Histological effects of a combination of heavy metals on Pacific white shrimp Litopenaeus vannamei juveniles.

Exposure to different levels of a mixture in equitoxic concentrations of Cd, Cu, Fe, Hg, Mn, Pb and Zn, equivalent to between 5 and 0.5% the individual respective 96-h median lethal concentrations (0.05-0.005AF: application factors) caused dose- and time-dependent damages to the hepatopancreas, gills, epipodites and midgut tissues of Litopenaeus vannamei juveniles. After between 1 and 3-4 weeks, there was between 50 and 100% loss of the hepatopancreas R cells and of the tubules regular structure. Gill alterations were observed between 2 and 3 weeks with 0.05-0.025 and 0.005AF, respectively. Epipodites showed time- and dose-dependent increasing degrees of melanization, and hemocytic enteritis was observed with 0.025 and 0.01AF. Exposure to 0.05 and 0.025AF caused also 50 and 12% mortalities after 2 and 3 weeks respectively, showing that even at these low levels the mixture of these metals may have a mid-term lethal effect. For this reason, the assessment of risk and of safe levels of toxic substances added to any natural environment through human or natural sources, should not neglect the effects on biological systems caused by the interaction of minute amounts of toxicants, which would be harmless unless present in combination.