Rotifers

Rotifers


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Food for Thought
Rotifers: an Alternative First Food

By Dougall Stewart
16-05-2002

Introduction

It is commonly acknowledged that the best first food for discus larvae is the nutritious slime layer that they produce. There have been many attempts to mimic this slime e.g. the use of microflake foods, egg yolk, or other high protein mixes.

One method that is currently under-published is the use of live natural organisms as a first food e.g. fresh water rotifers Brachionus calyciflorus.

Current methods of raising discus fry

  • Method 1

Currently the most popular method for rearing discus is to allow the newly hatched fry to feed from the slime coat of their parents. Then, when they are large enough, their diet is supplemented with baby brine shrimp or a microflake supplement (either as a powder or in suspension @ approx 30 microns or greater).

  • Method 2

An alternative to this method is to remove the eggs from the parents immediately after spawning and to hatch them artificially away from the parents. The subsequent fry are then fed on a high protein diet/mix of egg yolk and moved on to baby brine shrimp and or microflake at an appropriate stage.

Advantages and Disadvantages (quick summary)

The cost benefit dynamics of artificially rearing the fry away from their parents have been extensively written about elsewhere - However in brief:

  • The advantages of artificially rearing fry away from the parents:

  • parents are rested and do not have the ‘energy cost’ of rearing the young fry

  • the parents are less prone to physical damage by the over zealous fry ripping at their slime coat

  • more fry may survive especially if parents are egg/fry eaters

  • the fry are not exposed to flukes and other organisms from the parents.

  • The disadvantages of artificial rearing are:
  • Less than optimum growth rate
  • High mortality risk during the early stages; from pollution by egg yolk, other food mixes, blockages caused by baby brine shrimp shells etc.

An alternative method - Rotifers

One alternative method worth considering is the use of rotifers as a first food for discus fry that have been artificially hatched.

What is a rotifer and why are they important in aquaculture?

Rotifers are microorgansims (100 to 2500 microns) that are readily found in aquatic and semi-aquatic habitats. Some are marine inhabitants, however most species are freshwater organisms. Currently there are over 2000 described species (see Walsh & Brux, 2002 for more information on identification and species). They account for more than 50% of the zooplankton production in some freshwater systems and as such are a vital link in the food chain for most aquatic life. Their growing employment in aquaculture (and the methods used) was highlighted in an excellent article by Liu Fengqi (1996). Rotifers are known to have a very high reproductive rate, reaching population densities of 1000 or more individuals per litre of water.; and one gram of ‘rotifer eggs’ will contain approximately 2,000,000 cysts with a hatching rate of about 80% at 28C in 36 hours.

Yolk feed versus Rotifers

Lim and Wong (1997) demonstrated that in dwarf gouramis, a species known for its micro-fry, that during first food feeding, rotifer fed fry grew up to 33% faster and in addition they have a 44% greater survival rate compared to yolk fed cohorts.

Parent reared versus rotifer fed

Lim and Wong performed a second experiment which compared parental reared larvae growth and survival rates to rotifer fed larvae growth and survival rates.

Lim & Wong’s Method

  • Brown discus larvae that were obtained from a 18 month old male and a 14 month old female. The time to hatching was 60 hours, post fertilisation, at 28-30 °C and 20 % water changes were carried out on all tanks every 2 days

  • On the second day after hatching, all the fry except for 20 which were left with the parents’, were removed from the tank. 20 of the removed fry were placed into a 20 litre aquarium, and a further 30 fry were placed into a 30 litre aquarium

  • The fry that were left with their parents, fed from their body slime. The others were fed on rotifers at a rate of 5-6 ml per litre from day 4 to day 7

  • Between days 8 and 14 the larvae in all of the tanks were fed solely on Artemia nauplii

  • 20% of the tank water was changed every other day throughout the experiment

The results

  • Survival rate:
  • Parent reared 85%
  • Artificially reared, 20 liter tank 95%
  • Artificially reared, 30 liter tank 70%

  • Size and Shape:
  • Significantly less variation in overall size in both of the artificially reared tanks, compared to those that were parent reared
  • Similar sizes fry by the end of day 14 in all tanks

Ok - So what does this all mean?

Do we all rush out to our local pond, dip for rotifers and start growing an abundance of super discus. No!!! far from it, as with any live food care must be taken.

Therefore, to those that wish to experiment, perhaps you have a tank or two that you can set away and give this idea a go, I suggest that you get a ‘clean’ freshwater rotifer culture from a reputable supplier.

I have contacted Rohan Mak of ‘ZEBRAFISH MANAGEMENT LTD ’ in the UK and he tells me he is willing to import freshwater cysts for those that are interested; he already has, in stock, the marine rotifer Brachionus plicatilis which I strongly suspect can be used with similar results.

If you live elsewhere in the world and have a trusted supplier of rotifer and other microcultures please feel free to e-mail me their details. Dougall Stewart

Acknowledgements

I wish to thank:
Fred Goodall
for his encouragement, patience and for reading this text.

Walter Soestbergen
for his generosity in allowing this information to be shared with
discus enthusiast from around the globe.

References

[1] Walsh E., Brux, C. M., (2002), ‘Rotifer Project’, Department of Biological Sciences, University of Texas at El Paso in url: http://www.utep.edu/~rotifers/bio/home.html

[2] Fengqi, L, (1996), ‘Production and Application of Rotifers in Aquaculture’, Aquaculture. 22(3):16-22

[3] Lim, L.C., Wong, C.C., (1997), ‘Use of the rotifer, Brachionus calyciflorus Pallas, in freshwater ornamental fish larviculture’, Hydrobiologia, Belgium: Kluwer Academic Publishers, 358: 269-273

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