Microworms Profile and How to Culture as Live Food

In recent years, there has been a growing interest in life foods to diversify the diet of our aquarium pets. In this regard, microworms (Panagrellus redivivus) can be an interesting option to cultivate them at home.

These microworms are a rich source of nutrients, including proteins, fats, and vitamins, essential for the health of fish. Additionally, they are small in size making them great food for tiny fry and larvae.

In this article, I will provide a comprehensive overview of Panagrellus redivivus, including its life cycle and cultivation methods. Additionally, I will discuss the advantages and disadvantages of using microworms as live food compared to other options.

Interesting fact: Nematodes, the group to which these microworms belong, originated during the Precambrian or Cambrian explosion over 500 million years ago.

Description of Panagrellus Redivivus

photo source

Panagrellus redivivus is a free-living small milk-white worm ranging from 50 to 2,000 μm (l/16″) in length and 50 μm in diameter. It has a smooth and dense cuticle.

Note: “Free-living” lifestyle means that it is not parasitic and not associated with plants or animals.

Microworms are unsegmented and have circular body shape. They move by wriggling due to their longitudinal muscles.

They lack a circulatory system and respire through their skin. Oxygen passes through the skin and diffuses to the body cells that require it.

Their skin is divided into layers, providing stability and acting as a hydrostatic skeleton. They have a primitive nervous system that runs along the body in two main strands (dorsal and ventral) and allows them to respond to various stimuli.

This nematode species is dioecious, having separate male and female individuals, and is viviparous, giving birth to live offspring.

These microworms are found in nature in areas with high humidity and abundant decomposing organic matter.

Microworms as Live Food

Panagrellus redivivus has been considered as one of the promising live feed options for aquatic species, particularly during their fry/larval stage. This is attributed to the microworms’ small size and high reproductive capacity.

Note: For example, in the case of many newly hatched fry fish species, brine shrimp can be much too large to be ingested.

Several studies have been conducted to investigate the impact of micro worms on fish larvae and Penaeid shrimp larvae, Portunid crabs (especially Portunus pelagicus). The results have been somewhat contradictory, albeit generally positive in nature.

For example:

results of the experiments showed that when fed to Penaeus vannamei (Whiteleg shrimp), microworms caused significantly better growth than Artemia-fed and algae-fed treatments;
the use of Panagrellus redivivusas live feed for bighead carp and Asian catfish larvae resulted in much lower growth and survival than those fed Tubifex and Artemia nauplii;
according to another study, micro wormsexhibited better growth rates from zoea 2 to zoea 3 stage in the Blue swimming crab (Portunus pelagicus) when compared to diets based on Artemia. However, treatment with Artemia shows the highest survival rate during the zoea 4 to megalopa stages;
in catfish Synodontis petricola, there was no difference in growth between Artemia and nematode treatments;
the Artemia feed promoted the highest weight gain, followed by the alternating feeding regime. The single use of the nematode as live feed produced the poorest weight gain;
as feed for Betta splendens larvae, the Artemia feed promoted the highest weight gain. The single use of the micro worms as live feed produced the poorest weight gain.

It is also important to note that researchers acknowledged the need for additional investigations in instances when the use of micro worms produced less desirable results.

Another factor to consider is that these worms are unable to swim. They normally sink to the bottom of the tank, thereby making them less accessible for larvae feeding in the water column.

In addition, they emphasized that this could be due to low nematode density during feeding. There is also the hypothesis that these worms may simply not be the ideal food source for certain fish species, such as Betta fish indicating that the larvae do not have a specific capacity for efficient digestion of nematodes or an enzyme would not yet be fully available at this stage of larval development.

Nutritional Value of Microworms

The nutritional profile of Panagrellus redivivus is somewhat comparable to that of Artemia, containing:

Protein: 40-62%
Lipid: 15-20%
Carbohydrate: 31%

The nematodes are 76% water and 24% dry matter.

The amino acid composition of the nematodes is also similar to that of Artemia. However, fatty acid analysis of the lipid fraction revealed a pretty significant difference between Artemia and micro worms.

Micro worms, unlike Artemia, have their nutritional content, including lipid content and fatty acid composition, influenced by the composition of the culture medium in which they are grown!

Thus, it is possible to modify (enrich) the lipid content and fatty acid profile of the nematodes. According to the study, the pigment, delivered via nematodes, significantly improved larval coloration compared with that of placebo-pigmented nematodes.

Life Cycle and Reproduction of Microworms

The life cycle of Panagrellus redivivus consists of two stages:

the larval stage, which includes 4 molts post-hatch,
the adult stage.

Micro worms are gonochoristic (a species with separate male and female individuals), and they reproduce through ovoviviparity. This means that their eggs hatch inside the female’s body, and the juveniles are born live.

The life span of micro worms is about 9 – 10 days. The young reach sexual maturity in approximately 3 days.

Their size increases by 3 times during the first day and 5 to six times during the next 3 days.

Sexing:

According to the study, the sexes could be separated at the 2^nd molt by the development of a lobe of somatic cells in the gonad, anteriorly in males, and posteriorly in females.

Distinguishing between male and female individuals is difficult due to their small size, but females are typically smaller, but observations suggest that:

females are generally larger than males, males are more slender.
the males have curved tails.

Both females and males are present in approximately equal numbers.

Mating:

Microworms show both pre- and post-insemination coiling around the female and use their spicules to probe and open the female gonopore.

In most cases, the process of insemination is completed within one minute.

Interesting facts:

In males, the first mating occurs between 1 and 2 hours of molting.
The intervals between the 2^nd, 3^rd, and 4^th mating are approximately 2 hours.
The average number of daily matings ranges from 8.4 to 10.8. During their life, they mate 71 – 101 times.
Virgin male micro worms showed a rapid increase in mortality rate, with 24% mortality occurring within 24 hours of molting and over 99% mortality within 6 days.
In comparison, virgin female micro worms may have a significantly longer lifespan compared to copulating females, living almost twice as long.
Female Panagrellus redivivus are capable of copulating immediately after the final molt and their ability to copulate does not decline with age.
In Panagrellus redivivus, both males and females use sex pheromones.

Fertility:

Females release from 10 to 90 young every 1 to 1.5 days during their life span (10 days). Therefore, each female produces approximately 300 young.

Older females contained eggs in all stages of development.

How to Culture Micro Worms

Cultivating microworms is a straightforward process, making them one of the easiest live foods to produce. They consistently produce a reliable harvest and can withstand fluctuations in environmental conditions.

Microworms remain can survive slightly over 72 hours in saltwater and for 12 – 14 hours in freshwater.

List of Necessary Items:

Culture container.

Note: You will need any plastic container (up to 1 gallon (4 liters)) with a lid. Make sure to puncture a few holes in the lid beforehand using a needle to allow for proper air exchange.

Tip: Choose a container with taller sides to allow the worms to climb. This is important for harvesting.

Freshwater.

Just enough to make a paste.

Ground oatmeal.
Panagrellus redivivus
Soft brush.

Step-by-step process for culturing microworms:

Prepare the culture media using rolled oats. Use approximately 1 to 1-1/2 pints of rolled oats and one quart of water for each 8″ x 12″ container.
Cook the oats for 5 to 7 minutes, covered, and allow them to cool. The cooked oats should have a thick, paste-like consistency.

Note: If the oats are not cooked, reduce the water amount by approximately 2/3, but be aware of potential mold and insect infestation issues.

Spoon the media into the culture container, spreading it to a thickness of 1/2″ to 3/4″ (1 – 2 см).

Note: Microworms inhabit only the upper portion of the food substrate (0.08-0.2 inches or 2-5 mm). At the same time, a thinner layer dries out quickly, requiring more frequent transfer of the worm culture to a new substrate.

Sprinkle a tablespoon or more of baker’s yeast over the oatmeal and mix it in.
Spread the starter culture of nematodes over the surface of the media.
Set up the culture in a subdued (or ambient) lighting environment at a temperature between 68 – 85°F (20 – 30°C).
Mist the culture lightly with deionized water on a daily basis to replace lost moisture due to evaporation.
Make sure to keep the container closed to avoid contamination.

Microworms have the ability to tolerate temperatures ranging from below 32°F (0°C) up to 35°C (95°F). Although higher temperatures can lead to increased daily production, it will reduce their lifespan. At lower temperatures, cultures tend to have a longer lifespan but do not reproduce that much.

What else can we feed microworms?

These nematodes do not consume algae but will gladly consume:

corn meal,
white wheat flour,
baker’s yeast,
brewers yeast,
oatmeal cereal, etc.

Thus, you can easily combine these and other ingredients, for example:

cornmeal, baker’s yeast, and water;
oatmeal cereal, baker’s yeast, and water.

The results of experiments also showed that the population growth of nematodes in a Spirulina-enriched medium exhibited the highest number of individuals in the 2^nd week of cultivation, while the population grown in an oatmeal medium showed the highest number in the 5^th week of cultivation and did not increase further.

Maintaining the Culture of Microworms

Stir the surface of the culture once a week to ensure consistent production.
As the yeast consumes the oatmeal (or other food), the mixture may become thin and soupy, but this won’t affect microworm production.
Place a piece of sponge on the media to absorb excess moisture if necessary.
Over time, as the oatmeal is depleted and the nutrient supply for the yeast decreases, the microworm harvest will decline.
When the harvest diminishes significantly, it is time to start a new culture to maintain a steady supply of microworms.
If the top portion of your culture starts to become watery and brown, start a new culture.
Even if everything goes well, it is still recommended to change the media every 1-2 months.

To start a new culture, it will be enough to add one spoonful of worms from the old culture.

Harvesting Microworms

After 3 to 7 days, the surface of the media will start to shimmer with the movement of the microworms. The mixture will become thin and soupy.

You will also see that microworms start climbing up the sides of the container. It means that it is time to harvest.

Take a soft brush and gently collect the worms from the walls of the container.
Prepare a small jar of water and rinse the brush with the worms in it.
Shake the small jar and wait for the worms to settle at the jar’s bottom.
Pour off the cloudy water once it has become entirely clear.
Repeat the process until the water in the jar is entirely transparent.
Pour the worms into the aquarium.

The main reason why Microworms climb on the walls is that they try to escape from the excess carbon dioxide, making it easy to collect them with an old toothbrush, cotton swab, or brush.

Alternative ways to harvest microworms:

Put smooth-surfaced objects on the food substrate. Spray it with water. It will cause a large number of microworms to crawl onto their surface. This layer can be easily collected using a brush.
The nematodes can be separated from the medium by filtering them through a coarse sieve, covered with cotton discs of the type normally used for manual milk filtration.

Important: If the worms do not crawl along the edge, it means that the ventilation is too excessive or the temperature is too low.

As a rule, to obtain 0.22 pounds (100 g) of worms, 0.40 pounds (about 200 g) of oatmeal or 0.80 pounds (360 g) of oat flour is required.

How to Put Microworms in Aquarium

Some aquarists say that you can add these worms directly to the aquarium.

However, it is important to note that you should NEVER add microworms directly from the substrate without cleaning them first. The substrate contains a high level of bacterial contamination.

Storing Micro Worms

Microworms can be stored for extended periods of time due to their remarkable ability to survive in a dried state at low temperatures.

The worms, along with the substrate, slowly dry out at around 32°F (0°C). They can be kept in a consistently humid environment for up to 2 years.

When it’s time to rejuvenate the culture, the nematodes are brought back to life by gradually raising the air temperature and moistening the surface of the substrate.

In Conclusion

Live food promotes the health, growth, and survival rates of aquatic animals. Therefore, breeders seek new methods for maximum results with minimal investment.

Microworms are particularly beneficial for fish species whose fry are too tiny to consume newly hatched brine shrimp as their initial food source.

Nevertheless, it is generally recommended to use them in combination with other foods such as brine shrimp, rotifers, zooplankton, etc. to get the best results.

Microworms (Panagrellus redivivus)
Pros	Cons
Easy to culture	Do not swim (which can make them difficult for fish fry to notice.)
Simple setup	Other life foods are more nutritious
Fast reproduction	Can live in freshwater only for a few hours
Can be given for the smallest fry/larvae	Adult fish will ignore it
Cheap

References:

Choe, Andrea, Tatsuji Chuman, Stephan H. von Reuss, Aaron T. Dossey, Joshua J. Yim, Ramadan Ajredini, Adam A. Kolawa et al. “Sex-specific mating pheromones in the nematode Panagrellus redivivus.” Proceedings of the National Academy of Sciences109, no. 51 (2012): 20949-20954.
Biedenbach, James M., Linda L. Smith, Tamara K. Thomsen, and Addison L. Lawrence. “Use of the nematode Panagrellus redivivus as an Artemia replacement in a larval penaeid diet.” Journal of the World Aquaculture Society20, no. 2 (1989): 61-71.
B. Santiago, A. C. Gonzal, M. Ricci, S. Harpaz. Response of bighead carp Aristichthys nobilisand Asian catfish Clarias macrocephalus larvae to free-living nematode Panagrellus redivivus as alternative feed. 28 July 2003
Imran Affandi, Mhd Ikhwanuddin, Mohammad Syahnon, Ambok-Bolong Abol-Munafi, Growth and survival of enriched free-living nematode, Panagrellus redivivus as exogenous feeding for larvae of blue swimming crab, Portunus pelagicus, Aquaculture Reports, Volume 15, 2019,
Srinivasan, Jagan, Adler R. Dillman, Marissa G. Macchietto, Liisa Heikkinen, Merja Lakso, Kelley M. Fracchia, Igor Antoshechkin, Ali Mortazavi, Garry Wong, and Paul W. Sternberg. “The draft genome and transcriptome of Panagrellus redivivus are shaped by the harsh demands of a free-living lifestyle.” Genetics193, no. 4 (2013): 1279-1295.
Hechler, H. C. “Reproduction, chromosome number, and postembryonic development of Panagrellus redivivus (Nematoda: Cephalobidae).” Journal of Nematology2, no. 4 (1970): 355.
Sautter, Jürgen, Horst Kaiser, Ulfert Focken, and Klaus Becker. “Panagrellus redivivus (Linné) as a live food organism in the early rearing of the catfish Synodontis petricola (Matthes).” Aquaculture research38, no. 6 (2007): 653-659.
Rottmann R.W. (1998) Microworm Culture forAquarium Fish Producers. IFAS Fact Sheets FA-9. University of Florida, Gainsville, FL, USA, 2pp.
Couto, Márcia Valéria Silva do, Natalino da Costa Sousa, Higo Andrade Abe, Joel Artur Rodrigues Dias, Juliana Oliveira Meneses, Peterson Emmanuel Guimarães Paixão, Fernanda dos Santos Cunha et al. “Effects of live feed containing Panagrellus redivivus and water depth on growth of Betta splendens larvae.” Aquaculture research49, no. 8 (2018): 2671-2675.
Duggal, C.L. (1978). Copulatory Behaviour of Male Panagrellus Redivivus, Nematologica, 24(3), 257-268.
Duggal, C. L. Initiation of copulation and its effect on oocyte production and life span of adult female Panagrellus redivivus. 1978 Vol.24 No.3 pp.269-276