Acute Toxicity of Mercury and Nervous Tissue Damage in Postlarvae and Juveniles of Litopenaeus vannamei

Experiments were carried out to verify the existing information on the LC50 of waterborne Hg for L. vannamei postlarvae and determine that of juveniles. Additionally, the structure of different tissues of the organisms which survived the acute toxicity test was examined. The experiment with juveniles was performed after addition of 1.00, 0.75, 0.50, 0.25 and 0.10 mg/L of Hg (using HgCl2). For postlarvae, additions were 0.5, 0.4, 0.3, 0.2, 0.1 and 0.01 mg/L. Acute toxicity tests determined that, after 96 h of exposure to Hg, the respective LC50 values for juveniles and postlarvae of Litopenaeus vannamei were 0.50 and 0.25 mg/L. Although lower that the only value of LC50 known for L. vannamei postlarvae, these lethal concentrations are one order of magnitude higher than those known for larval and juvenile stages of other crustaceans, suggesting that L. vannamei has a high resistance to the toxic effects of waterborne Hg. The only histological damages in survivors to 96 h of exposure were observed in nervous tissues and consisted in the presence of necrotic cells. The degree of damage severity was related to the concentration of dissolved Hg, and the numbers of necrotic cells decreased progressively with increasing distance from the nerve ganglia. A safe level of mercury exposure for L. vannamei postlarvae would be 2.5 μg/L.