Microworms (Panagrellus redivivus ...or are they P. silusiae?). More tasty nematodes, this time living on yeast and bacteria in a thick oatmeal batter or just on a round of bread that you've seeded with yeast and moistened. Microworms are only a little larger than Turbatrix; they can be handled by fish fry still too small to manage brine shrimp nauplii. They tend to sink, which makes them perfect for bottom-hugging fry, and they survive for about 12 hours in the aquarium, which helps you to offer a continuous buffet.

Microworms were first isolated in Norway, before World War II. Microworms are closely related to that most famous nematode of all, Cænorhabditis elegans, the first animal to get its complete genome mapped. A few males are produced, but most microworms are self-fertile females. The young are born live and are ready to reproduce in a couple of days. Consequently, microworms multiply almost as fast as bacteria. Individual nematodes have a life-span of a couple of weeks. Mistakenly, I used to culture them in the refrigerator, which was extremely trying for my housemates. But they don't need coolness, McDaphnia pointed out to me, and yeast thrives at summer temperatures up to 100^oF, which microworms can handle. At temperatures over 80ºF, the culture reproduces faster but gives out and goes foul quicker. I lost my microworm cultures in the heatwaves of July and August 2002.

Culture. Low screw-top babyfood glass jars make good microworm cultures. So do empty covered margarine dishes. But I prefer even lower Petri-dish sized clear plastic containers you can find at the local Hobby Hovel. You'll often be warned to use a ventilated top, but as long as you're opening the jars every couple of days, their oxygen supply won't give out. Don't overdo it at first with the yeast, which competes for oxygen until the microworms get going. When carbon dioxide levels rise, microworm metabolism seems to slow. The humidity encourages the worms to climb the clean container sides. A half-inch layer of cooked oatmeal or pablum or a round of stale (but not moldy) white bread, cut out with the jar and pressed to the bottom, moistened amd lightly seeded with dry yeast (such as Fleischmann's baker's yeast), and a teaspoonful of old culture that has at least some live microworms in it, will start the culture going. If microworms are swarming on the lid of a going culture, you can make a clean new culture just by switching lids. Don't make the medium too deep; microworms need some oxygen, and the deeper anoxic medium will develop bacterial fermentation, producing alcohol (toxic to the nematodes) and stink. Yeast remains viable longer in the refrigerator as long as it's kept dry (but don't freeze it). Don't make the oatmeal too soupy, either; the culture will liquify it with metabolic water as they go.

If mold is a problem try adding one third white vinegar to the water, and re-culture using clean worms from the lid. Or you could try a pinch of salt. Three Israeli scientists experimenting for the Ministry of Agriculture to discover the optimum growth rates of P. redivivus for use in aquaculture, found that "rearing nematodes in culture media of various salinity levels can render them more suitable for fish larvae that require similar salinity levels for optimal growth. Nematodes were reared at salinity levels ranging from 0 to 36 ppt, and a level of 18 ppt (roughly half the salinity of seawater) was found to be optimal for nematode growth. At 36 ppt growth was substantially reduced. A brackish rearing medium was also found to be more effective for the elimination of contamination in nematode culture. This was fresh to me. I've never met an aquarist who added a pinch of marine salt to water that was going to be used for setting up a microworm culture.

Microworms can switch to an alternative metabolism when oxygen levels get too low to be useful. But they won't multiply as fast, and other anaerobic organisms might overtake the culture. Keep it aerated. Within a couple of days or so at the warm temperatures of a fish-room cupboard, the yeast is working the oatmeal, and yeast-fed microworms are swarming on the clean sides of the jars. Hold the culture so that it reflects light, and you should see the surface seething. Scoop them off with a wet Q-tip or a wetted watercolor brush (my preferred tool) or a razor blade or your fingertip or one of those little rubber spatulas that get the last smidge of mayonnaise (better mark it "WORMS" though), and swish it in the tankwater (you don't want to get that yeasty culture in the aquarium, eh). If the jar is low enough, microworms will even swarm onto the lid. With about a teaspoon of water swilled round inside the lid, you can pour concentrated clean microworms right into the fry tank.

A good microworm culture has a funky yeasty odor you'll learn to recognize. When the culture is getting thinner than a milkshake or developing a powdery fungal crust, or if it's smelling sour and foul, or when you notice there are fewer nematodes on the glass, it's time to reculture from the best current culture. I wash out the culture jars and disinfect them with a teaspoonful of full-strength Clorox shaken inside the closed jar, which I rinse well and leave to air dry. I keep several cultures going, so that I have a back-up in case of a crash. Figure a culture will last ten days or a couple of weeks, but not as long as a month. Even if the old culture is really nasty, some microworms will usually survive on the surface. I've been known to reculture from really past-it microworm culture by dropping a shred of paper towel on the surface and plucking it up with tweezers, to lay it onto the new culture.

Oliver Hartwig asked "Microworms: the perfect live food?" in Aquarium Fish, Dec. 1990.

Mike Edwardes has some microworm culturing technique among his "Foods for Fry" at his website

Compare Adrian Tappin's somewhat different microworm culturing techniques at his "Home of the Rainbowfish" site: He's less fastidious than you might be about housefly maggots in his microworm culture (a welcome food for larger fishes). Tappin gives a nutritional profile for microworms that I'd never seen before.

Dan Carson's unusual method for culturing microworms with barely moistened yellow cornmeal and yeast in covered "server-saver" refrigerator dishes is meticulously described in his article, originally for the Honolulu Aquarium Society, archived at www.geocities.com/Heartland/Meadows/2948/micro.html

And the University of Florida Cooperative Extension Service's extensive library has a good microworm culture article.