Microworms (Panagrellus redivivus)
For most of the fish species an aquarist might raise, live food is considered a necessity, especially during the first weeks of feeding. The main reason for this is that most fish fry respond only to independently moving prey items. Brine shrimp (Artemia) are probably the best known example of a suitable fish fry feed, and they have historically been used almost exclusively as an initial food for fry. However, brine shrimp do have some disadvantages as a fry food. Newly hatched brine shrimp are a fairly large food source, too large for many fish fry to ingest. Brine shrimp will survive in fresh water for a few hours, but will eventually die of osmotic stress. Furthermore, the costs of brine shrimp cysts have become substantial.
Biology
Microworms (Panagrellus redivivus, formerly known as Anguillula silusae) may serve as
a suitable alternative to brine shrimp. These nematodes are about 50 µm in diameter and just over 1 millimetre in length, barely visible to the naked eye. Because of their size and shape, they can be fed to fish that are too small to take a brine shrimp nauplii. Microworms remain alive in fresh water for twelve hours or more (Rottmann, 2002).
Microworms reproduce sexually. Microworms are live bearing, releasing 10 to 40 young every 1 to 1.5 days for a 20 to 25 day life span. Therefore, each female produces approximately 300 young. Young microworms will reach sexual maturity in about three days. Their size increases by three times during the first day and five to six times during the next three days. As for their nutritional value, the nematodes consist of 76% water and 24% dry matter. About 40% of the dry matter is protein and 20% is fat (Rottmann, 2002).
Microworm culture
Microworms are one of the simplest live foods to culture. They produce a dependable harvest and are tolerant of environmental variables. If you search the internet, you’ll run into several methods of culturing these critters. All these methods will probably work, with their own advantages and disadvantages. What I will show you here is my method, one of the ones that seem to work.
So, what do we need? First of all a watertight container, I use 12 * 17 cm plastic refrigerator boxes with snap on lids. Next, get yourself some oatmeal, yeast, and water. Now, put a 1 cm layer of oatmeal in the plastic container. Add water to the oatmeal until it is just submerged. Sprinkle some yeast over the oatmeal and mix it in. After the oatmeal has taken up the moisture you should be left with a thick paste. You then take a starter culture of nematodes and spread this over the surface. After a couple of days the surface of the medium will appear to shimmer with the movement of the microworms. In about a week the nematodes will start climbing up the sides of the container and are ready for harvesting. With a knife, a small paint brush or just your fingers you can scrape the worms off the side of the container. Scrape of as many worms as you need and put them into a beaker with a small amount of water You can then use an eyedropper to feed your fry. Below you see some pictures of the different stages of the culture (from left to right; a new started culture, the culture at day 7, and an culture of a few weeks old).
References
Rottmann, R.W., 2002. Microworm culture for aquarium fish producers. http://edis.ifas.ufl.edu/FA022, 19 January 2008.