Rotifers can survive very low levels oxygen levels. However, a drop in DO, even for a short period, can significantly stress the rotifer. At RMI we routinely measure DO using the % oxygen setting (calibration setting). We use this setting because it is the “partial pressure” of oxygen that the rotifer experiences and is unaffected by temperature and salinity.
Rotifer stress is more visible during enrichment when the rotifers are exposed to very high levels of lipid rich feeds. At there time O2 must be set properly and monitored closely. If your enrichment protocol extends for several hours and O2 is not kept near saturation throughout the enrichment process, rotifer stress will be apparent in the form of a higher percentage dirty, shrunken (lorica is angular rather than round and plump) and “wasted” (clear lorica) rotifers.
Natural saturation of oxygen is 21% (equilibrium with the atmosphere). At RMI we maintain O2 levels at 17-27%. High levels of O2 seem to do no harm. Lower levels of O2 (<15%) seem to increase susceptibility to stress and reduce vigor and productivity
At RMI we consume O2 at a rate of 3 lpm per billion rotifers in culture (“RMI Mini-L 160” at 9000 rotifers per ml). This number is affected by the size of the O2 bubble, the amount of normal aeration and rotifer density and feed rate. Every system must be adjusted differently.
Rotifers (Brachionus) are brackish water animals. Brachionus (both B. plicatilis and B. rotundiformis) can survive full salinity and even hyper-saline conditions (to at least 45ppt). Brachionus can also survive at very low salinity and have been acclimated to 2ppt for some freshwater applications. However, Brachionus do best in a range of 15-20ppt. Only at this range will they achieve maximum productivity.
Brachionus rotifers prefer normal marine salinity 7.8-8.2 and may thrive at a slightly higher pH. Typically, feed and rotifer waste reduce pH below this level. If pH drops below 7.0, the low pH is mildly stressful and productivity.
It is a common practice to culture rotifer at a ph below 7.0 to reduce ammonia toxicity (see ammonia below). Below a pH of 7.0, ammonia is almost completely non-toxic. Above pH 8 controlling ammonia toxicity become critical.
Brachionus plicatilis (L-Type) grow best at moderate temperature (20°C-24°C). They can be grown successfully at temperatures as low as 16°C and as high as 28°C. B. plicatilis will swim well at 10°C and will even swim (though slowly) at 4°C. The tolerance of B. plicatilis to cooler temperatures is especially useful for temperate and coldwater fish and cold storage of rotifers for later feeding.
Brachionus rotundiformis (S- and SS-Type) grow best between 27C and 32C. They can be grown successfully at temperatures as low as 20 °C and as high as 34°C.