Algal food Zooplanktonic food

Utilization of rotifer Brachionus spp. as a live food organism for
hatchery-based seed production

What is Brachionus rotifer?

    About 2,500 species of rotifers have been known from global freshwater, brackish water, and seawater.
Brachionus is one of the most common genera among the known 2,500 rotifer species. The genus is important zooplankton species as a primary live food source for the early life of both marine and freshwater animal species.
Body of Brachionus is covered by a distinct cuticle, bilateral symmetry and sexual dimorphism. The body is
comprised of four regions: head with corona, neck, body, and foot. The foot is an appendage that extends from the
body ventrally. It possesses two toes.

Two marine Brachionus rotifer species

     Formerly called S (small) and L (large) typed rotifers were reclassified. The L typed rotifer is Brachionus plicatilis O. F. Muller 1786. And the S type is Brachionus rotundiformis Tschugunoff, 1921. These two rotifers were also different in karyotypes. The chromosome number of Brachionus plicatilis is 2n=22, Brachionus rotundiformis is 2n=25. While the so-called SS (super small) type is considered as a subspecies of Brachionus rotundiformis by morphological taxonomic keys, reproduction patterns, allozyme analysis and male mate recognition patterns.  

Rotifer species of live food organisms

   The euryhaline (marine) species - Brachionus rotundiformis, Brachionus plicatilis
   The freshwater species ? Brachionus calyciflorus, Brachionus rubens

Why rotifer as a live food organism for fish larvae

    With the remarkable developments in larval rearing technology of industrial useful fishes, demand for the rotifer is further increasing. The reasons for this are the following biological characteristics: 1)small body size and round shape, 2)slow swimming speed and habit of staying suspended in the water column, 3)easily enriched with external nutrients resources, and 4)high reproduction rate and high density cultures. The usage of the marine Brachionus as live food source for marine fish larval rearing started 43 years ago by acclimation euryhaline Brachionus to seawater by a pioneer scientist Takashi Ito of Japan. At that time, rotifers were known as harmful zooplankton for outdoor eel fish cultures. Because of high rotifer population density depletes dissolved oxygen caused mass mortality of cultured eels by denaturation of culture water. 

Mass culture methods of Brachionus rotifers

     Various mass culture methods have been suggested for stable rotifer culture and successful fish larval rearing. Of them two have been commonly used.

      1)  Batch culture (repeated stocking method or total harvesting method) 

The batch cultures, part of the harvested rotifers is used as inoculum for the next batch cultures. Culture periods are usually short, ranging from 2-7 days. A consistent high yield can be achieved from each batch culture. Volume of culture tanks usually performed in small tanks forms 10-50 tons. However, this culture method is very laborious.

     2)  Semi continuous culture (thinning out method)

The semi-continuous culture method, a certain volume of rotifer is daily harvested from the culture tank, which is replaced with food suspension medium. This culture is usually conducted in tanks ranging from 50-100 tons.

High density cultures

     Ultra high density culture (over 20,000 ind./ml) of rotifer by feeding condensed freshwater Chlorella has been recently developed, in which smaller size of culture tanks (dawn to 1 ton) are available for mass production of the rotifer.

Algal foods for rotifer culture

     Nannochloropsis, Nannochloris, Dunaliella, Tetraselmis, yeast and enriched freshwater Chlorella are the commonly used food for mass culture of rotifer.  

Optimum rotifer culture conditions

1)  Water temperature

The growing temperature for Brachionus rotundiformis is higher than for Brachionus plicatilis. The lower temperature limits for Brachionus rotundiformis and Brachionus plicatilis are 20? and 10?, respectively.

2)   pH

Swimming activity and respiration rate are not significantly different at pH 6.5-8.5. However, alkaline water depresses swimming activity of the rotifers more than acidic water does.

3)   DO

Marine Brachionus rotifers require oxygen concentrations above 1.0mg/l for their growth, and some can tolerate anaerobic or near-anaerobic conditions for short period of time. Negative growth rates of Brachionus rotifer are observed at 0.9mg/l.

Biological affects of the contaminating organisms in the mass culture tanks

   In mass culture of rotifers, there are many factors affecting rotifer population growth. Among them is the biological environment, such as contamination with rotifer and other organisms in the mass culture tanks. Copepods (Tigriopus japonicus, Acartia), Anostracan (Artemia) and ciliate protozoans (Euplotes, Vorticella and Holosticha) often co-existing in the marine Brachionus rotifer culture tanks. There were no contaminating organisms that contributed to an increase in rotifer population growth.

Axenic culture of rotifers

    Population growth of axenic cultured rotifer is maintained with a high density and stable growth compared with the general culture style of non-axenic culture.

Protocol for axenic culture of Brachionus rotifers
 1) Establish mono specific cultures of Brachionus rotifers
 2) Collecting of eggs from maternal rotifer females
 3) Rinse eggs with sterilized seawater (SSW) and 10% AM9 antibiotic solution
 4) Repetition of 3)
 5) Soak eggs in the 100% AM9 antibiotic solution for 90 minutes
 6) Rinse eggs with SSW
 7) Repetition of 6) for 60 minutes
 8) Shake eggs every 10 minutes
 9)  Rinse eggs with SSW
10) Transfer culture tank with axenic situation microalgae suspension medium
11) Sterility test with modified STP