Microworms: nutritious nematodes for fry
Microworms (Panagrellus redivivus ...or are they P. silusioides?). More tasty nematodes, this time living on yeast and bacteria in a thick oatmeal batter, or a loose wheat flour paste or just on a round of bread that you've seeded with yeast and moistened (with less dependable results). Grits, cornmeal, instant potato flakes all seem to work for the necessary starch base. Dried baker's yeast sprinkled from the packet supply the yeast.
Microworms are easy and tolerant. They are about a millimeter long, only a little larger than Turbatrix and equally threadlike; they can be handled by fish fry with a very small gape, ones still too small to manage brine shrimp nauplii or immature Grindal worms that you've sieved through a daphnia net or just selected by waiting a few moments for the heavier, larger worms to drop to the bottom of the rinsewater. They provide a concentrated nutritional package that's even more easily cultured than Artemia. Though sometimes aquarists debate their nutritional value, an FAO manual finds that they have a nutrition profile comparable to newly-hatched Artemia. Panagrellus are about 76% water and 24% dry matter, of which about 40% is protein and 20% fat, Dr Jesse Chappell of Auburn State University tells us in his culturing notes.
Even full-grown small tetras and rasboras will dash about after them. Microworms tend to sink, which makes them perfect for bottom-hugging fry, and they survive for about 12 hours in the aquarium, longer at cooler temperatures, which helps you to offer a continuous buffet: watch guppy fry go for them in a YouTube video.
Though there are twelve species of Panagrellus, found in insect frass or spoiled cider, or the bookbinder's vinegary paste where Carolus Linnaeus found them in 1767, the aquarist's microworms were first isolated in Norway, before World War II, and formally described in 1945. 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, kept at room temperature, are ready to reproduce in a couple of days. They produce a dozen or three every day or so, and their individual life span is two or three weeks. Consequently, microworms multiply almost as fast as bacteria.
Culture. 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°F, which microworms can handle. At temperatures over 80°F, the culture reproduces faster but gives out and goes foul quicker. I lost my microworm cultures in the heatwaves of July and August 2002 and again in 2010. When they are stored just above freezing, their metabolism slows enough that a starter culture can be held for months. Now, cool and secure within the smallest size of styrofoam beer cooler, and insulated with crumpled newspaper from contacting a frozen cold pack I've stashed in there above them, my microworms doze through the city's nastiest heat spells.
Low screw-top babyfood glass jars make good microworm culture containers. So do empty covered plastic margarine dishes. But I prefer even lower Petri-dish sized clear plastic containers that you can find at the local Hobby Hovel. You'll often be warned to use a ventilated top, or drill yours with minute holes, but if the holes are small, masses of microworms may clog them anyway; as long as you're opening the jars every couple of days, their oxygen supply won't give out. 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 moderately aerated.
A half-inch layer of cooked oatmeal or pablum or a round of stale (but not moldy) white bread, cut out with the inverted jar and pressed to the bottom, moistened and the surface 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 by rinsing microworms off the lid in a teaspoonful of water, swished round it, or even just by switching lids. Rock the seeded culture a little in your hand, or use the bowl of your plastic picnic spoon to spread the worms thinly over as much of the fresh medium's surface as possible: all the action is in the uppermost couple of millimeters, and a surface teeming with microworms doesn't let mold get a purchase. Don't make the medium too deep; microworms need some oxygen, and the deeper anoxic layers of 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).
The oatmeal should be thick but stirrable; don't make the oatmeal too soupy, either; the culture will liquify it with metabolic water as they go. A good microworm culture has a funky yeasty odor you'll learn to recognize. A gentle stir occasionally with the plastic picnic knife in your fish cupboard may extend the productive life of the culture ...or it may just cross-contaminate the cultures. I tend to leave them alone.
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 of the closed container encourages the worms to climb the clean container sides.
Writing the date on a stick-on label helps you keep track of how long your cultures are lasting, and which are the oldest ones that most need renewing.
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 glistening surface shimmering and seething, as in this YouTube video. Scoop them off clean sides or top with a wet Q-tip or a wetted aquarium-only 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, like my Petri dishes, 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. Or you can take them up in an eyedropper and closely control how much you are feeding.
Keeping a jump ahead of mold. Cooking the pabulum first eliminates mold spores. If mold is a problem for you as it is for me, begin by sterilizing your scrubbed culture jars with full-strength Clorox swished all round the closed jar and left for an hour, then thoroughly rinsed and let dry, and rinsed out again just before use.
For a medium less friendly to mold, try adding a capful of white vinegar to a half cup of the cooked pabulum; the worms acidify the culture as they go, and the vinegar seems to give them a running start. And 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'd 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. So now I drop the pH with cider vinegar and add a pinch of salt when I prepare the culture medium and I re-culture using clean worms from the lid of the best current culture; this is keeping mold at bay.
Reculturing. When the culture is darkening or getting thinner than a milkshake or developing a powdery fungal crust, or if it's just smelling sour and foul, or when you notice there are fewer nematodes up 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 back-ups in case of a crash. I make two identically prepared cultures at a go, and still always one develops better than the other, reinforcing my amateur status. I used to figure a good, mold-free culture will last ten days or a couple of weeks, but not usually as long as a month; now with salt and vinegar they go a full six weeks. 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.
Starters of microworms are usually available for a few dollars at your local fish club's auction meeting. You might take along a burgeoning culture of your own and contribute it, just to make sure everyone is supplied. Or buy one through AquaBid's "Live Aquarium Food" section, at a fixed "Buy Now" price.
Links. Oliver Hartwig asked "Microworms: the perfect live food?" archived from Aquarium Fish, Dec. 1990 at Fish Channel. Jim Atchinson's article "Microworms" is very detailed. There are lots of microworm tips and good photos in the lively and wide-ranging Aqualand (Des Moines, IA) website: "Keep your baby fish alive with microworms".
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.
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, "Food for fry: raising microworms", originally written for the Honolulu Aquarium Society. Wayne Schmidt offers a method for harvesting microworms by the spoonful: that's a lot of microworms.
And R. W. Rottmann, "Microworm culture for aquarium fish producers", archived in the University of Florida Cooperative Extension Service's extensive library, tells how it's done on a really large scale.