Mercury (Hg2+) effect on enzyme activities and hepatopancreas histo structures of juvenile Chinese mitten crab Eriocheir sinensis* Abstract We studied the effects of mercury (Hg2+) on antioxidant and digestive enzyme activities in terms of LC50 value and on hepatopancreas histo structures of juvenile Chinese mitten crabs Eriocheir sinensis in 40-day exposure to various concentrations of Hg2+ (0, 0.01, 0.05, 0.10, 0.20, and 0.30 mg/L). The results show that the activities of superoxide dismutase (SOD), glutathione peroxidase (GPX) and catalase (CAT) significantly increased in the concentrations of 0.01 and 0.05 mg/L, while that of enzyme decreased in 0.10, 0.20 and 0.30 mg/L treatments. Meanwhile, Hg2+ disrupted the histo structures of the hepatopancreas, causing decreases in activities of pepsin, tryptase, amylase, and cellulose, which are synthesized in the hepatopancreas. Moreover, as the Hg2+ concentration increased, the survival rate of the crabs decreased, worst at 56.57% in 0.30 mg/L. Therefore, although crabs are able to tolerate low levels of mercury pollution, high levels lead to cellular injury and tissue damage in hepatopancreas, which then loses some of its vital physiological functions such as absorption, storage, and secretion. Keyword: Eriocheir sinensis; mercury; hepatopancreas; antioxidant enzymes; histo structure; digestive ; enzymes 1 INTRODUCTION Mercury is a nonessential metal for aquatic animals (Bano et al., 1989). Due to anthropogenic input and environmental transport of mercury, pollution of the aquatic environment has increased significantly in the past decade (Mason et al., 1994; Botton, 2000). Waterborne mercury penetrates into aquatic animals, accumulates in different tissues, and induces adverse effects at the molecular, cellular and tissue levels (Bianchini et al., 1996; St-Amand et al., 1999; Chou et al., 2002). The Chinese mitten crab, Eriocheir sinensis (Edwards, 1853), is one of the most economically important crab species in China, with annual production reaching 500 000 tons and valued at US$2.2 billion in 2005. Previous investigations have described acute toxicity (Bianchini et al., 1996) and morphological changes in the gills of E. sinensis exposed to mercury (Zhao et al., 2008). However, the mechanism of mercury toxicity to E. sinensis is still largely unknown. Previous studies show that ambient mercury exposure leads to the generation of reactive oxygen species (ROS) including O2-, OH-, O2-and H2O2 (Ali et al., 2000). ROS are very unstable and highly reactive, and can induce alterations in the structure of cellular macromolecules, such as proteins, lipids, nucleic acids, etc. (Michiels et al., 1988). ROS formation can also induce cellular damage (Verlecar et al., 2008). Superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) are the primary enzymes responsible for scavenging ROS. Homeostatic mechanisms act to equilibrate the amount of ROS generated under stress conditions in order to protect an organism from adverse effects(Pan et al., 2006; Yan et al., 2007). In addition, the activity of these enzymes is regulated by health status and environmental stress (Winston et al., 1991). Although several studies have reported ROS formation, antioxidant enzyme activity, and scavenger responses in crustaceans (Sridevi et al., 1998; Yan et al., 2007), little is known about the relationship between ambient Hg2+ and the antioxidant defensive systems of Eriocheir sinensis.* Supported by the National Basic Research Program of China (973 Program) (No. 2007CB407306); Basic Scientific Research Operation Cost of State-level Public Welfare Scientific Research Institute of Chinese Research Academy of Environmental Sciences (No. 2007KYYW08) * Corresponding author: xhwangnankai.edu.cn The hepatopancreas in various crustacean species functions as the major digestive gland, and it is very sensitive to heavy metals and other water-born pollutants (Bhavan et al., 2000; Yang et al., 2007; Li et al., 2007). The excretion of digestive enzymes, which reflects digestive activity, is one of the most important physiological functions of the crustacean hepatopancreas, and is affected by copper (Cu2+) (Yang et al., 2005). Histo structural analysis of the hepatopancreas has been used as a practical means of assessing the physiological adaptation of Macrobrachium rosenbergii to copper (Li et al., 2007). Hg2+ is more toxic than Cu, or other metal ions, to Macrobrachium nipponensis and Eriocheir sinensis (Zhang et al., 2008; Zhao et al., 2009). Canli et al. (1995) reported that both organic, and inorganic, mercury were mainly taken up from seawater through the gills of Norway lobsters, Nephrops norvegicus. However, little is known about the histo structural changes within the hepatopancreas of E. sinensis after Hg2+ exposure. The objectives of this study were to examine survival rates, the activity of three antioxidant enzymes