You need a unique sample for your teaching lesson? Have a look into your rain gutter.

It’s real life: Sometimes the good things are closer than expected. Your rain gutter not only disposes the water from your roof, but also collects detritus from its colonizers such as mosses and lichens. It’s an extreme biotope: wet and cold in winter, even freezing, dry and hot in summertime. Only few microorganisms survive this extreme change.

One of them just recently came into spotlight. In April 2019 the Israeli space probe Beresheet* crashed onto the surface of the moon; 585kg of weight caused a small caldera. Aboard there were several thousands of Tardigrada, also known like Water Bears. As they had been transported in dried condition, and apparently no water seems to be available on the moon by fog or rain, the probability of life on the moon in 2020 still is quite low.

No need to travel so far to meet this astonishing group of animals. Take a sample of detritus from your rain gutter, if dry, first let it soak in rainwater. After a few days at room temperature it is time to check the water sample. Tardigrada may be fixed on the surface of moss leaves, so it makes sense to place a moss stem on the glass slide and to strip off the leaves by using two tweezers, one fixing the stem, one stripping off the leaves against the direction of growth.

A dead Tardigrada: Sectoral structure of body clearly visible.

About Tardigrada...

Water bears are easily recognized by their body shape: mostly less than 1mm long, with 8 legs and retractable claws at the end. Their movements look clumsy and gave reason for the naming (Latin tardus - slowly; gradus - step).

Their survival strategy is cryptobiosis, an extreme reduction of metabolic processes in case of dehydration, low oxygen level and low temperatures. Literature tells that Tardigrada survive at a temperature of -243°C. It seems that a European winter should be no problem for them.

Tardigrada do have a significant 3-dimensionality. 

The discovery of cryptobiosis in the 19th century lead to a general discussion about the “criteria of life”: Defined body shape, movement, metabolism, irritability, reproduction, growth/development. A topic definitely worth to be discussed in any biological education.

Tardigrada are eutelic organisms, characterized by a defined and fixed number of somatic cells. It means that growth of the organism is not based on multiplication of cells by cell divisions, but on the physical growth of each single cell.

Moving specimen can hardly be captured with one shot.

There are much more facts which deserve astonishment and give reason to take the Tardigrada as a subject for at least one biology lesson. Following the zeitgeist of our era, most Tardigrada are vegetarians. Good to know that a trip to the crash site of the Beresheet* space probe on the moon is not necessary to get a nice sample.

*In Hebrew Beresheet means Genesis; in reference to the Bible.  Furthermore, "STAR TREK 2: The Wrath of Khan" (1982), a Genesis projectile was sent out to create life on a dead planet.