3.1 Heavy metal contents in three different tissues of field-collected crayfish
The contents of the selected heavy metals (Cr, As, Hg, Cd, and Pb) in the hepatopancreas, abdominal muscle, and gonad of crayfish collected from 7 sampling sites were summarized in Table 2. Hexavalent chromium was a group 1 carcinogen with multiple complex mechanisms by which it triggers cancer development. Increased levels of DNA adduct formation, chromosome breaks, and oxidative stress were the main mechanisms of Cr VI causing cellular damage (DesMarais & Costa 2019). The highest content of Cr was 0.5959±0.0026 mg/kg d.w in abdominal muscle of crayfish from Dongting Lake. This result was consistent with these observations that the average of Cr content in sediment from Dongting Lake (88.29 mg/kg) was higher than Cr contents from Poyang Lake (70.77 mg/kg), Taihu Lake (41.5 mg/kg), and Chaohu Lake (63.36 mg/kg) (Yuan et al. 2011; Jiang et al. 2012; Tang et al. 2010; Li et al. 2013). The lowest content of Cr was 0.0029±0.0029 mg/kg in gonad from Anqing. The contents of Cr in abdominal muscle were significantly higher than those in other tissues collected from 5 sampling sites (Chaohu Lake, Waishan Lake, Dongting Lake, Jianli, and Anqing). Kuklina et al (2014) also found that Cr was more concentrated in abdominal muscle. However, the content of Cr in abdominal muscle was far lower than those in exoskeleton, hepatopancreas, and gills of crayfish after exposure to waterborne Cr (Bollinger et al. 1997). For fish, Cr was investigated to enrich in bladder and liver (Wei et al. 2014). Possible reason was that the enrichment of Cr might be specific and change according to the surrounding environment.
Organic As was non-toxic whereas inorganic As was toxic. Trivalent form (As2O3) and pentavalent form (As2O5) are two oxidation states of inorganic As. However, As (Ⅲ) is 60 times more toxic than As (Ⅴ) (Ratnaike et al. 2003). In this study, the highest content of As was 2.7379±0.0306 mg/kg in hepatopancreas of crayfish collected from Taihu Lake, while the lowest content of As (0.1411±0.0035 mg/kg) was observed in abdominal muscle collected from Chaohu Lake. It was reported that the content of As in crayfish muscle collected from Chaohu Lake (0.1402-0.2410 mg/kg d.w) was higher than those in crayfish muscle collected from Poyang Lake (0.010-0.080 mg/kg d.w.) and Xiang River (0.037-0.141mg/kg d.w) (Wei et al. 2014; Jia et al. 2017). These results may indicate that As is more easily enriched in crayfish than other aquatic invertebrates. Results showed that As contents in hepatopancreas and gonad were markedly higher than those in muscle. According to the United States Food and Drug Administration (1993), the content of inorganic As could be estimated as 10% of total As. Based on this parameter, the contents of inorganic arsenic in all the samples did not exceed the safety limit set by the China National Standards Management Department (CNSMD).
Since the Minamata incident in Japan in 1950s, methylmercury in the aquatic environment and aquatic organisms has raised global concerns (Harada et al. 1995). Peng et al (2016a) reported that MeHg constituted 92-99% of mercury in crayfish muscle. The highest content of Hg was in muscle from Dongting Lake (0.0613±0.0022 mg/kg d.w) and the lowest content of Hg was in gonad from Taihu Lake (0.0065±0.0003 mg/kg). These results were consistent with the content of Hg in the muscle of crayfish collected from 23 cities in China (58.1±19.2 μg/kg). Notably, unlike the other four heavy metals, the total contents of Hg in hepatopancreas and gonad were significantly lower than that in muscle. The same results were also observed in previous researches (Stinson & Eaton 1983; Goldstein et al. 1996; Wei et al. 2014).
Table 2 Heavy contents (Cr, As, Hg, Cd, and Pb) (mg/kg ww) of crayfish (hepatopancreas, muscle, and gonad) sampled from 7 locations in China (mean ± SD)
Location
|
Tissues
|
Heavy metal content (mg/kg w.w)
|
Cr
|
As
|
Hg
|
Cd
|
Pb
|
Chaohu Lake
|
Hepatopancreas
|
0.0965±0.0220
|
0.5660±0.0656
|
0.0116±0.0048
|
0.3086±0.0372
|
0.0319±0.0051
|
Muscle
|
0.2123±0.0031
|
0.1411±0.0035
|
0.0484±0.0015
|
0.0011±0.0003
|
0.0156±0.0005
|
Gonad
|
0.2058±0.0022
|
1.1402±0.0039
|
0.0201±0.0019
|
0.0017±0.0005
|
nd
|
Poyang Lake
|
Hepatopancreas
|
0.1508±0.0039
|
1.0742±0.0187
|
0.0188±0.0006
|
4.5671±0.0407
|
0.0518±0.0006
|
Muscle
|
0.1122±0.0049
|
0.2106±0.0044
|
0.0279±0.0018
|
0.0016±0.0008
|
0.0141±0.0006
|
Gonad
|
0.1607±0.0042
|
1.5155±0.0144
|
0.0101±0.0008
|
0.0015±0.0011
|
nd
|
Weishan Lake
|
Hepatopancreas
|
0.0181±0.0006
|
1.7004±0.0155
|
0.0096±0.0019
|
0.4277±0.0046
|
0.0561±0.0006
|
Muscle
|
0.1936±0.0058
|
0.1976±0.0022
|
0.0197±0.0024
|
0.0006±0.0004
|
0.0240±0.0008
|
Gonad
|
0.0602±0.0053
|
1.3303±0.0197
|
0.0054±0.0021
|
0.0002±0.0002
|
nd
|
Taihu Lake
|
Hepatopancreas
|
0.0723±0.0009
|
2.7379±0.0306
|
0.0107±0.0007
|
0.8850±0.0099
|
0.0400±0.0008
|
Muscle
|
0.0811±0.0013
|
0.1718±0.0032
|
0.0158±0.0002
|
0.0004±0.0002
|
0.0168±0.0007
|
Gonad
|
0.0998±0.0029
|
1.0660±0.0267
|
0.0065±0.0003
|
0.0014±0.0005
|
nd
|
Jianli
|
Hepatopancreas
|
0.0868±0.0016
|
1.9231±0.0245
|
0.0183±0.0018
|
2.1747±0.0236
|
0.1443±0.0010
|
Muscle
|
0.1564±0.0167
|
0.2085±0.0136
|
0.0302±0.0014
|
0.0012±0.0006
|
0.0195±0.0033
|
Gonad
|
0.0118±0.0434
|
1.2270±0.1511
|
0.0117±0.0011
|
0.0015±0.0010
|
nd
|
Anqing
|
Hepatopancreas
|
0.0877±0.0347
|
0.7771±0.1315
|
0.0125±0.0007
|
0.6907±0.1346
|
0.0464±0.0110
|
Muscle
|
0.2984±0.0052
|
0.1863±0.0092
|
0.0475±0.0035
|
0.0010±0.0003
|
0.0062±0.0006
|
Gonad
|
0.0029±0.0029
|
1.3641±0.0328
|
0.0150±0.0005
|
0.0013±0.0005
|
nd
|
Dongting Lake
|
Hepatopancreas
|
0.0447±0.0075
|
2.1492±0.1494
|
0.0190±0.0004
|
1.3947±0.1367
|
0.2141±0.0020
|
Muscle
|
0.5959±0.0026
|
0.2945±0.0052
|
0.0613±0.0022
|
0.0014±0.0004
|
0.0373±0.0047
|
Gonad
|
0.2656±0.0434
|
1.2510±0.1255
|
0.0168±0.0013
|
0.0089±0.1432
|
0.0589±0.0066
|
* ND = not detected
Cadmium was one of the global health problems that affected many organs and in some cases it could even cause deaths. Long-term exposure to cadmium through food, soil, water, and air might cause cancer and organ system toxicity such as cardiovascular, skeletal, reproductive, urinary, respiratory systems, and central and peripheral nervous (Rahimzadeh et al. 2017). The highest Cd content was detected in hepatopancreas of crayfish collected from Poyang Lake (4.5671±0.0407 mg/kg d.w). The contents of Cd in hepatopancreas of crayfish collected from Poyang Lake, Taihu Lake, Dongting Lake, Anqing, and Jianli exceeded the threshold values in the national food safety standards of China. The contents of Cd in sediment were 0.13-1.49 mg/kg in Poyang Lake, 1.71 mg/kg in Dongting Lake, and 0.20-2.88 mg/kg in Taihu Lake, which exceeded the content of class three (1 mg/kg) from the Chinese Environmental Quality Standard for sediment (Zhang et al. 2012; Qin et al. 2012; Hu et al. 2015). These evidences indicated that cadmium pollution was serious in these areas. Hepatopancreas is the main organ of cadmium accumulation and detoxification in crayfish (Kouba et al. 2010). The contents of Cd in abdominal muscle and gonad (<0.0089 mg/kg d.w) were far below the Cd contents in hepatopancreas in this study. The contents of Cd in hepatopancreas exhibited an apparent positive correlation between accumulation time and exposure contents while muscle did not (Zhang et al. 2014). This might be due to the existence of metallothionein proteins in hepatopancreas which could bind Cd for detoxification (Ploetz et al. 2007). The present study showed that the contents of Cd in muscle were lower than those reported in other studies, including 1.2-60.6 μg/kg d.w Cd from 12 provinces in China (Peng et al. 2016b), and 0.08 mg/kg d.w Cd from Lake Washington (Stinson & Eaton 1983).
Lead toxicity is a major public health problem in developed and developing countries. Both acute and chronic exposure to lead have the potential to cause many deleterious systematic effects, including immune imbalances, frank anemia, hypertension, cognitive deficits, vitamin D deficiency, infertility, gastrointestinal effects, and delayed skeletal and deciduous dental development (Mitra et al. 2017). The highest content of Pb was observed in hepatopancreas collected from Dongting Lake (0.2141±0.0020 mg/kg d.w). The contents of Pb in abdominal muscle ranged from 0.0062 mg/kg d.w to 0.0373 mg/kg d.w, which was consistent with the values (mean 0.023 mg/kg d.w) reported by Peng et al (2016b). The contents of Pb in gonad were not detected, except those in Dongting Lake. The contents of Pb in hepatopancreas were slightly higher when compared with the those in abdominal muscle and gonad. The same results were also reported by previous studies (Wei et al. 2014; Jia et al. 2017). Roldan and Shivers (1987) indicated that Pb could store in metal-containing vacuoles of hepatopancreatic cells.
Metals showed different affinity to organs, which might be due to the different functions of organs and metabolic roles of metals (Ashraf 2005). As presented in Table 2, it was concluded that the hepatopancreas was the primary organ for Cd, As, and Pb deposition, the abdominal muscle was the ideal organ for Cr and Hg deposition, and the gonad was the primary organs for As deposition. The maximum limit required of these heavy metals for crayfish in the national food safety standards of China were 2 mg/kg w.w (Cr), 0.5 mg/kg w.w (Inorganic As), 0.5 mg/kg w.w (MeHg), 0.5 mg/kg w.w (Cd), and 0.5 mg/kg w.w (Pb), respectively (GB 2762 2017). The values of Cr, As, Hg, and Pb contents in these three different tissues of crayfish all met national food safety standards of China. However, the content of Cd in hepatopancreas exceeded the lowest limit. However, the studies couldn’t just focus on the heavy metal content of crayfish. Health risk assessment is essential.
3.2 Health risk assessment
Fig. 2 presented the MPI values for heavy metal in three tissues collected from seven sampling points. Considering the MPI in different tissues, the distribution of the heavy metals was in the ascending order of abdominal muscle < gonad < hepatopancreas for the seven sites. Generally, the muscle is weak to accumulate heavy metals. The liver and gill, as metabolically active organs, have a great tendency to store high levels of heavy metals (Monikh et al. 2013). Considering the MPI in same tissues, the order of MPI in hepatopancreas was as follows: Jianli > Poyang Lake > Dongting Lake > Taihu Lake > Anqing > Weishan Lake > Chaohu Lake; the order of MPI in abdominal muscle was as follows: Dongting Lake > Chaohu Lake > Jianli > Anqing > Poyang Lake > Weishan Lake > Taihu Lake; the order of MPI in gonad was as follows: Dongting Lake > Chaohu Lake > Poyang Lake > Taihu Lake > Jianli > Anqing.
The THQ provided an indication of the risk level associated with pollutant exposure. This method of risk estimation had recently been used by many researchers and had been shown to be valid and useful (Yi et al. 2011). In this study, the index of THQ was introduced to estimate potential health risk of chronic exposure to heavy metals in crayfish. As shown in Fig. 3, the average THQ of individual heavy metal in abdominal muscle followed the order 1 > Hg > Cr > As > Cd > Pb for all sampling sites. It was worth noting that hepatopancreas was the favorite food of crayfish for Chinese, while the heavy metals tended to be enriched in the hepatopancreas, so it could not be ignored when evaluated. The average THQ of individual heavy metal in hepatopancreas followed the order 1 > Cd > As > Hg > Cr > Pb (Fig. 4), suggesting that risk of heavy metals exposure via crayfish consumption in these locations is extremely low.