Furthermore, Cd can directly activate endogenous endonuclease, resulting in cleavage of the DNA with the appearance of a characteristic ladder pattern. In this process, caspase-9, the key enzyme, can cleave and activate downstream effector caspases, such as pro-caspase-3, and the active executioner caspases are responsible for cleaving of their target substrates to induce apoptosis. Many studies concerning Cd-induced damages in testis have been performed on rodents. However, studies on Cdinjury in the testis of a crustacean are limited. Sinopotamon henanense, a representative of decapod crustacean, is widely distributed in freshwater areas of the Yangtze River drainage, Huaihe River drainage and Yellow River Valley of China. The background level of Cd was 0.019 mg/L in the Yangtze River water system and the content of the Cd was 0.187–0.72 mg/g in the suspended particulate matters of the Yangtze River water system. In some valleys near the Cd-rich mines, the Cd content in the sediments attained 231.7 mg/kg. Furthermore, Cd contamination caused by human activity is much more prominent than by the natural erosion process in some places. The dissolved Cd due to acidic pH resulted in the secondary pollution in aquatic environments. Freshwater crab lives in the sediments of streams and has the capability of accumulating heavy metals. Therefore, the crab is a suitable bioindicator for aquatic heavy metals pollution. Previous studies in our group have shown that distinct ultrastructural changes appeared in the testis after an injection of a dose of CdCl2 in S. henanense. The present study was designed to investigate the cytotoxic effects of Cd on the testis and explore the probable mechanisms of apoptosis. The biochemical, physiological, and morphological alterations of the testis were investigated in S. henanense after acute Cd exposure. Spermatogenesis is a complex multi-temporal process, including proliferation and differentiation of spermatogonia, meiosis and spermiogenesis. In this process, any of the affected areas are likely to cause spermatogenesis impairment, and even infertility. Cd is an important heavy metal widely used in NiCd batteries, metal plating, pigments, plastics and alloys. This metal stimulates free-radical production, resulting in oxidative deterioration of lipids, proteins and DNA, as well as initiating various pathological conditions in humans and Crizotinib animals. Cd exerts adverse effects on structures and functions of reproductive organs directly at the testis level or by altering post-testis events such as sperm progress motility and/or function, all of which may culminate in hypogonadism and infertility. In the present study, we investigated the effects of Cd on oxidative stress and apoptosis of testes germ cells in crabs. Oxidative stress, which is induced by reactive oxygen species such as superoxide anion, hydrogen peroxide and hydroxyl radical, is known to play a critical role in testis injury.. Similar to H2O2, MDA was also increased with increasing Cd concentrations.