Correlation between cardiac response and ammonia nitrogen of the razor clam (Sinonovacula constricta)

The razor clam (Sinonovacula constricta), a marine bivalve with high economic and ecological value, has been exposed to severe ammonia nitrogen (AN) environment due to its habits of benthic living. Therefore, an ammonia tolerant strain is needed for the sustainable development of the razor clam. Here, a non-invasive heartbeat detection method was used to investigate the correlation between cardiac response and AN of S. constricta. It was found that the electrocardiogram of 391 clams showed a sinus rhythm, and heart rate (HR) was negatively correlated with shell length, height and wet weight (P < 0.05). Furthermore, HR showed an obvious trend of first increasing and then decreasing under the continuous increase of AN concentration, which can be defined as the AN concentration at this inflection point as the breakpoint of AN (BPAN). To verify the effectiveness of BPAN as an index of ammonia tolerance for razor clam, 76 clams were selected as sensitive group (SG) and tolerance group (TG), then they were subjected to acute ammonia stress (similar to 226.67 mg/L). Results showed that the hemolymph ammonia content in both groups showed a significant increase trend after AN stress (P < 0.05), and the survival rate of the TG was significantly higher than that of the SG at 84 h and 96 h (P < 0.05). In addition, the stable heartbeat time of SG and TG after AN stress were positively correlated with shell length, height, width and wet weight (P < 0.05), and negatively correlated with HR (P < 0.05). The further study found that the continuous increase of AN concentration at 20 degrees C, 25 degrees C and 30 degrees C had different degrees of influence on BPAN and cardiac parameters of the clam. Based on these, the establishment of BPAN provides a new perspective for the breeding of new ammonia-tolerant variety of the razor clam. This study provides a basis for the establishment of cardiac activity and environment tolerance indexes of marine mollusks.