they seem to be doing it to camouflage themselves.” So who knows, that’s also the reason A. coprologos carefully covered the house with excrement.

Angustopila coprologos decorates his house with grains; probably feces. Image: Contributions to Zoology 2022; 10.1163/18759866-bja10025.

With the discovery of the two new species, the microsnail stock has expanded again. “That remains a nice feeling,” says Schilthuizen, who has already discovered more snail species. “That way you fill in the gaps in the civil registry of biodiversity.” Schilthuizen is convinced that there are still many holes. “I think globally we don’t even know half of the microsnails. This is of course also because the creatures move slowly and therefore hardly move. There are species that only occur in an area that is a few square meters in size and have evolved there on the spot. Before we have been to all those places, we are many years further.”