Alternation of immune parameters and cellular energy allocation of Chlamys farreri under ammonia-N exposure and Vibrio anguillarum challenge

The complex interactions among host, pathogen and environment are believed to be the main causes for the mass mortality of cultured scallops. In the present study, the temporal variations of immune parameters and cellular energy allocation (CEA) of Chlamys farreri under ammonia-N, Vibrio anguillarum as well as their combined treatment were investigated to better understand the energetic mechanisms of scallop in immune defense. After 1 d exposure to ammonia-N, V. anguillarum and their combination, the superoxide anion level and superoxide dismutase (SOD) activity in the serum of scallops increased substantially. At 24 d post exposure, the mRNA expression levels of isocitrate dehydrogenase (IDH), heat shock protein 70 (HSP 70), HSP 90 and glutamine synthetase (GS), as well as the malondialdehyde content remarkably increased, while SOD activity was depressed significantly (P < 0.05). The glycogen reserved in the tissues from scallops exposed to the combined stress for 1 d, 12 d and 24 d were significantly lower than those in the control (P < 0.05). The CEA values in all the examined tissues including gonad, gill, hepatopancreas and adductor muscle were significantly lower than those of control (P < 0.05) when exposure to ammonia-N, V. anguillarum and their combined treatment for 12 and 24 d. Furthermore, the combined stress also had a significant impact upon CEA in all the examined tissues in scallops post 1 d exposure (P < 0.05). The above results demonstrated that SOD, IDH, HSPs and GS in hemolymph of treated scallops are necessary, but not sufficient to the complete protection against stress-induced cellular damage along with the treatment duration. Immune defense against the combination of pathogen invasion and environmental stress can impose greater costs on scallop's energy expenditure than a single stressor, and the combined treatment preferentially consumed more available glycogen in scallops for immune defense. Hence, in addition to be used in immunological evaluation, CEA is also a powerful tool to provide valuable insights into possible mechanisms of mass mortalities in cultured scallops.