Do you know the water fleas? You know they live in biological pools? But no one has ever been stung because these small aquatic crustaceans have no organ to sting.
There are about 650 species described in the world and the vast majority have sizes between 0.2 and 6 mm. They live especially in freshwater habitats and occupy the water column in huge numbers. As their bodies are almost transparent, even in very transparent water they are difficult to detect.
And they are themselves responsible for the water to be so transparent. Through their antennas they use for catching food, constantly cleaning the water. Each individual can filter out about 30 milliliters per day. It looks like little, but 1.000 animals already clean 30 liters a day. And per cubic meter live about 6.000 individuals (value detected in biological pools of public use in Germany). In other words, a biological pool of medium size can host up to 1,2 millions! What is especially important, many species of water fleas feed on bacteria. For example, from the species Daphnia magna it is known that daily can ingest up to 5.6 million cells of the bacterium Escherichia coli.
We are so happy to swim together with thousands of water fleas because they are working as true hygienic police.