The late ones travel on the back of a snail! …

The dispersal abilities of animals contribute to their local genetic variability and the persistence of species. Mammals or birds usually move quickly and far away to provide this dispersal. On the other hand, microorganisms, less able to colonize large areas, sometimes have to show ingenuity to expand their presence and thus increase genetic diversity within the species. One of the strategies implemented is foresight, which designates an interaction between two organisms in which one (the host) is transported by the other (the host). Until now, scientists did not know if small invertebrates such as tardigrades could practice foresight.

Local transport: the guarantee of a non-hostile destination

Species that have been foretically dispersed in at least 13 animal threads, 25 classes and 60 orders have been observed, Zofia Książkiewicz and Milena Roszkowska report in Scientific reports. Most known phoronts are arthropods, which live in terrestrial habitats; for example, mites have been observed traveling on beetles, bumblebees, or centipedes. Please note that the forestry relationship is generally non-destructive, unless the transport causes damage to the host’s skin.

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Grasshoppers are organisms less than a millimeter long. They are known for their extreme endurance in hostile environments, due to their ability to enter anhydrobiosis, a state of almost complete dehydration of the body. Thus, they can survive dry and “dormant” for several years in waterless environments, and then resume their activity as soon as they rehydrate. When they are in this state of “vitrification”, they can be easily dispersed by the wind or carried by birds over long distances. However, the mechanisms responsible for small-scale dispersion remained poorly explored. Zofia Książkiewicz and Milena Roszkowska therefore studied the subject.

An ideal candidate for transporting tardigrades should have a moist seed layer, as tardigrades require high humidity to stay active. It must also share the same habitat and be active at the same time as the tardigrades. From these conditions, land snails seem to be perfect vehicles: they become more active as soon as moisture increases and their skin is permanently hydrated by mucus.

The choice of a guest known for his slowness may seem surprising at first glance, but the screws move faster and at much greater distances than the tardigrades: 25 m / h for the wooden screw vs. 23 mm / h for tardigrades. In addition, snails can traverse impenetrable environments for tardigrades, especially man-made spaces such as roads or lawns. But above all, with a screw, the tardígrads have the guarantee of being driven to an environment that suits them; however, carried by the wind, there is no guarantee that they will land in a suitable environment.

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A sometimes fatal journey for the tardigrades

As part of their study, the two biologists investigated whether, on the one hand, snails had a significant effect on the dispersal of active tardigrades and, on the other hand, whether snail mucus interacted with the tardigrades of a snail. which could affect their recovery after anhydrobiosis. They studied two relatively common species, which coexist in nature: Milnesium inceptum (a representative of the tardígrad philum) i Cepaea nemoralis (also called wooden snail or hedge).

The two researchers examined the interaction between the two species using various laboratory experiments. They placed about ten degrees Celsius in a square of silicone (3 cm on each side by 0.5 cm thick) inside ventilated boxes, some of which also contained a forest screw. Grasshoppers could sometimes take refuge in moss prey in their natural habitat; other boxes contained only drops of water in which the tiny creatures could float. After 72 hours, the scientists counted the number of tardigrades inside and outside the silicone square.

Graphic representation of the three experimental conditions. (A) 10 degrees squared silicone square (control); (B) 10 square degrees of silicone and a screw placed in the box; (C) 10 square degrees of silicone, a screw placed in the box, and an additional piece of foam added as a “barrier” between the bars and the screw. Credits: Z. Książkiewicz et al., Scientific reports (2022)

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The experiment showed that tardigrades were only able to escape their silicone square in the presence of snails; in his absence, none moved. ” Therefore, we assume that the screws were responsible for transporting the tardigrades. However, it should be noted that transport efficiency may depend on the type of substrate “It seems that transport can be difficult if the substrate allows the late blight to adhere firmly to it.

In addition, in the boxes containing foam, they moved less late, suggesting that these tiny animals are picked up “randomly” by the movements of the snails and that their chances of being transported in this way decrease if they nest in the moss. .

However, this forest is not without dangers for water bears: only 34% who took advantage of the trip survived 24 hours after rehydration. The mucus of the snail consists mainly of water, and tardigrades rehydrate immediately in contact with it. However, it also dries very quickly, too quickly for late graders to have time to adjust. In conclusion, the snail-tardigrade interaction can have both positive and negative effects on tardigrades. This study showed that snail transport is possible, but biologists do not know if this strategy is actually implemented in nature, or how often. Only future observations in the natural environment will eliminate the doubt.

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