Abstract—The acute toxicity of melathion (MAL), atrzine (ATR) and Cadmium (Cd) in zebrafish (Danio rerio) were investigated by acute toxicity (LC 50 ), and the results showed that the 96 h LC 50 followed the order: MAL>Cd>ATR. Furthermore, the accumulation of ecotoxicity in zebrafish body and correlated biochemical responses in brain and muscles were explored and it was found that MAL, ATR and Cd could be rapidly accumulated in fish body shortly after exposure. The activities of superoxide dismutase and catalase in fish body increased after 96h exposure. The acute toxicity caused protein reduction in brain and muscles, and elevation of total lipid dry weight in fish body. Besides, some dose-dependent inhibition of AChE activity was found in brain and muscles of zebrafish exposed to higher concentrations of acute toxins. These can be employed in biomonitoring programs because they showed good correlation with toxins and were sensitive to different concentration patterns. Index Terms—Acute toxicity, Danio rerio, heavy metal, organophosphorus I. INTRODUCTION With the uncontrolled globle development of industry from last century, both humans and other lifes are suffering directly or indirectly from various environmental problems, among which, water pollution is continuously posing serious threatens to us. Water sources can be directly polluted by domestic drainage, industrial effluents, medical waste and agricultural chemicals and then bioaccumulated harmful substance in aquatic organisms potentially pose a threaten to wildlife and humans [1]. Freshwater contamination with heavy metals has become a major concern over the past decades, not only because of their threat to public and water supply but also due to their damage to aquatic life, especially fish. Thus, heavy metal is considered to be one of the most important contamination sources strongly affecting fish population, growth and reproduction [2]. Instead of atmospheric sources, heavy metals often enter the environment mainly through human activities circulate in food webs and accumulate in living organisms [3]. Non-essential heavy metals are harmful and sometimes even lethal toxins that may lead to fertility decline, cellular damage, cell death and dysfunction of a variety of organs when accumulated in tissues [4]. Nevertheless, the ecological toxicity of heavy metals on the environment is still in the dark and unknown up to present. Therefore, more information is urgently required to unravel their acting mechanism and better understand the importance of these pollutants. Pesticides play an important role in maintaining agricultural production through the protection of all types of crops from pest attack and vector-borne diseases, but some pesticides have adverse effects on fish and other non-target animals. Organophosphate (OP) compounds are extensively used in the fields of agriculture as well as public health, making up 50% of the insecticides use world wide, because such compounds can degrade promptly due to their non-persistent nature [5]. Wacksman et al. pointed out that OP could inhibite the activity of acetylcholinesterase (AChE), and consequently, acetylcholine accumulated in the cholinergic receptors of the central and peripheral nervous systems [6]. Despite the fact that AChE could be used to indicate exposure to OP pesticides in non-target taxa [7], yet there is increasing evidence showing that other categories of chemicals can also affect these enzymatic activities in fish, such as heavy metals and complex mixtures of pollutants [8]. SODs are a metalloenzyme group that catalyse the dismutation of the superoxide anion radical to hydrogen peroxide (H 2 O 2 ) plus water, thus protecting organism against oxidative damage. The primary role of CAT is to decompose H 2 O 2 , reducing the oxidation caused by H 2 O 2 [12]. Antioxidant defense in cell plays a pivotal role in sustaining homeostasis of the organism, because it’s the regime of antioxidant defense that removes reactive oxygen produced by the metabolism of extraneous chemicals in the body well under normal physiological conditions. Some of these enzymes involved in antioxidant defense can serve as good molecular bioindicators for contaminant-mediated oxidative stress as well as indicating the magnitude of the response in populations subjected to pollutants such as heavy metals and other toxins. Fish, as the top predator of aquatic communities, are used broadly in the health assessment of aquatic ecosystems, because they had the largest magnitude of contaminant bioaccumulation in the food webs. Generally, for this reason, fish are used to assess the quality of aquatic systems and Biochemical Changes in the Tissues of Zebrafish (Danio rerio) Exposed to Organophosphorous Pesticides and Heavy Metal Cadmium Lixiao Wang, Abeer Ghazie Azize Al-sawafi, and Yunjun Yan International Journal of Environmental Science and Development, Vol. 8, No. 10, October 2017 707 doi: 10.18178/ijesd.2017.8.10.1043 Richetti, et al. manifested that AChE is an important biomarker for many environmental pollutants in zebrafish [9]. In ecotoxicology, cellular and biochemical parameters are strongly recommended to assess the effects of toxic chemicals to model organisms. Oxidative stress were quite important in the molecular mechanism related to OP-induced toxicity, and both superoxide dismutase (SOD) and catalase (CAT) are proper biomarkers of antioxidant [10], [11]. The Manuscript received June 23, 2017; revised August 31, 2017. The authors are with Huazhong University of Science and Technology, China (e-mail: yanyunjun@hust.edu.cn).