The dispersal abilities of animals contribute to their local genetic variability and the preservation of the species. Mammals or birds can usually move quickly and far to provide such dispersal. On the other hand, microorganisms less capable of colonizing large spaces must sometimes be ingenious to expand their presence and thus increase genetic diversity within a species. One of the implemented strategies is phoresy, which refers to the interaction between two organisms, in which one (foront) is transported by the other (host). Until now, scientists didn’t know if tiny invertebrates like tardigrades could practice phoresia.
Local transport: no hostile direction guarantee
Phoretically dispersed species have been observed in at least 13 animal phyla, 25 classes and 60 orders, report Zofia Ksienzkiewicz and Milena Roszkowska in Scientific reports. Most known foronts are arthropods found in terrestrial habitats; for example, ticks have been observed to travel on beetles, bumblebees, or centipedes. Note that phoretic relationships are generally non-destructive, unless transport causes damage to the host’s skin.
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Tardigrades are organisms smaller than a millimeter. They are known for their extreme stability in aggressive environments, due to the ability to enter into anhydrobiosis – a state of almost complete dehydration of the body. Thus, they can survive for several years in a dry and “dormant” state in an environment without water, and then resume their activity as soon as they are rehydrated. When they are in this state of “vitrification”, they are easily blown away by the wind or carried by birds over long distances. However, the mechanisms responsible for small-scale settlement remained poorly understood. Therefore, Zofia Ksienzkiewicz and Milena Roszkowska took up this issue.
An ideal candidate for transporting tardigrades should have a moist seed coat, as tardigrades require high humidity to stay active. It must also have the same habitat and be active at the same time as the tardigrades. Based on these conditions, land snails seem to be ideal vehicles: they become more active as soon as the humidity rises, and their skin is constantly moistened with mucus.
The choice of a host known for its slowness may seem surprising at first glance, but snails move faster and for much longer distances than tardigrades – 25 m / h in a tree snail versus 25 m / h. 23 mm/h for tardigrades. In addition, snails may traverse environments that are impervious to tardigrades, especially man-made spaces such as roads or lawns. But, above all, with a snail, tardigrades are guaranteed to land in the right environment for them – and vice versa, if they are blown away by the wind, there is no guarantee that they will land in the right environment.
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Sometimes a fatal journey for a tardigrade
As part of their study, the two biologists investigated, on the one hand, snails have a significant impact on the dispersal of active tardigrades, and on the other hand, whether snail mucus interacts with tardigrades in a way that potentially affects their recovery from anhydrobiosis. They studied two relatively common species coexisting in nature: Milnesium inceptum (representative of the tardigrade) and Head nemoral (also called wood or hedge).
The two researchers studied the interaction between the two species through various laboratory experiments. They placed about ten tardigrades on a silicone square (3 cm side and 0.5 cm thick) inside ventilated boxes, some of which also contained a wood snail. Tardigrades could sometimes hide in moss taken from their natural habitat; other boxes contained only drops of water in which tiny creatures could swim. After 72 hours, the scientists counted the number of tardigrades inside and outside the silicone square.
Graphical representation of the three experimental conditions. (A) 10 tardigrades in a silicone square (control); (B) 10 tardigrades in a silicone square and a snail in a box; (C) 10 tardigrades in a silicone square, a snail placed in a box, and an extra piece of Styrofoam added as a “barrier” between the tardigrades and the snail. Credit & Copyright: Z. Ksienzhkiewicz et al., Scientific Reports (2022)
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The experiment showed that tardigrades could only get out of their silicone square in the presence of snails; in their absence, no one was displaced. ” Therefore, we hypothesize that snails were responsible for the transport of tardigrades. However, it should be emphasized that the efficiency of transport may depend on the type of substrate. “, – the researchers write. It turns out that transportation can be difficult if the substrate allows tardigrades to firmly attach to it.
In addition, fewer tardigrades moved in the foam boxes, suggesting that these tiny animals are picked up “by chance” by snail movements and that their chances of such movement are reduced if they nestle in the moss.
However, this phoresis is not without dangers for water bears: only 34% of those who took the trip survived 24 hours after they were rehydrated. Snail mucus is mostly water, and tardigrades immediately rehydrate upon contact with it. However, it also dries out very quickly, too quickly for the tardigrades to adapt. In conclusion, snail-tardigrade interactions can have both positive and negative consequences for tardigrades. This study showed that snail transport is possible, but biologists do not know if this strategy is actually implemented in nature and how often. Only future observations in the natural environment will dispel doubts.