Behavior of trace metals in Mytilus edulis during a reciprocal transplant field experiment

Marine mussels Mytilus edulis were transplanted reciprocally from relatively pristine Sequim Bay to a metal-contaminated site at Tacoma (both Washington State, USA) and vice versa. We studied temporal patterns of trace metal accumulation by the mussels, metal incorporation into subcellular compartments (low molecular weight, metal-binding proteins, lysosome-like vesicles) and condition index (a measure of animal health) in a field experiment. Analysis of seawater from the Tacoma site showed elevated concentrations in all the metals examined in this study; i.e. copper, zinc, cadmium, mercury, and silver. Values fluctuated with time; peak concentrations were 105 ppb copper, 113 ppb zinc, 5.4 ppb cadmium, 0.0189 ppb mercury, and 0.13 ppb silver. These values were 255 X , 188 X, 103 X , 47 x and 138 x the respective metal concentrations in Sequim Bay. With the exception of cadmium, the tissues of indigenous mussels at the Tacoma site reflected the high seawater metal concentrations. Mussels transferred from Sequim to Tacoma accumulated metals rapidly and approached or exceeded the metal concentrations in Tacoma mussels. General patterns were complicated by fluctuations in metal concentrations. Loss of metals following reciprocal transfer was relatively rapid in copper and silver values which were close to background levels after 4 wk and slower with zinc and mercury (24 wk or longer to background). Patterns of accumulation and loss and organs for concentration were specific for individual metals. Gills and digestive gland, for example, varied in their abilities to concentrate different metals and were often more sensitive indicators of metal bioconcentration than the whole organism. Gills were also examined for copper, cadmium and zinc on low-molecular-weight metal-binding proteins, high-molecular-weight proteins (< 70,000 M,), and pelleted fraction. A highly significant correlation (r = 0.87, p C 0.01) existed between copper in whole tissues and copper associated with the 3 fractions identified above. Changes in copper in the subcellular compartments paralleled changes in the whole tissues in this study. X-ray microanalysis provided evldence for the localization of zinc as being primarily in the kidneys and was useful for examining the metal cornposltion of organs which could not be easily excised and analyzed using other procedures. Measurement of condition index indicated that Tacoma mussels were stressed by field conditions; condition index was reduced in both transplanted and indigenous groups of mussels at Tacoma. The time at which the reduction was sustained over time coincided with the time at which the trace metals in Tacoma seawater had risen to extremely high concentrations.