Zebra mussels feed on suspended particles (unicellular algae, bacteria, and fine organic detritus) using a complex arrangement of cilia. Water enters the animal through an incurrent siphon and is carried over the gill where suspended particles are filtered by cilia and are sorted according to size. Accepted particles are combined with mucus and passed to the mouth. Rejected particles are combined with mucus and ejected as pseudofeces. Zebra mussels can effectively remove particles less than 1 mm in diameter whereas most other bivalves cannot filter objects less than 3 mm This enhanced filtration capability enables zebra mussels to feed on planktonic bacteria that are unavailable to native mussels.
This method of feeding is common in all freshwater and marine bivalves. An individual zebra mussel can filter up to 8.5 l of water a day. Because of their mode of feeding and their ability to achieve high densities, zebra mussel filter feeding can increase water clarity locally. Phytoplankton, fine organic matter, and clay or silt particles are filtered out of the water and ingested or deposited as pseudofeces. Deposition of silt into zebra mussel feces and pseudofeces can greatly increase sedimentation rates in natural habitats and raw water systems. Zebra mussels are often used in Europe as water clarifiers at treatment plants.