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Disappointments, empty nets and the polychaeta

Most people tend to avoid talking about failure - us included. Everything had looked good so far back in summer 2017: we found a cosy hotel close to Rovinj (Croatia), fairly easy to reach from our hometown.

Previous journeys to Croatia had proven that it is not tremendously difficult to spot marine tardigrades in the Adriatic Sea: at almost at any place where we had collected sand samples we had found maritime tardigrades. Once collected in analog era 35 mm (slide) film containers the samples could easily be transported back home and screened during the months to follow. But not his time - in Rovinj.

[ View from our hotel in Rovinj, Croatia ]

Fig. 1: View from our hotel balcony towards the Adriatic Sea. In the evening it was possible to visually enjoy the dolphins - just by means of a modest spotting scope. We thought that the tardigrade collection would be just routine, working automatically, like in the years before.

In the evenings we took part in several fascinating boat excursions.

[ Dolphin in the Northern Adriatic Sea, near Rovinj, Croatia ]

Fig. 2: No, this is not an oxidized sports' trophy but a dolphin enjoying life in the vicinity of our hotel. The photograph was taken by us during a dolphin spotting excursion, from a small boat.

With a deja-vue feeling we collected several sea water/sea sand samples, taken from shallow depths, between 1 and 2 m. A final check through the low-mag stereo microsope showed the appropriate mixture of medium sized sand grains with some shell gravel. Besides nematodes of various sizes, ciliates amongst sea urchin spine fragments, elastic flatworms, a few diatoms, foraminifera, tiny snails, mites etc. Exactly the mixture where we do expect marine tardigrades as well. Just as an example have a look at one of the ciliates, a perfectly green one:

[ Green ciliate from the Adriatic Sea ]

Fig. 3: Green marine ciliate from the Adriatic see. It looks similar like a freshwater Paramecium bursaria. A few tenths of millimetres long.

Back at home, as we thought, we were going to find the tardigrades, few in number, and viciously hidden, as usual. But alas, every time we saw a shivering sand grain, the animal behind was a so-called polchaete - i.e., a bristle bearing worm with a multiply segmented body. Its movement characteristics are similar to those of the tardigrades.

[ Polychaete, from the Adriatic Sea ]

Fig. 4: Polychaete from the Adriatic Sea near Rovinj. These tiny animals are crawling among the sand grains, in a similar way as the marine tardigrades do. Body length of the individual shown here: a little less than 0.5 mm.

We did come across polychaeta with many more segments but we will not show any of them here: just remember that our focus is on tardigrades ... And, funny enough, we feel that a website dedicated to the polychaeta wouldn't be as popular as one about tardigrades. Quite normal, human animal racism. But the polychaeta are many and as far as we know they cause no harm to humans at all.

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Video clip: Polychaeta from the Adriatic Sea near Rovinj. When looking closer the difference between the polychaeta and the tardigrades becomes obvious, beginning with the "duplicate" eye spots, furthermore the position of the head appendices, and of course lots of bristles (in positions where the tardigrades have claws or toes instead).

We would like to wish you all the best for the upcoming year 2020 and hope that your tardigrade "nets" will not end up as empty as the ones reported here!

© 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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