Culturing Microworms for Fry

How to culture microworms (Panagrellus redivivus), a small nematode that bridges infusoria and baby brine shrimp as a live food for growing fish fry, with medium, timing and harvest.

What microworms are

Microworms are the small free-living nematode Panagrellus redivivus (also called the sour paste or beer mat nematode). They are about 50 micrometres in diameter and just over 1 mm long, barely visible to the naked eye. They are widely used in aquaculture and the hobby as a live food for fish fry and larvae, and serve as a useful step between tiny infusoria and the larger baby brine shrimp.

Starting a culture

Microworms are easily cultured at home on a cereal medium. A common method is to spread a thin layer of cooked oatmeal, cornmeal or a flour-paste porridge in a shallow lidded container, sprinkle a little dry yeast on top, and inoculate with a microworm starter culture. Kept at room temperature, around 20 to 25 deg C, the worms multiply quickly: the females mature in about three days and deliver live young rather than laying eggs.

Harvesting and feeding

Within a few days, masses of worms climb the container walls above the medium, where they can be wiped off with a finger or cotton bud, rinsed in a little water, and added to the fry tank. Microworms survive for some time in the water, giving fry the chance to find and eat them. Feed only what the fry will consume, since uneaten worms eventually die and foul the water.

Maintaining the culture

A culture stays productive for roughly one to two weeks before the medium sours and smells; at that point it should be refreshed by starting a new container from a small amount of the old culture. Because cultures can crash or become contaminated, it is wise to keep at least one backup culture going at all times.

Related cultures

Several similar nematode and small-worm cultures are used the same way and on similar media, including Walter worms and banana worms (close relatives of microworms) and vinegar eels (Turbatrix aceti), which are cultured in diluted vinegar and stay suspended in the water column, making them useful for fry that feed away from the bottom.