Microworms are nematodes of the species Panagrellus redivivus and are called microworms because to most people, that is a perfect summary – very small worms. Microworms are described as free-living (i.e. non-parasitic) nematodes which are ovoviviparous and have a very fast reproduction rate. The generation time of a microworm culture is as low as 5 to 6 days and each egg sac contains as many as 80+ larva1.
They measure approximately 0.05mm wide by 1.5mm long which is one of the reasons they are so popular with fish keepers around the world. Their extremely small size means they are readily eaten by the babies of a wide variety of fish. Also, the nutrient composition and amino acid profile of microworms is very similar to that of freshly hatched baby brine shrimp.
Some aquarists and fish breeders stand by baby brine shrimp as the best food for fish fry, but studies have shown that baby fish fed a diet high in microworms had a final mean body weight and total length that were similar to fish fed a diet that was high in BBS1.
Microworms have many qualities that make them more appealing than brine shrimp as food for small-mouthed fish. The price of BBS can vary widely on a yearly basis because of the quality and supply of harvests in the few areas of North America where they can be collected2. BBS can be very difficult to hatch for new fish keepers as the hatch rate is very dependent upon many factors, not all of which are fully understood. Finally, BBS are too large for the mouths of many baby fish so may not easily be eaten until the fish are a few days to a few weeks old.
Microworms excel where BBS fail. Microworm cultures do not rely on annual harvesting; they can be grown in your own home, constantly throughout the year. Microworms are, therefore, very cheap when compared to BBS. Beginners find that microworms are extremely easy to culture as all that is needed are a few items you will already have at home. The small diameter of P. redivivus make them much easier for baby fish to consume when compared to the much larger BBS.
Microworms can be grown in solid or liquid cultures and need constant low temperatures (room temperature is fine) and a sufficient oxygen supply. Under maximal conditions, as high as 130 million microworms per pound of culture can be harvested2! Now, just imagine how much this amount of microworms will benefit the health of your baby fish!
1. Santiago et al. 2003. J. Appl. Ichthyol. 239-243.
2. Ricci et al. 2003. Appl Microbiol Biotech. 60:556-559.