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Are tardigrades able to smell?

External contribution by Marien-Gymnasium, Werl, Germany

Annotation: We are glad to present a third-party experiment here once more. The authors would like to emphasize that the tardigrades were not maltreated in order to perform the experiments.
The copyright owners of the text and the images are the pupils Clara, Emely und Alissa and their teacher Marco Hagedorn from Marien-Gymnasium, Werl, Germany.

Mosses are by no means the only rewarding tardigrade habitat. When looking for tardigrades you might as well have a closer look at litter. Within the framework of the German year 2017 "Jugend forscht" competition it could be demonstrated that the tardigrades are able to migrate from soil into litter. And we think that they are following this pathway in order to look out for nutrition. Possibly they feed on tiny fungi on the leaf surfaces.
Because of this migration the pupils Clara, Emely and Alissa wondered whether the tardigrades might be able to smell the litter when coming from the soil, possibly following the smell or, alternatively, whether the direction of the migration might just be a coincidence.

There is a statement in the July 2009 Water Bear web base magazine indicating that the tardigrade ability to smell might be simply taken for granted: "In spite of its tiny size a tardigrade is able to see, to smell, to run, to crawl and to kiss." (http://www.baertierchen.de/wb_jul09.html). We wanted to check this out by means of the two following experiments.

1. The filter paper experiment

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Fig. 1: Filter paper was used for this experiment in order to create some kind of artifical litter. One liter of leaf juice was used to wet the filter papers on the left side whereas the filter papers on the right side were merely treated with pure water.

For this experiment we used 16 filter papers (two groups of 8) and fixed them to the soil by means of toothpicks.The first group was soaked with leaf litter water, the second group with pure water. The leaf litter water was prepared as follows: a 5 liter container was filled with litter and two liters of water were added. This mixture was stirred for 5 minutes taking care that the leaves were actually quenched. After those 5 minutes the leaves were removed and the mixture poured through a 20 µm sieve (this mesh size will make sure that no live tardigrades are transported into the solution).

After three days the filter papers were screened for tardigrades. For this sake the filter papers were flushed with pure water and the tardigrades transferred to Petri dishes for the final evaluation (counting) under the dissecting microscope.

Four redundant experiments were performed and 63 tardigrades found. The distribution among the two groups of filter paper was as follows:

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Diagram 1: Percentages of tardigrades on pure water treated filters (left, marked as "mit Leitungswasser") and leaf juice filters (right, marked as "mit Laubwasser").

2. The Agar experiment

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Fig. 2: Agar plates for experiment 2

For the Agar experiment one Petri dish is filled with 2% Agar solution (in water). For this sake 1 g of Agar is mixed with 50 ml of water. The mixture has to be boiled for a short moment in order to become homogeneous. A second Petri dish is prepared in the same manner, but this time with leaf litter water instead of pure water (same as in experiment 1). The two Petri dish preparations are then cooled down to room temperature. Now a "combination plate" with two halves of Agar can be built (cf. fig. 2, on the very right side). In case of a gap between those halves one can apply additional liquid Agar solution in order to properly bridge between the two areas. Now as many tardigrades as possible are collected from litter (we had about ten in each experiment) and then pipetted to the center of the split Agar preparation. We waited until the next day and performed an evaluation (we had made sure in preliminary tests that the tardigrades are in fact able to move on the Agar surface area). A previous "Jugend forscht" experiment in the year 2014 had revealed that the tardigrades are able to move with a speed of up to 74 cm/h. So there should be enough time for the tardigrades - at least in theory - to walk along on the 5 cm diameter Petri dish.

Overall 7 experiments with 54 tardigrades were performed - see our results in diagram 2:

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Diagram 2: Percentage of tardigrades on the pure water Agar side (left) and the leaf litter water Agar side (right).


In both experiments (filter paper and Agar experiment) the tardigrades were evenly distributed among the two different areas. Of course, one interpretation of the outcome might be that the tardigrades are not able to smell at all and that the quotation mentioned above was falsified by the experiments.
But one should be careful when jumping to quick conclusions. Possibly our leaf litter juice wasn't as attractive as thought, possibly soaked with too little or too much leaf litter juice.
We were wondering too whether we should check the other statement about tardigrade kisses as well. But perhaps we should better be discrete and respect their privateness instead?

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
The Water Bear web base is a licensed and revised version of the German language monthly magazine  Bärtierchen-Journal . Style and grammar amendments by native speakers are warmly welcomed.

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