Some five years ago, I got involved in fermenting coconut water and overripe bananas to make vinegar. If you make your own vinegar, and prior to pasteurisation, you’d have these free-living (non-parasitic) nematodes swimming near the surface of the vinegar. I took a video under the microscope.
Popularly known as vinegar eels, Turbatrix aceti feed on the microbial culture of vinegar and other similarly fermented foods. They are non-parasitic and harmless. After several hours, the vinegar on the slide began to dry up and this happened to the Turbatrix aceti.
Vinegar eels are also harvested and fed to fish fry. If you are interested in using vinegar eels this way, there’s good information at Living Food Cultures.