**1. Introduction**

Cadmium (Cd) is one of the most toxic heavy metals for humans; the main source of nonoccupational exposure to Cd includes smoking, air, and food and water contaminated by Cd (Nagata et al., 2005). In addition, herbal medicine is another source of Cd. The World Health Organization (WHO) estimates that 4 billion people or 80 percent of the world population, presently use herbal medicine (Naithani et al., 2010). Several articles have reported of adverse effects of these herbal preparations due to the presence of high level of heavy metals such as Cd, lead, chromium, nickel, etc. (Naithani et al., 2010). Saeed et al. (2010) investigated twenty five herbal products. The results revealed that the concentrations of some heavy metals, including Cd, were far greater than the permissible limits proposed by the International Regulatory Authorities for herbal drugs. Acute or chronic exposure of Cd causes respiratory distress, lung, breast and endometrial cancers, cardiovascular disorders and endocrine dysfunction (Åkesson et al., 2008; Chang et al., 2009; Messner et al., 2009; Nagata et al., 2005; Naithani et al., 2010; Navas-Acien et al., 2004).

In addition, Cd is a common inorganic contaminant of coastal sediments and waters due to anthropogenic pollution and natural sources (Ivanina et al., 2008, 2010; Sokolova et al., 2004). It can be accumulated in aquatic animals (e.g. crabs, shrimps, oysters and mussels) after entering through different way such as respiratory tract, digestive tract, surface penetration etc. (Dailianis & Kaloyianni, 2004; Dailianis et al., 2009; Ivanina et al., 2008, 2010; Li et al., 2008b; Sokolova, 2004; Sokolova et al., 2004; Wang L. et al., 2001, 2002a,b, 2008; Wang Q. et al., 2003; Zhao et al., 1995). It is seriously harmful to the growth of aquatic life and survival, resulting in decline of their populations. At the same time, as aquatic food products, these animals exposed to Cd might threaten human health.

#### **1.1 Cd accumulation and distribution in crabs and shrimps**

Cd in waters can be absorbed by aquatic organisms via respiratory system, digestive system and body surface without significant excretion (Rainbow & White, 1989; van Hatton et al., 1989). And we can get valuable information for evaluating the level of Cd pollution in waters and sediments by assaying Cd concentration in crabs and shrimps.

Toxic Effects of Cadmium on Crabs and Shrimps 223

Small amounts of Cd can be detoxicified into non-toxic substance by metallothionein in the organism (van Hatton et al., 1989). Excessive Cd will damage the body, however, as it will combine with protein molecules having sulphur, hydroxyl and amino group, and restrain some enzyme system activity. In addition, because the affinity of Cd with sulfhydryl groups is stronger than zinc (Zn), it can replace the enzyme-bond Zn and cause the enzyme to lose

**1.2.1 The influence of Cd on antioxidant enzymes system in crabs and shrimps**  One of the mechanisms for Cd toxicity to animals is the oxidative damage. On one hand, Cd can cause the body to produce excessive active oxygen. On other hand, it can change the expression and vitality of antioxidant enzymes. Antioxidant enzymes mainly include the superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), glutathione enzyme turn sulfur (GST), etc. They can effectively scavenge active oxygen in the body and avoid oxidative damage to the body (Wang L. et al., 2007). Numerous studies have been published on the influence of Cd on antioxidant enzymes in terrestrial creatures, while reports about shrimps and crabs are rare. In one study the Cd concentration was 0.025 mg/L and 0.05 mg/L in water, and SOD, CAT and GPX activities in *Charybdis japonica* could be stimulated after 0.5 d, and then reduced during the experimental period (Pan & Zhang, 2006). When crabs (*S. yangtsekiense*) were exposed to the reagent with a dose range of 7.25-116.00 mg/L for 24, 48, 72 and 96 h, the activities of SOD, CAT and GPX increased initially and decreased subsequently (Li et al. 2008; Wang L. et al., 2008; Yan et al., 2007). After Immersing the juvenile crab *E. sinensis* in 2.0 mg/L water, the activities of SOD, CAT and GPX in hepatopancreas were all initially decreased, and then recovered to some degree during the duration of the study (Liu et al., 2003). This showed that low concentration of Cd stimulated antioxidant enzymes

**1.2 The influence of Cd on the enzyme activity in crabs and shrimps** 

activity while high concentration inhibited antioxidant enzymes activity.

**1.2.2 The influence of Cd on metabolic enzymes in crabs and shrimps** 

tissues were damaged and the enzyme released into serum (Lu et al., 1989).

observed above (Tang et al., 2000).

hepatopancreas and that is opposite in the abdominal muscles (Devi et al.,1994).

Glutamic-pyruvic transaminase (GPT) and glutamic-oxalacetic transaminease (GOT) are the important aminotransferase in the protein metabolism. Low concentration of Cd stimulated the activity of GPT and GOT in *Scylla serrata* while high Cd concentrations showed apparent inhibition. The results showed the obvious dose-effect relations (Tang et al., 2000). Effects of Cd on GOT and GPT activity are also tissue-specific. GPT and GOT activity decreased significantly in the heart, gills and hepatopancreas after *Macrobrachium rosenbergii* was poisoned by Cd, but increased in the green glands. This may be because green gland is excretory organ with strong detoxicification (Zhao et al., 1995). GPT activity in serum of *E. sinensis* increased with increasing Cd concentration after poisoning. That might be because

Lactic dehydrogenase (LDH) plays an important role in the carbohydrate metabolism. The crab *Uca pugilator* were immersed in 2.0 mg/L water for 24 h, 48 h, LDH activity reduced in

Alkaline phosphatase is a kind of low-specific phosphomonoesterase which plays an important role in nucleinic acid, protein and lipid metabolic. The influence of Cd on enzymatic activity in *S. serrata* also exhibited dose-effect relationship that was similar to that

its function (Müller & Ohnesorge, 1982).

#### **1.1.1 The difference of Cd accumulation and distribution in different tissues**

Experiments have confirmed that Cd absorption and accumulation by crabs and shrimps had obvious differences among the various body segments. Accumulated Cd was distributed to all organs with the highest proportions of body content being found in the exoskeleton, gills, hepatopancreas, and so on.

The first organ in which Cd accumulates is the exoskeleton. Cd has similar chemical properties to calcium (Ca), the main component of the exoskeleton, such as the same charge number, the similar ion diameter and electronic number. Therefore, the Cd in waters can replace the Ca entering the body via exoskeletons (Jennings & Rainbow, 1979). The gill is a respiratory organ for crabs or shrimps. It plays an important role in the absorption and transport of heavy metals (Silvestre et al., 2004; Silvestre et al., 2005a) and is the target organ of Cd in waters. The hepatopancreas are detoxicating organs in crabs and shrimps which can change the toxic heavy metal into non-toxic compounds and reduce the toxicity of the heavy metal in the body. Thus the Cd concentration is higher in the hepatopancreas.
