Assessment Of Common Submerged Macrophytes In East Taihu Lake As Feed Resources For Eriocheir Sinensis

The contents of nutritional components and heavy metals from five submerged macrophytes, Elodea canadensis, Vallisneria natans, Potamogeton malaianus, Ceratophyllum demersum L. and Hydrilla verticillata, in East Taihu Lake were determined. The results showed that the crude protein ranged from 17.99%²3.00%, crude fat from 3.37 to 6.12%, ash from 29.82 to 34.71%, total sugar from 8.49 to 23.29%, total amino acids from 103.32 to 134.12g·kg^-1, and the essential amino acids from 39.43 to 49.60 g·kg^-1 in these five submerged macrophytes on the basis of dry weight. The results also revealed that a wide range of mineral elements, such as Mn, Cu, Fe, Zn, K, Ca were found in these plants, and Ca was as high as 13.28 95.57 g·kg^-1. Even more importantly, lower contents of heavy metals, including Pb, Hg, As, Cr, Cd were detected in these plants, which met the national formula feed safety limit for aquatic animals.The dry powder of these plants were incorporated, respectively, into the basic diet at a level of 15% (w/w) to feed Chinese mitten crab (initial body weight: 9.37±0.15 g) every day for 8 weeks. The feeding experiment was conducted in an indoor recirculating aquaculture system. In order to provide scientific basis for submerged macrophytes as feed resources of Chinese mitten crab, body growth rate, immunity functions, hepatopancreas structure and flavor were analyzed.These feed macrophytes produced different effects on the survival rates of Chinese mitten crab, which were listed in the order of importance as E. canadensis > H. verticillata > V. natans > C. demersum L.> control group > P. malaianus, and there were no significant difference among these groups (p>0.05); The effects on the average growth rate were listed in the order of importance as H. verticillata > E. canadensis > control group > V. natans group > C. demersum L. > P. malaianus, and P. malaianus group was lower than the control significantly (p<0.05), the other submerged macrophytes groups were no significant difference compared with control (p>0.05). As for their effects on FCR, they were listed in decreasing order as E. canadensis > H. verticillata > V. natans > the control > C. demersum L. > P. malaianus and P. malaianus group was higherthan the control significantly (p<0.05), the other submerged macrophytes groups were no significant difference compared with control (p>0.05). With respect to the nonspecific immunity and disease resistance of Chinese mitten crab, No significant difference was found in transaminase activities in serum among all groups (p>0.05). The superoxide dismutase (SOD) activities of treatment groups with submerged macrophytes were higher than the control significantly (p<0.05). The activity of lysozyme (LSZ) from groups by feeding H. verticillata, E. canadensis, or V. natans, were higher than the control significantly (p<0.05), P. malaianus group and C. demersum L. group also present higher lysozyme activity than the control, but no significant difference was detected (p>0.05). In this condition, submerged macrophytes groups enhanced the resistance of Chinese mitten crab to Aeromonas hydrophila infection to a certain extent except P. malaianus group. The submerged macrophytes groups maintained a high digestive enzyme activity except P. malaianus group. The histology of hepatopancreas showed that basement membrane was complete, the structure of cell was normal, there were no obvious pathological changes in hepatopancreas. TEM results showed that the micro-villous structure of the control, E. canadensis group, H. verticillata group and V. natans group were tight and neat, but the microvilli of liver cells from P. malaianus group and C. demersum L. group were sparsely distributed, even some have been broken off, and caused slight damage. The incorporation of submerged macrophytes into basic diet could increased the levels of free amino acids and delicious amino acids in muscles and hepatopancreas of Chinese mitten crab, and supplements with E. canadensis and H. verticillata produced these effects significantly (p<0.05). No significant decrease in SFA or increase in MUFA, PUFA, UFA were found in muscles and hepatopancreas among all groups (p>0.05).It might be suggested that the main submerged macrophytes in East Taihu Lake are rich in nutritional components, and all meet the safety standard for fish feed. Except P. malaianus, the other four submerged macrophytes had no effect on the growth of Chinese mitten crab, and they could enhance the immunity and digestion of Chinese mitten crab to a certain extent. H. verticillata, E. canadensis, V. natans could enhance the flavor of Chinese mitten crab by elevating the contents of free amino acids and delicious amino acids in muscles and hepatopancreas of Chinese mitten crab. All the tests showed that H. verticillata, E. canadensis, V. natans and C. demersum L. could be exploited as feed resources for Chinese mitten crab.