The smallest land snail in the world – the shell of the snail species from Vietnam is smaller than a grain of sand –

Housed midget: In Vietnam, researchers have discovered the smallest land snail known to date. Its housing is only half a millimeter in size, making it smaller than a grain of sand. The snail species Angustopila psammion could thus be at the lower limit of what is biologically possible for snails. Why this species is so extremely miniaturized is still unclear, but it could make it easier to use special niches or escape from predators.

In the animal kingdom there are some groups that have produced extremely miniaturized representatives. The smallest reptile, a chameleon, measures only 13 millimeters, the smallest frogs are even only eight to nine millimeters in size. These amphibians, native to Madagascar, are also the smallest land vertebrates in the world. And even among the dinosaurs, which otherwise attracted attention because of their gigantic size, there were miniature specimens: A dinosaur preserved in amber was only 14 millimeters long.

Miniature land snail

The snails have also set a new record in miniature format: the shell of the Angustopila psammion species found in Vietnam is only around 0.48 millimeters high and 0.6 millimeters long. With a shell volume of just 0.045 cubic millimeters, it is the smallest of all known land snails. Only a few sea snails, such as the species Ammonicera minortalis, are even smaller.

“It is something very special to discover such a record holder in the animal world,” says co-author Adrienne Jochum from the Senckenberg Research Institute and Natural History Museum in Frankfurt. “We assume that the size of the snail we found is already at the lower limit of adult land snails. The animals cannot get much smaller because there must be a certain number of neurons that make a snail functional. In addition, the snail shell must also offer enough space for at least one egg.”

Why is the snail so small?

Jochum and her colleagues discovered the tiny creature in a cave in North Vietnam. When they washed and sift through sediment samples from Cap La Cave, they found numerous tiny snail shells. Analysis of its shape and characteristics revealed that it was a new species. It is still unclear whether this tiny land snail lives in the dark dry cave or whether the shells just fell in there through gaps in the cave ceiling.

“Our discovery immediately raised the question of the evolutionary mechanisms that lead to some snails being so small,” explains Jochum. “The most plausible possibility is that the tiny snails can use previously unoccupied niches – because of their size, they can search for food in confined spaces as well as eat food particles that are of no interest to larger animals.” The small size also gives the snails the advantage hiding from their predators; often they are even so small that they are not interesting as food.

The newly discovered snail Angustopila coprologos decorates its porcelain-like shell with grains of droppings. © Senckenberg

Mini snail with droppings on the house

However, the team led by Jochum and first author Barna Pall-Gergely from the Center for Agricultural Research in Budapest discovered another mini snail from the genus Angustopila: In Laos they found the slightly larger species Angustopila coprologos. This has the peculiarity that it decorates its shell with a radial pattern of tiny, brown grains of excrement.

The research team can only guess what purpose this rather unappetizing garnish serves. It is known from other, larger land snails that they often cover their shells with bark, lichen, clay or soil particles in order to camouflage themselves and thus avoid the attention of predators such as birds or beetles. “Such optical camouflage makes no sense for the extremely small snails that live in limestone crevices,” says Jochum.

Rather, she suspects that the droppings could be used to attract mates or to protect the snails from drying out in the dry underground. The droppings could then act as “mini-sponges” that retain moisture. “In any case, it is surprising that these tiny snails develop such complex mechanisms, about which we – so far – still know little,” concludes Jochum. (Contributions to Zoology, 2022; doi: 10.1163/18759866-bja10025)

Source: Senckenberg Research Institute and Natural History Museums

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