Heavy metals affect the circulating haemocyte number in the shrimp Palaemon elegans

Environmental contamination by heavy metals produced by either anthropogenic or natural activities represents a threat to many species of aquatic animals worldwide. This study investigates the effect of short-term (96 h) exposure to dissolved heavy metals on the number of circulating haemocytes in the shrimp, Palaemon elegans (Rathke). Changes in haemocyte counts were determined in relation to time of exposure and with heavy metal concentration, relating the results to toxicity. It was found that immersion in artificial seawater containing Hg, Cd, Cu, Cr, Zn or Pb caused a decrease in the haemocyte count during the first 8 h exposure, although the haemocyte number returned to the initial (time 0) levels over the following 16 h immersion. In each case, the decrease in circulating haemocyte count induced by these metals was significantly different from the controls. The greatest decrease in haemocyte numbers (haemocytopenia) was induced by Pb, followed, in descending order, by Zn, Hg, Cr, Cu and Cd. The lethal level of haemocytopenia for the shrimps, defined as the number of haemocytes ml remaining in moribund animals (i.e. threshold of mortality) was found to be significantly lower than the levels tolerated by surviving shrimps (i.e. the limit of survival). The percentage of haemocytes remaining in the circulation at the threshold of mortality as a function of the number at time 0 was 56.6 +/- 8.8%. By contrast, the equivalent value for the threshold of survival was 63.7 +/- 12.4%. Importantly, the percentage decrease in haemocyte counts tolerated by P. elegans appears to vary with the metal. Animals treated with Pb or Zn survived with a lower number of circulating haemocytes than animals exposed to the other heavy metals.