CLEARANCE OF LABORATORY-CULTURED BACTERIA BY FRESH-WATER BIVALVES - DIFFERENCES BETWEEN LENTIC AND LOTIC UNIONIDS

Nine species of unionids cleared laboratory-raised Escherichia coli fr om artificial pond water. The six unionid species collected from river s had higher clearance rates than the three species collected from pon ds, when clearance was normalized to millilitres per gram of dry tissu e mass per minute. Analysis of variance indicated that all lotic union ids examined form a group with similar clearance rates. When normalize d 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 c irri of the unionid species collected from ponds have smaller cirri (< 16 cilia per cirral plate). There was a strong correlation between cir ral surface area (mm(2)) per milligram of dry tissue and clearance rat e among the unionid species studied. As a comparison, Corbicula flumin ea and Dreissena polymorpha were also examined and both tended to clea r bacteria more rapidly than the lotic unionids.