The body fluid osmolarity of individual rotifers was measured at 12 external concentrations ranging from 32 to 957 m-osmol/1. Brachionus plicatilis is essentially an osmoconformer, since a change in the concentration of the medium results in a corresponding change in the concentration of the body fluids. Most animals were, however, slightly hyperosmotic throughout the range tested.
The lowest body fluid osmolarity was 59 m-osmol/1 at an external concentration of 32 m-osmol/1. It appears that B. plicatilis is unable to tolerate the low concentrations that are frequently associated with acid water environments and this is responsible for the restriction of this species to alkaline and brackish waters.