Culturing Marine Rotifers for Freshwater Applications
Rotifers fall into two salinity categories - marine and freshwater. “Marine” rotifers are actually brackish water in origin and can survive and grow in salinity ranging from 4 ppt to 40 ppt.
There is much more information about culture of marine species, and they are therefore the preferred choice. The drawback is that rotifers grown in 20-30 ppt salinity are shocked when put into fresh water. They are not killed, but they may stop swimming for a time until they adjust to the lower salinity. But there is an easy solution - grow the marine rotifers at a lower salinity (15 ppt is actually the optimum). At 5 ppt marine rotifers such as Brachionus plicatilis will grow well, although they won't be quite as prolific. When the 5 ppt rotifers are put in fresh water (0 ppt) they will stop reproducing but remain healthy, alive, and will maintain their enrichment levels (for at least 1 hour).
To cultivate 5 ppt rotifers it is best to drop the salinity gradually, preferably lowering it no more than 10 ppt per day if you want to maintain production. While you are lowering the salinity the egg count and reproductive rate will be reduced, but once the rotifers have been at the target salinity for 48 hours they should rebound quickly. Always make sure that the ammonia (NH3) level is kept low (preferably less than 0.5) to avoid stressing the animals.