Background  Papia undulata is a famous economic shellfish in the South China Sea, mainly distributed in the coastal areas of Fujian, Guangdong, Guangxi and other provinces. Dry transport is the main method of the transportation of P. undulata. Long-term air exposure will affect the flavor of P. undulata and even lead to its death.

Objective  To investigate the effects of different air exposure time on immune related indices of P. undulata, a marine shellfish.

Methods  P. undulata was exposed to air for 0 h (control group), 6 h, 12 h and 24 h, respectively, and seawater culture was resumed after air exposure and the time nodes of seawater culture for 2, 6, 12 and 24 h, respectively, were restored. The indices of acid phosphatase (ACP), alkaline phosphatase (AKP), superoxide dismutase (SOD), catalase (CAT), malondialdehyde (MDA) and nitric oxide (NO) were determined after homogenization of the axilla unguiculata foot tissue at low temperature. Results Air exposure time within 24 h had no significant effect on the activities of hydrolase (ACP and AKP) (P>0.05), but had significant effects on the activities of antioxidant enzymes (SOD and CAT) and the content of oxygen free radicals (MDA and NO). Air exposure had a certain inhibitory effect on the activity of antioxidant enzymes, and the longer the air exposure time, the greater the fluctuation of SOD and CAT activities, and the more serious the oxidative damage. The longer the air exposure time, the higher the MDA content, the more serious the damage of P. undulata membrane, and the trend of NO content first increased and then decreased, which may be related to the protective effect of a large amount of NO produced by short time air exposure on the body.

Conclusion  The results indicate that the axe-foot tissue is not the main metabolic site of P. undulata, but the key site of antioxidant processes. The longer the air exposure time within 24 h, the greater the influence of antioxidant enzyme (SOD, CAT) activity and oxygen free radical (MDA, NO) content related immune indices, but had no significant effect on hydrolase (ACP, AKP) activity, indicating that the axe-foot tissue is the key site of antioxidant process in the body of P. undulata.

Significance  The results of this study provide reference for the screening of immune physiological indices and physiological response mechanism of P. undulata under air exposure stress, and provide basic data for the preservation of P. undulata during long-distance transportation.