Nine species of unionids cleared laboratory-raised Escherichia coli from artificial pond water. The six unionid species collected from rivers had higher clearance rates than the three species collected from ponds, when clearance was normalized to millilitres per gram of dry tissue mass per minute. Analysis of variance indicated that all lotic unionids examined form a group with similar clearance rates. When normalized on the basis of gill surface area, rates of clearance by all of the lotic unionids become remarkably similar to one another regardless of mass, but differ significantly from those of the lentic unionids. The cirri found on the laterofrontal cells of the gills of lotic unionids tend to be complex, containing > 25 cilia per cirral plate, while the cirri of the unionid species collected from ponds have smaller cirri (< 16 cilia per cirral plate). There was a strong correlation between cirral surface area (mm2) per milligram of dry tissue and clearance rate among the unionid species studied. As a comparison, Corbicula fluminea and Dreissena polymorpha were also examined and both tended to clear bacteria more rapidly than the lotic unionids.
