Fig. 3. Uptake of E. coli from water by zebra mussels and depuration in the presence of low external concentrations of E. coli in two experiments. (A) Sewage was added at time = 1 h. At time = 13 h, the sewage-containing water was replaced by sterile water. Mussels and water were sampled at indicated intervals. (B) Similar to (A), except that bacteria enumeration was done on water and two groups of mussels at each time. Bacteria in one group of mussels was enumerated after shucking, as in (A); in the other group, whole mussels, with shells, were enumerated