Scientists have found experimental evidence that tardigrades piggyback on snails to disperse over greater distances. But that doesn’t always end well…

Tartars are multicellular, but tiny animals: often only half a millimeter long and about a fifth of a millimeter wide. Despite their small size, they have increasingly attracted our attention in recent decades. And that can mainly be explained by the fact that tardigrades have turned out to be virtually indestructible; UV radiation, dehydration, extreme heat, freezing cold, lack of oxygen, dangerous cosmic radiation, being shot with a gun (really tried!): the tardigrade cannot be broken. Well, almost not. Because in a new study, scientists seem to have found something that in some cases is a particularly bad idea even for the indestructible tardigrade: hitchhiking.

eight legs

Most small and soft organisms have to do without legs, but the tardigrade is an exception. The tiny organism is blessed with eight legs, with which it can even run – albeit in a very remarkable way. But those short legs don’t take the tardigrade – even running – very far. However, that did not stop the tardigrade from conquering the world: today the organism can be found on all continents on earth and in the most diverse habitats. It is probably partly due to passive forms of transport, where tardigrades, for example, are carried along by the wind or water and can travel enormous distances. But there is another option, researchers now write in the magazine Scientific Reports† Possibly the tardigrades also cover considerable distances for them by hitchhiking with…snails.

At first glance, a snail may not seem like the ideal means of transport. But from the point of view of a tardigrade it can be very different. First of all, previous research has shown that tardigrades move at about a speed of 23 millimeters per hour, which suddenly makes the snail a pretty smooth transporter. In addition, tardigrades must always have a layer of water around their bodies to remain active (see box). And snails are moist. To top it off, snails often live in areas where tardigrades are active and – unlike tardigrades – they can move over obstacles (posed by humans), such as curbs. In short: piggybacking with a snail is – again: from the point of view of a tardigrade – not so illogical.

Active or a ton
To function, tardigrades – on land – need a layer of water around them. It requires certain (very humid) conditions that are limited in some habitats. And that means that tardigrades can only be active to a limited extent in those habitats. That also severely limits the number of millimeters they can actively travel. When the conditions are unfavorable for an active life, that does not mean the end for the tardigrade. It has another remarkable ‘superpower’: in shorter and longer periods of drought it goes into anhydrobiosis. This causes the tardigrade to slowly dry out almost completely (almost all the water that is normally present in the tardigrade’s tissues disappears) and the metabolism is virtually shut down. The tardigrade shrivels and looks like a barrel. That is why such a dried out tardigrade is also called a barrel.

But can tardigrades actually hitch a ride with snails? The researchers put it to the test. They collected snails belonging to the species Cepaea nemoralis and rinse them completely clean. They then put the snails in the same petri dish as active tardigrades belonging to the species Milnesium inceptum (and naturally occurring in the same habitat as C. nemoralis† It soon became apparent that snails were sliding over the tardigrades and – when the bottom of the petri dish was smooth – the necessary tardigrades remained ‘sticky’ to the snail. Some of those tardigrades were then moved several centimeters (a considerable distance for a tardigrade). “It’s probably just a coincidence,” researcher Zofia Książkiewicz tells Scientias.nl† “Beards cannot ‘decide’ to use this mode of transport; active tardigrades are simply on the route the snail is currently following.”

pooped

This is not the first time that researchers have investigated the interaction between snails and tardigrades. In the past, tardigrades were also found in the droppings of snails. Presumably the tardigrades were eaten by the snails and then pooped out further on. And the tardigrades could retell that too; after pooping, they simply resumed life – in their new place of residence. But now researchers have experimentally shown that snails can also transport tardigrades in a somewhat less intimate way.

Mortality

But piggybacking on a snail may not be without risks, as the experiments also show. This is especially true for tardigrades in anhydrobiosis. “Since snails and tardigrades are active under the same (wet) conditions, we thought that transport by snails was mainly possible for active tardigrades. On the other hand, snails can also be active in slightly less humid conditions than the tardigrades require. And that’s why we also investigated the impact that the snail slime has on the ‘barrels’,” explains researcher Milena Roszkowska. And the results are not very promising. For example, only 34 percent of the tardigrades in anhydrobiosis appeared to survive contact with the mucus – and thus their interaction with the hitchhiker so attractive at first sight. “Because the slime is mostly water, we think the slime caused the tardigrades to rehydrate.” In other words, they started coming out of the anhydrobiosis. “But that didn’t quite work out before the mucus dried.” The result? Tartars that died in very strange poses – due to the fact that they tried in vain to revert to anhydrobiosis in response to drought. “In addition, it may also be the case that the antibacterial properties of snail slime affected the microbiome of the tardigrades and thus influenced mortality.”

In the wild

These are intriguing findings that – and it should be emphasized – are based solely on laboratory tests. Whether something similar happens ‘in the wild’ remains uncertain. “We do believe that it can also happen in the natural habitat,” says Książkiewicz. Of course also prompted by the fact that the species studied occur naturally in the same habitat and therefore regularly bump into each other. According to her, the main question is to what extent it happens, how far the tardigrades manage to make it this way and what impact it has on the tardigrades. “Such short-distance transport can have a significant impact on genetic diversity within a population,” notes Książkiewicz.

However, getting answers to such questions is not so easy. “Of course it would be perfect if we had data on wild populations in their natural environment,” Roszkowska said. “But that is – due to the small size of the tardigrades – almost impossible. We should therefore actually start with another laboratory study in which we determine the distances over which the tardigrades are transported. And perhaps we should also investigate how quickly the tardigrades transported by snails settle in sterile mosses. But such research would be very labour-intensive.” And so it seems that we have to accept that some questions remain unanswered at least for the time being.