| Mollusc culture, a traditional marine aquacultute industry in China, playsan important role in the economic development of the costal provinces. However,since the summer of 1997, large-scale mortality of cultured scallop has causedcatastrophic losses to scallop aquaculture, and become a major constraint for thedevelopment of the culture industry. To improve health management practices, moreknowledge on the internal defense in scallop is necessary. So the followingexperiments were conducted: (1) comparison on scallop immunity between C. farreriand A. irradians before and after the environmental challenge;(2) comparison of theimmunocompetence of scallop C. farreri with different sizes. The results aresummarized as follows:Flow-cytometric method was adopted to determine hemocyte mortality,phagocytosis capacity and the respiratory burst. There were slightly difference inhemocyte mortality between C. farreri and A. irradians with the values of 4.95% and4.83%, respectively. Much like the hemocyte mortality, the reactive oxygens of A.irradians (1.94) detected under natural condition had no significant difference with C.farreri (1.56). But the phagocytosis rate of A. irradians (26.73%) was significantly higherthan that of C. farreri (19.89%) (p<0.05). Meantime, the activities of SOD,ACP andMDA in the haemocytes and the hepatopancreas were also analysed. The activities ofSOD in haemocytes and hepatopancreas of A. irradians (995.43 and 113.99) wereboth significantly higher than those of C. farreri (446.64 and 40.6) (p<0.05). Thesame trend was detected concerning the activities of ACP in both haemocytes andhepatopancreas between the two scallops. However, the content of MDA of the twoscallops in both haemocytes and hepatopancreas showed no significant difference, A.irradiant were 3.37 and 92.46, C. farreri were 2.17 and 28.96. All above resultsindicated that A. irradians had a higher potential capacity to destroy invadingpathogens and protect organism from oxidative damage than C. farreri.Several immune parameters (the hemocyte mortality, phagocytosis capacity andthe respiratory burst) and the enzyme activities (SOD, ACP and MDA) of C. farreriand A. irradians were measured after immersed in the different concentrations of Pb2+suspension (0, 0.2, 0.3 and 0.5 mg L-1). The results showed that thePb2+ exposures (0,0.2, 0.3 and 0.5 mg L-1) increased the hemocyte mortality compared to the controlgroup in both scallops. With the increase of the Pb2+ concentration, the hemocytemortality rate of C. farreri rose significantly, and was higher than that of A. irradians.Under the Pb2+ stress, the phagocytosis rate of C. farreri decreased sharply and waslower than the control group, while which of A. irradians increased even above thelevel of control group. The respiratory burst of C. farreri enhanced under Pb2+ stress,but with the increase of the Pb2+ concentration, the respiratory burst of it decreasedinstead. Whereas the respiratory burst response of A. irradians to the Pb2+ stressincreased significantly all the way. The activity of SOD increased after the Pb2+exposure in both scallops, and the A. irradians showed a higher activity of SOD thanC. farreri. Much like the SOD, the activity of ACP also went up with the increase ofthe Pb2+ concentration in both scallops. The content of MDA in all experimentalgroups increased significantly compared to the control groups, and the value of MDAincrement in C. farreri was higher than that in A. irradians. In conclusion, Pb2+ couldinfluence the immunocompetence of C. farreri and A. irradians greatly, and couldincrease their susceptibility to pathogen. Additionally, the results indicated that A.irradians was more tolerant against Pb2+ pollution than C. farreri.C. farreri and A. irradians were subjected to temperature rise from 18℃ to 30℃for three days. Several haemocyte functions were assessed before and after thetemperature elevation only in A. irradians, as a high mortality was observed in scallopC. farreri after temperature elevation. The haemocyte mortality after temperature risehad no significant difference compared with the control group, although there was atrend of increasing percentage of dead haemocytes. And a significant decrease ofphagocytosis rate was quantified after the temperature increased, suggesting that theimmunity potential was suppressed at high temperature. In contrast, the amount ofaccumulated ROS during the respiratory burst increased significantly aftertemperature elevation (P<0.05), indicating an intensive immune response to hightemperature. The results indicated that high temperature had profound influence onhaemocyte, and it might be one of the most important environment factors inducingthe outbreak of disease.The phagocytosis capacity and the enzyme activities of SOD,ACP and MDA inhaemocytes and hepatopancreas from the C. farreri at the same development stage butin different sizes were analysed. The results showed that the phagocytosis rate variedslightly between the two groups, the small size group was 22.82, and the big sizegroup was 20.96, respectively. The activities of SOD in haemocytes andhepatopancreas of the small size group were both significantly higher than those ofthe big size group (P<0.05). Similarly, the activity of ACP varied significantlybetween the two groups in haemocytes, the small size group was 44.3, and the big sizegroup was 2.91 (P<0.01). However, the activity of ACP varied slightly between thetwo groups in hepatopancreas, the small size group was 485.33, the big size groupwas 455.75. The MDA content in haemocytes was very low in both groups, the bigsize group was 2.67, the small size group was 3.17. The results revealed that there wasa negative relationship between the growth performance and immunity defense inscallop C. farreri.
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