Type III functional response in the zebra mussel, Dreissena polymorpha

Distinguishing between functional responsetypes requires observations at low food concentrations, but surprisingly these tend to be rare. A paucity of feeding observations at low food concentrations is especially acute for dreissenid mussels, despite their importance as invading species in fresh waters. We assessed the functional responseof zebra mussels( Dreissenapolymorpha) via feeding experiments and behavioral observations conducted at low food levels. Critically, food levels were chosen to be in the vicinity of minimum concentrations found in lakes with established D. polymorpha populations. Results of two feeding experiments show clear evidence of a Type III functional responsefor D. polymorpha feeding on Ankistrodesmus falcatus— clearance rateincreased with increasing food concentration at low food levels. Musselswere always open and feeding during these experiments. An independent setof behavioral observations further showed that the fraction of musselsactively feeding (valves open, sipho...