Rotifer resting eggs: An alternative for rotifer mass cultures in fish farms - Lessons from a comprehensive study on production, storage and hatching of resting eggs

Rotifers of the Brachionus plicatilis complex species serve as the first live food for marine fish larvae. The essential daily supply of rotifers depends on a successful culture of billions of rotifers, as there are no current alternatives for this live food in the large scale production of farmed fish. This problem is not faced with Artemia, the second source of live food, as they are supplied as cysts and hatched according to demand. Rotifers produce resting eggs (RE) that can be stored for long periods of time and hatched when needed, but their use in aquaculture is limited, despite great advantages. Unlike Artemia cysts which are harvested from natural sources or salt works, rotifer RE originate from cultures. A systematic and comprehensive study was carried out to identify the main factors affecting RE production, storage and hatching, with the aim of increasing the efficiency and reducing the costs of production, thus facilitating their wider use in hatcheries. The current study was performed with a clone displaying high propensity towards RE production. RE production was affected by the food type, with a 2-5 fold difference between Nannochlorosis sp. from different sources, at optimal feeding regimes. We uncovered a central role for lipids in promoting RE production. Remarkably, addition of a lipid enriched solution to yeast, increased the number of RE by 10.4-fold, reduced the percent of damaged RE by 5.8-fold and production was similar to one type of algae. The food type did not affect either the length of the obligatory dormant period nor the hatching performance. RE started to hatch at 22-35 days after their formation but the peak was reached after similar to 44-62 days. RE that were dried close to the time of their formation did not hatch and the hatching potential was gradually obtained towards the end of the obligatory period. Wet RE stored for 2-65 months at 24-26 degrees C, exhibited a similar hatching performance. Dried RE, however, could be stored for only similar to 9 months at the same conditions, but for longer periods at 4 degrees C. Rotifers that hatched from RE can serve for mass production as RE production in these cultures accounted for 4.5% of the total number of females. An automated continuous culture system is proposed and discussed. The current study also provides novel insights into biological attributes on the production and survival of RE, as an important component of the natural food web in the aquatic environment.