Factors affecting survival of rotifers (Brachionus plicatilis O.F. Müller) at 4°C

The possibility of maintaining rotifers (Brachionus plicatilis O.F. Müller) at 4°C for extended periods of time at high densities and at a low cost was examined. It was found that the dynamic processes involved in the maintenance of a rotifer population (hatching and production of eggs) continue to operate at 4°C, but their rate is reduced to one-tenth of that occurring at 25°C. The survival of rotifers and the percent of eggs carried by them were affected by the frequency of changing the culture media, the salinity in which they were cultured, and the amount of food provided and its type (algae, yeast or Topal). Higher survival rates were observed in rotifers maintained in the dark. Statistical analysis revealed that in most cases, the time spent at 4°C significantly affected both survival and the percent of eggs. On several occasions, time interacted with the tested effect, that is, there were differences in the direction of the trend in time between the tested groups. In such a case, the contribution of each of the studied effects could not be evaluated over the total period of the experiments. It had to be analysed separately for each of the observation days; the most relevant being the last day of the experiment. Results of the present research indicate that the cheapest system would involve maintaining rotifers (at least 1000 ml^-1) cultured in 10‰ seawater at 4°C with yeast as food. Media would have to be changed every 4-8 days, and the cultures should be maintained in the dark. These rotifers may be used directly, following proper enrichment with unsaturated fatty acids, as food for marine fish larvae, or alternatively to initiate new cultures. The present paper proposes a new method for the preservation of rotifers for periods extending to days or weeks. This would allow more flexibility in managing the supply and demand in a marine hatchery. Alternatively, stored rotifers could be distributed from one main location to small hatcheries or other users lacking facilities for culturing rotifers.