Their shell is less than half a millimeter in size and the entire animal is smaller than a grain of sand: Biologists have discovered the smallest known land snail species in Vietnam. The tiny shells of the newly discovered species Angustopila psammion now raise the question of why this species of snail is so small and what evolutionary advantages this species has brought from its miniaturization.
Snails are the most species-rich group of molluscs and the only one that has also produced terrestrial species. Even if the snails have a common basic structure, they have produced a wealth of different forms in the course of evolution – from large, sometimes colorful sea snails to small and large land snails with and without shells.
Tiny shells in cave sediment
A team led by Barna Pall-Gergely from the Center for Agricultural Research in Budapest has now discovered a very special species of snail during a research trip to Southeast Asia. In search of snail species found there, the scientists also sampled sediment collected under rocks and in caves. These samples were placed in a tank of water so that pebbles sank to the bottom, but the hollow snail shells floated on the surface.
The team made an exciting discovery in the sediment from the Cap La Cave in northern Vietnam: the samples contained numerous tiny snail shells, which were on average only 0.48 millimeters high and 0.6 millimeters long. They were smaller than a grain of sand. Based on the shape and shell characteristics, the biologists were able to determine that these shells must have come from a still unknown species – named Angustopila psammion. What is special, however: With a shell volume of only 0.045 cubic millimeters, this species is the smallest known land snail in the world. Slightly smaller gastropods such as the species Ammonicera minortalis are only known from the sea.
Biological use still unclear
“It is something very special to discover such a record holder in the animal world – especially since this is of course not a matter of course given the size,” 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.”
This begs the question of why these snails are so tiny. However, biologists still do not have a clear answer. “The most plausible thing seems to be that the tiny snails can use previously unoccupied niches – due to their size, they can search for food in confined spaces as well as eat food particles that are of no interest to larger animals,” explains Jochum. “In addition, the small size gives the snails the advantage of hiding from their predators. In fact, they are often so small that they are not interesting as food.”
A snail with excrement jewelry
On their research expedition through Southeast Asia, Jochum and her colleagues in Laos discovered another unusual species of snail. The species Angustopila coprologos, which belongs to the same genus as the mini record holder, is hardly larger, but has an unusual shell decoration: The animal decorates its fine, porcelain-like shell with grains of excrement, which are arranged in a radial pattern – like pearls on a necklace.
However, it is still unclear why this snail does this. It is known that other, larger land snails often cover their shells with bark, lichen, clay or soil particles in order to adapt to their environment and to 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, the researcher suspects that the droppings could serve to attract sexual partners.
It is also conceivable that the snail uses the droppings as protection against drying out. Because the granules retain water, they could help the animal retain moisture in its mostly dry environment. “In any case, it is surprising that these tiny snails develop such complex mechanisms, about which we – so far – still know little,” concludes Jochum.
Source: Senckenberg Research Institute and Natural History Museums; Specialist article: Contributions to Zoology, doi: 10.1163/18759866-bja10025