Asplanchna sp. rotifer swimming. Asplanchna rotifers are strong swimmers and feed predatory on other rotifers, ciliates and copepods but are also likely to feed on large algae. Inside the huge, sack-like body of this individual developing eggs and one young embryo are visible. The embryo is identifiable by its small red eyespot. Microscopic contrast technique : Differential interference contrast. Magnification 90x when printed 10 centimetres wide.