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

Environmental contamination by heavy metals produced by either anthropogeni c or natural activities represents a threat to many species of aquatic anim als worldwide. This study investigates the effect of short-term (96 h) expo sure to dissolved heavy metals on the number of circulating haemocytes in t he shrimp, Palaemon elegans (Rathke). Changes in haemocyte counts were dete rmined 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 haem ocyte count during the first 8 h exposure, although the haemocyte number re turned to the initial (time 0) levels over the following 16 h immersion. In each case, the decrease in circulating haemocyte count induced by these me tals was significantly different from the controls. The greatest decrease i n haemocyte numbers (haemocytopenia) was induced by Pb, followed, in descen ding order, by Zn, Hg, Cr, Cu and Cd. The lethal level of haemocytopenia fo r the shrimps, defined as the number of haemocytes ml(-1) remaining in mori bund animals (i.e. threshold of mortality) was found to be significantly lo wer than the levels tolerated by surviving shrimps (i.e. the limit of survi val). The percentage of haemocytes remaining in the circulation at the thre shold 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 tolerate d by P. elegans appears to vary with the metal. Animals treated with Ph or Zn survived with a lower number of circulating haemocytes than animals expo sed to the other heavy metals. (C) 2001 Academic Press.