In 1995-1996, Corbicula fluminea was collected at monthly intervals at two sites in the delta of the lower Paraná river (Argentina), at the confluence of the San Antonio and Vinculación rivers, and from the Paraná de las Palmas river close to its outlet into the Río de la Plata estuary. The San Antonio site is located in the vicinity of sources of industrial and sewage effluents, whereas the more remote Paraná de las Palmas river sampling location is considerably less polluted. Growth rates of caged individuals at the two sites also monitored during the same period, and Cu, Cd, Pb and Zn were measured monthly in the water and in tissue of animals. Throughout the year concentrations of all metals in water, as well as Pb and Zn in tissue, varied irregularly and their levels at the two sites were roughly similar; on the other hand, Cu and Cd in tissue of animals from the polluted site were consistently and significantly higher than in those from the remote site. Changes in the size-frequency of clams from Paraná de las Palmas river throughout the period surveyed clearly indicated a highly structured population with a single well defined reproduction period in October-November. In contrast, shell lengths at San Antonio changed very little throughout the 13 month period, with over 88% of the clams varying between 18 and 20 mm in length; no discernible cohorts were recorded. Growth rates of caged clams were very significantly lower at the polluted site, with the remote site latter yielding comparable values to those obtained from the remote site cohort analysis data. Highest growth rates (around 10 mm) were attained by the smallest (about 7-8 mm) clams reared in Paraná de las Palmas river waters, whereas 23 to 25 mm organisms maintained in San Antonio waters grew only 2 to 3 mm in the same period. Of the 57 animals which survived throughout the caging experiment, those below 10 mm at the outset increased, on average, 8.2 mm in length after 13 months, whereas clams less than10 mm in length grew only 4.2 mm. The condition index (i.e., dry tissue weight divided by shell length) of clams from San Antonio was consistently about 50% lower than that of clams from the Paraná de las Palmas river.It is concluded that delayed growth, poorer condition index, and the abnormal population structure at San Antonio are due to enhanced pollution in this area as compared with the Paraná de las Palmas river, and that all three traits constitute a more useful biomarker of contaminants.

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