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this is my first message here. Let me tell you why I came to this stackexchange:

A couple of years ago, I took an empty transparent jar (5 liters) and put in it some water (taken from a pond), some dirt and some randomly chosen acquarium-plant. And waited. It was (still is) an amazing experience! The jar went through a number of phases but is still alive, very green, surviving well without any maintainance for more than 2 years (maybe I have added 1-2 liters during this period of time, due to some very low evaporation).

Recently, a friend gave me as a gift a small 15liter acquarium and I decided to do another experiment of this kind. The larger acquarium (compared to the 5L jar) will also give me the possibility to observe better the environment and it's evolution.

Now 2-3 months have passed. The water is still quite yellow-ish, but from previous experience with the jar this has to be expected for the first year or so. [the jar became transparent/green from yellow all of a sudden after about 1 year of life]

However the jar is full of life, the plants are doing fine and there are a lot of little creatures.

QUESTION: could you help me identify one of these creatures? It is a very very small one and I'd like to know more about it. In this video you can see a number of them going around a small piece of cotton that fall down in the acquarium yesterday.

LINK: https://www.dropbox.com/s/jrkoa5kswghiizm/Video%2011-01-2018%2012%2037%2055.mov?dl=0

thanks for any suggestion!

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  • $\begingroup$ Those are probably Ostracoda, commonly known as "seed shrimp" a small freshwater crustacean. $\endgroup$
    – Alejo
    Feb 12, 2018 at 7:21

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The creatures in the video do indeed show the shape, size, coloration, and typical behavior of ostracods ("seed shrimp"), a type of freshwater crustacean (class Ostracoda) that is distributed worldwide (Martens et al. 2008). They lay eggs either on plants or directly in the water as plankton. Some researchers consider them to be potentially useful as bioindicators (e.g. Ruiz et al 2013), or even keystone species (Lawrence 2002). You can find many photos of ostracods on the Internet.

Lawrence, J. et al. 2002. Microb. Ecol. 44: 199
Ruiz, F. et al. 2013. Int. J. Environ. Sci. Technol. 10: 1115
Martens et al. 2008. Hydrobiologia 95:185
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