Microworms are well known to be an excellent food source for first feeding fish larvae. The species most commonly cultured in the aquarium hobby is believed to be Panagrellus redivivus, a member of the nematode family, Panagrolaimidae. The Panagrolaimidae family include opportunistic bacterial-feeders and some notable specialists of fermenting liquids or decomposing wood. I suspect, however, that there are probably a number of different nematode species being cultured in the aquarium hobby as "microworms".
The first Panagrellus species to be described is currently known as Panagrellus redivivus. It was described by Linnaeus (1767) as Chaos redivivus. This species was more commonly known as the 'sour paste nematode' in reference to its isolation from book-binding glue, or as described by Linnaeus in 1767, "habitat in aceto and glutine bibliopegorum". The generic name Panagrellus was not established until 1938, when Gerald R. Thorne described a nematode isolated from wounds of a cottonwood tree in Utah. Based on observation of new diagnostic morphological features, Thorne erected the genus Panagrellus, describing Panagrellus pycnus, as a new species. Controversy over the acceptance of the generic name Panagrellus over Chaos prevailed for many years. However, because Chaos redivivus had a rather vague description and no known type specimens, the name Panagrellus was eventually accepted based on modern taxonomic standards.
Panagrellus has a worldwide distribution, with species described from almost every continent except Australia and Antarctica. Currently 12 species are recognised, with P. pycnus as the type species and eleven other named species: P. ludwigi, P. nepenthicola, P. silusioides, P. redivivus, P. redivivoides, P. ventrodentatus, P. dorsibidentatus, P. dubius, P. filiformis, P. ceylonensis and P. leperisini. These species have been delimited and described using Linnaean or phenetic species concepts, based on morphological or morphometric data. Only the morphology of the spicules and occasionally the structure of the vulva in females are useful for diagnosis and identification of species. This lack of distinctive morphological features makes diagnosis of Panagrellus species rather problematic (Stock & Nadler, 2006).
Panagrellus redivivus is a bacteriophagous, ovoviviparous, free-living soil nematode, being one of the few that do not lay eggs, but hatch juveniles internally. It has four larval stages before becoming adults. The first larval stage is intrauterine, but the remaining stages are free-living. It is gonochoristic, producing equal numbers of males and females (Stock & Nadler, 2006). Bacteriophagous nematodes are already known as a potential food source for fish larvae. Panagrellus redivivus is a nematode which is easy to rear in large quantities in culture. The worms feed on bacteria which are precultured. They have a short life cycle and a high fecundity. Panagrellus redivivus are a tiny nematode about 0.5 to 2.0 mm in length and 0.05 mm in diameter. They reproduce sexually and are livebearers; releasing 10~40 young every 5~7 days for a 26~36 day life span. The young reach sexually maturity in approximately three days. Their size increases by three times during the first day and five to six times during the next three days.
Microworms have been cultured by aquarists since the early 1930's as a live food for a variety of fish species. Their small size and ease of culture has received renewed attention in recent years with rising costs and declining hatch rates of brineshrimp eggs sold in the aquarium hobby. Microworm has as good if not better nutritional profile to that of brineshrimp, containing 48% protein, 21% lipids, 7% glycogen, 1% organic acids, and 1% nucleic acids. Approximately 70% of the lipids are fatty acids and the remainder is phospholipids.
Microworms are one of the simplest live foods to culture. When cultured under the right conditions they will multiply in vast numbers. They are a valuable live food and tolerant of environmental variables. Microworms like it warm and a temperature range of 20~25°C is about right. As the temperature begins to rise or fall below this range; their production rate will decline. However, they can maintain their life cycle at temperatures from 5°C up to and including 37°C. They have the added advantage of staying alive for six to eight hours in freshwater, by which time they should all have been eaten.
I have cultured microworm for many years and have tried several different culture mediums; bread soaked in beer, yeast blends, and a host of other foods. There are almost as many different culture methods for microworm as there are aquarists, each having their own successful anecdote. I will outline some of the more successful methods I have used. What you have to do is find one that suits your particular requirements. Starter cultures are available from biological supply companies or fellow hobbyists.
Microworm can be cultured in almost any shallow, flat, watertight container with a snug-fitting lid. This prevents contamination by insects and other bugs, and also prevents the culture from dehydrating. Small holes should be punctured in the lid for air circulation and the containers stored in a well ventilated room. Microworm should be able to be harvested daily for about 28~56 days using the same culture medium. However, it largely depends on the culture medium used. It is a good idea to have at least two cultures running at the same time. Start your second culture about two weeks after the first. You may find that a culture will sometimes rapidly decline in production of worms. Having a second culture in production will ensure that you have worms available at all times.
Culture medium can be prepared from almost any grain flour, yeast, and water. However, research has shown that the type of culture medium used has a dramatic influence on worm yields. One such trial was conducted using three mediums - wheat flour, oatmeal, and cornmeal. Yield of worms in wheat flour was significantly greater than in oatmeal or cornmeal. Production of worms stopped after day 20 in cornmeal, day 33 in oatmeal, and day 53 in wheat flour. The addition of yeast during initial media preparation was found to have no effect on worm yields. However, the addition of yeast on a weekly basis to the wheat flour medium gave a significant greater yield of worms than did untreated wheat flour. The wheat flour was mixed with water to form a smooth paste and placed in the culture container. After inoculation with live worms, the addition of 5 ml of a yeast solution, consisting of 7 gm bakers or brewers yeast (Saccharomyces cerevisiae) dissolved in 70 ml water; was lightly sprayed over the medium every 7 days. The addition of yeast should also inhibit the growth of nematophage fungi.
One method I have used is oatmeal (porridge). Use one part oats with one part of water. Place the mixture into the culture container and spread to a thickness of 15~25 mm and microwave on high setting for three minutes. The mixture is then allowed to cool to room temperature. Any media on the sides of the container should be removed with a damp cloth. After the mixture has cooled, place the starter culture on top of the porridge. Within 3~6 days you should see the surface moving. If you use a magnifying glass, you will observe hundreds of tiny worms.
You can increase the production of worms by sprinkling dry yeast powder over the surface of the mixture. You do not have to add the yeast until after about two weeks, then once a week should be sufficient. If the culture medium becomes very watery, you can add a slice of bread to the container to soak up the moisture. The addition of bread has a similar effect as does the bakers yeast.
Yet another method requires only a slice of white bread and brewers yeast. This culture method produced the best results for me in terms of the number of worms produced. Firstly, cut the crusts off the slice of bread and place it squarely on the bottom of the container. Mix 5 grams of brewers' yeast with ¼ cup of water and pour the mixture onto the bread, making sure that the bread is completely saturated. It is important that there should be very little excess fluid in the container when the container is tilted.
Next add the starter-culture of microworm, by spreading it over the surface of the bread. Replace the container lid securely, and place the container in a warm area. Within three to four days, the culture should be thriving with worms migrating up the side of the container. As the bread is consumed another slice can be added to keep the culture active. After the addition of around 3 or 4 slices of bread the culture will need to be replaced. If the culture becomes too wet, more bread should be added to absorb the excess moisture. Remember the wetter the culture, the lower the production of worms.
During the warmer months of the year I often found another small worm in the culture as well. This is because the odour of the culture will attract the common housefly, which lays its eggs through the small holes of the container lid. The eggs then hatch and the larvae develop and grow on the culture medium. This may seem a little unpleasant to some people, but these worms are ideal for larger rainbowfishes, which love them. Their development doesn't appear to have any detrimental effects on microworm production.
Ricci et al. (2003) developed a system for the mass production of Panagrellus redivivus. It consisted of autoclavable plastic bags filled with sponges soaked with medium. This system enlarges the usable surface inside the growing system by using crumbled polyether polyurethane sponges to create an interstitial space. This space allowed optimal reproduction conditions and served as a living habitat for the nematodes. It also guaranteed sufficient aeration. The bags were inoculated with Saccharomyces cerevisiae to guarantee a monoxenic culture. The system was aerated and kept humid during the 11~13 days of incubation at 25°C.
Worm harvesting is a very simple procedure. Wait until the worms are climbing the container walls and you will be able to collect them by running your finger around the walls. If you find this method a little unpleasant, then you can use a small stiff brush. The worms can then be fed directly to the larvae by swishing your finger or the brush in the aquarium water. Do not dip your finger or brush into the culture medium to collect worms, as any culture media residues should be minimised in order to avoid pollution of the aquarium water. Another method of harvesting is to lay wooden ice-block sticks (or similar objects) on the surface of the culture. The worms will crawl onto the sticks and you can then simply swish the stick in the aquarium water. Yet another method is to use a damp (thick) paper towel cut to fit over about half of the culture surface. To harvest the worms just use a spoon or spatula to gently scrape the worms right off of the paper towel, making sure you don't tear the towel (Wedekind, 2008).
Do not forget uneaten worms will die and pollute the aquarium water, particularly in a small aquarium. If left unattended, it can decimate an entire batch of fish larvae in a matter of hours. To prevent this problem, try feeding the larvae three or four times per day in small amounts rather than one or two large ones. In all, microworm offers a cheap, simple and nutritious food for feeding the larvae of most freshwater fish species. It is also suitable for feeding juveniles and adults of some of the smaller fish species such as Iriatherina werneri and Pseudomugil species.
Microworm can be fed alone or in combination with other foods such as brineshrimp nauplii, rotifers, zooplankton, egg yolk, dry diet, etc. Studies of fish larvae fed microworm are not significantly different from those fed brineshrimp. A feeding program utilising a combination of food items is better able to meet the nutritional requirements of all freshwater fish larvae.
The use of different culture media to produce microworms affects the reproduction of the nematodes as well as their nutritional composition. Studies on enriched media for microworm have shown encouraging results. The nutritional quality of microworm can be enhanced by the use of the direct enrichment technique. Enrichment is simply carried out by adding the product to the culture medium. In one report (de Lara et al. 2007) the microworm were cultured in two media: one with oatmeal and the other with spirulina enriched oatmeal, in 15x15x5 cm plastic containers with 200g oatmeal and 300 millilitres (ml) purified water. Five grams of spirulina was used in the medium. The results show that growth of the microworm population in the spirulina-enriched medium presented the highest abundance of individuals on the second week of culture, whereas the population grown in the oatmeal medium showed the highest abundance on the fifth week of culture but did not reach the number of organisms attained by the population cultured in the spirulina-enriched medium.
The amino acids content of the populations from both media were compared to those reported for brineshrimp fed with spirulina, observing that the amounts were higher for most amino acids in microworm cultured in the spirulina-enriched medium. The composition of fatty acids in the microworm cultures in both media depicted significant differences for the linoleic, arachidonic, and eicosapentaenoic fatty acids, which were found in a higher percentage than reported for microworm cultures in oatmeal supplemented with sunflower oil. This information shows that spirulina accelerates growth of microworm populations and allows the presence of amino and fatty acids.
Rouse et al. (1992) used a culture medium which was fortified with a 10% fish oil emulsion, obtaining nematodes that had significantly higher total lipid content and elevated levels of highly unsaturated fatty acids (HUFA). Additional investigations concerning the effect of an added oil source (fish oil or sunflower oil) on body composition, average yields and multiplication factors of the nematodes were conducted by Schlechtriem et al. (2004). Corn oil and yeast were added in varying combinations to culture media of Panagrellus redivivus to effect a change in growth and fatty acid content. The addition of corn oil or yeast to cultures increased nematode growth over standard media. Combinations of corn oil plus yeast increased nematode growth by 68%.
Literature
de Lara, R., T. Castro, J. Castro and G. Castro (2007) Nematode culture of Panagrellus redivivus (Goodey, 1945) with Spirulina sp. enriched oatmeal. Revista de Biología Marina and Oceanografía 42(1): 29-36.
Focken, U., C. Schlechtriem, M. vonWuthenau, A. Garca-Ortega, A. Puello-Cruz and K.Becker (2006) Panagrellus redivivus mass produced on solid media as live food for Litopenaeus vannamei larvae. Aquaculture Research 37:1429-1436.
Lavens, P; Sorgeloos, P. (eds.) (1996). Manual on the production and use of live food for aquaculture FAO Fisheries Technical Paper. No. 361. Rome, FAO. 295 pp.
Mohney, L. L., Lightner, D. V., Williams, R.R. and Bauerlein, M. (1990). Bioencapsulation of therapeutic quantities of the antibacterial Romet-30 in nauplii of the brine shrimp Artemia and in the nematode Panagrellus redivivus. Journal of the World Aquaculture Society, 21(3): 186-188.
Ricci, M., A. P. Fifi, A. Ragni, C. Schlechtriem and U. Focken (2003) Development of a low cost technology for mass production of the free living nematode Panagrellus redivivus as an alternative live food for first feeding fish larvae. Applied Microbiology and Biotechnology, 60(5):556-559.
Rottmann, R. W. (1988). Microworm Culture for Aquarium Fish Producers. University of Florida, Institute of Food and Agricultural Sciences, USA.
Rouse, D. B., Webster, C.D. and Radwin, I. A. (1992). Enhancement of the fatty acid composition of the nematode Panagrellus redivivus using three different media. Journal of the World Aquaculture Society, 23(1): 89-95.
Schlechtriem, C. (2004) The suitability of the free-living nematode Panagrellus redivivus as alternative live food for first feeding fish larvae. Ph.D. thesis, University of Hohenheim / Shaker Verlag, Aachen.
Schlechtriem, C., D. R. Tocher, J. R. Dick and K. Becker (2004) Incorporation and metabolism of fatty acids by desaturation and elongation in the nematode, Panagrellus redivivus. Nematology, 6(6):783-795.
Stock, S. P. and Nadler, S. A. (2006) Morphological and molecular characterisation of Panagrellus spp. (Cephalobina: Panagrolaimidae): taxonomic status and phylogenetic relationships. Nematology 8(6): 921-938.
Thorne, G. (1937) A Revision of the Nematode Family Cephalobidae Chitwood and Chitwood, 1934. Proceedings of the Helminthology Society, Washington 4: 1-16.
Wedekind, B. (2008) Experiments with Microworms: How to raise and collect them easily.
© Copyright Adrian R. Tappin
Updated December, 2008.
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