(Zofia Książkiewicz and Milena Roszkowska)
A group of Polish zoologists have found that tardigrades can travel on snails.
Although these mollusks are known for their slowness, they still move much faster than tardigrades.
In addition, snails are able to transport their passengers to suitable wet habitats.
However, snail slime can be dangerous for tardigrades when they are in suspended animation.
Tardigrades (Tardigrada) are known for their extraordinary survival skills.
In a state of anhydria, these invertebrates can tolerate extreme temperatures, high doses of ionizing radiation, and even being in a vacuum.
However, for active life, they need at least a thin film of water around the body, which binds them to wet habitats and limits their opportunities for settling.
Most tardigrades move independently only for very short distances.
They settle mainly passively, due to wind and water currents that carry individuals that are at rest.
(Zofia Książkiewicz and Milena Roszkowska)
Now zoologists Zofia Książkiewicz and Milena Roszkowska from the Adam Mickiewicz University in Poznań have suggested that tardigrades can also practice phoresia, the use of larger animals as vehicles.
This is exactly what many small arthropods do, such as the larvae of some beetles, mites and false scorpions.
According to a number of observations, tardigrades settle on the feathers of birds, as well as with their droppings and nesting material.
However, according to the researchers, the ideal transport for tardigrades would be land snails, whose bodies are constantly wet due to mucus.
Despite the fact that these molluscs are known for their slowness, they still move much faster than tardigrades (whose speed is limited to 23 millimeters per hour) and are able to cross obstacles that are insurmountable for them.
Previously, it was shown that tardigrades swallowed by snails can theoretically settle with droppings.
However, Ksiyonzhkevich and Roszkowska were interested in cases when tardigrades travel on snails on horseback.
The researchers focused on tardigrades Milnesium inceptum and wood snails (Cepaea nemoralis), which are found in the same habitats.
They collected representatives of both species and transferred them to the laboratory.
First, the authors tested whether snails could, in principle, carry tardigrades.
To do this, tardigrades were placed in a drop of water in a Petri dish with a smooth bottom and the snails were allowed to crawl over this drop.
Three tardigrades managed to attach themselves to the mollusks and ride them several centimeters.
Nevertheless, when the experiment was repeated using cups with an emery-treated bottom, the tardigrades were no longer found on snails.
Probably the reason is that the tardigrades held on tightly to the scratches on the bottom.
At the next stage, the authors tried to experimentally evaluate the role of snails in the distribution of tardigrades.
To do this, they attached a square silicone frame with a side of three centimeters and a height of half a centimeter to the bottom of a plastic box scratched with emery.
2.5 milliliters of water was poured inside the frame, and 7.5 milliliters of water was poured onto the bottom around it.
Ten tardigrades at a time were then placed inside the frame and tested to see if they could get out.
In total, Ksienzhkevich and Roszkowska conducted 90 tests.
In 30 control experiments, the tardigrades had to leave the frame on their own.
In the 60 remaining tests, a wood snail was planted in a box outside the frame, and in half of the cases a piece of moss was placed inside the frame.
After 72 hours, the researchers counted the number of tardigrades inside and outside the frame.
None of the tardigrades from the control group managed to get out of the frame.
However, in the boxes where the snails were present, the researchers found live and dead tardigrades all over the bottom.
(Zofia Książkiewicz and Milena Roszkowska)
Apparently, they crossed the borders of the frame on snails.
At the same time, the authors note that the presence of moss reduces the effectiveness of this method of settling.
If we take into account only live tardigrades, then there is no significant difference between tests without snails and tests with snails and moss.
This is due to the fact that only a few tardigrades stay on the surface of wet moss, where they can meet the snail.
In contrast, in the moss-free experiments, all tardigrades were on an open plastic surface and could come into contact with the mollusk.
The researchers suggest that settling on snails is much more effective than using wind or water.
The fact is that snails, like tardigrades, prefer high humidity and therefore are able to deliver their passengers to habitats with ideal conditions for them.
Given that tardigrades themselves travel very limited distances, even traveling a few tens of centimeters can have a positive impact on their genetic diversity.
Ksienzhkiewicz and Roszkowska also admit that in nature snails carry not only adults, but also their eggs or egg-filled exuvia skins.
However, further studies are needed to confirm this idea.
In an additional experiment, the duo placed tardigrades in Petri dishes with a scratched bottom and dehydrated them, causing them to fall into a state of anhydria.
In total, they used sixteen cups of five individuals each.
After seven days, one wood snail was launched into the cups and allowed to crawl freely for a minute.
After that, the authors checked the condition of the tardigrades, and a day later they added water to the cups.
The researchers were interested in how the mucus of the mollusks affects the ability of tardigrades to return to an active state.
The authors found that tardigrades begin to recover from anhydria immediately after contact with mucus.
However, their return to full activity after subsequent treatment with water was longer than usual.
In addition, after rehydration, only 34% of the individuals in contact with the mucus survived, although this figure is normally 98%.
Probably, the mucus dried out too quickly, so that after contact with it, the tardigrades did not have time to return to a state of anhydria.
Thus, contact with snails can be both beneficial and detrimental to tardigrades.
Source:
- Scientific Reports: https://www.nature.com/articles/s41598-022-08265-2
- Live Science: https://www.livescience.com/tardigrades-hitchhike-on-snails