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Micro aquarium population explosion!

In north-eastern Germany, at the southern borderline of the Baltic Sea there is a national park called "Nationalpark Vorpommersche Boddenlandschaft". When looking at Google Maps using the cooordinates 54.471098, 12.526624 (just copy-paste and zoom out a little bit) you will see the small town of Prerow on an island extending its "nose" northwards into the Baltic Sea. Exactly at the marked position, in shallow Baltic Sea water we took one of our usual sand samples. Actually we felt that it didn't appear to be a nice tardigrade habitat: when stirring the sand surface area a little bit it strongly smelled like hydrogen sulfide. Nevertheless we took the sample home to Munich. A casual inspection under the microscope revealed lots of really tiny snail shells most of which were empty. No wonder, the hydrogen sulfide ...
Afterwards we placed the sample on a window sill and forgot about it. Only when cleaning up a few months later we noticed that the hydrogen sulfide smell had actually disappeared.

[ micro aquarium with sand and sea water from the Baltic Sea ]

View into the micro aquarium (a preserving jar with about 7 cm diameter) from top. Shells and snail shells, and some sand.

So, before disposing of the contents we decided to look into it once more. A good idea. Actually the sand was teeming with maritime tardigrades!

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High population density of maritime tardigrades within the micro aquarium. The sand grains were swirling around, under the dissecting microscope the situation looked similar to a modestly boiling water. Video (~ 1 MB).

Then an extremely hot summer followed. The temperature in the micro aquarium rose to 23 °C.

[ micro aquarium with sand and sea water from the Baltic Sea ]

Baltic Sea water micro aquarium. The thermometer is reading 23°C !

We became nervous and wondered whether our tardigrades would be able to cope with this situation. Actually we had read in an historical article edited by the Naples' Sea Aquarium that cold, oxygen saturated water was really essential for a stable micro aquarium.

[ Aquarium Neapolitanum ]

Zoological Station of Naples, Italy: Aquarium Neapolitanum. Naples 1925 (it contains many interesting and wise suggestions on how to run a micro aquarium).

But the tardigrades didn't care about the theory and continued to live within the tiny preserving jar, without the addition of any food, probably just in equilibrium with some tiny traces of micro algae!

Thanks to the high population density we saw increasing chances to find some of their mysterious eggs, too. In fact the eggs of those maritime tardigrades were always considered as virtually non-existent though it was clear that they were present within the bodies of the females. But nobody was able to trace them once they had been deposited among the sand grains. Even the famous tardiologist Prof. Ernst Marcus noted, slightly frustrated:

"Though we have seen [...] the Ductus deferentes filled with spermatozoa and though mature females were seen in great number, we were not able to find out anything about mating and egg deposition. We assume that the eggs must be deposited freely as we were not able to find a single cuticula ..."

Learn more about this topic in our next issue.


Ernst Marcus: Zur Anatomie und Ökologie mariner Tardigraden. Zoologische Jahrbücher / Abteilung für Systematik, Ökologie und Geographie der Tiere, 53 (1927) p. 537.

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
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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